data communication essentials
DESCRIPTION
Data Communication Essentials. Excerpted from Joe Conron’s data communications course. A Communications Model. Source generates data to be transmitted Transmitter Converts data into transmittable signals Transmission System Carries data Receiver Converts received signal into data - PowerPoint PPT PresentationTRANSCRIPT
Data Communication Essentials
Excerpted from Joe Conronrsquos data communications course
A Communications Modelbull Source
ndash generates data to be transmittedbull Transmitter
ndash Converts data into transmittable signalsbull Transmission System
ndash Carries databull Receiver
ndash Converts received signal into databull Destination
ndash Takes incoming data
Simplified Communications Model - Diagram
Key Communications Tasksbull Transmission System Utilizationbull Interfacingbull Signal Generationbull Synchronizationbull Error detection and correctionbull Addressing and routingbull Recoverybull Message formattingbull Securitybull Network Management
Networking
bull Point to point communication not usually practicalndash Devices are too far apartndash Large set of devices would need impractical
number of connectionsbull Solution is a communications network
Simplified Network Model
The Network Core
bull mesh of interconnected routers
bull the fundamental question how is data transferred through netndash circuit switching dedicated
circuit per call telephone net
ndash packet-switching data sent thru net in discrete ldquochunksrdquo
Network Core Circuit Switching
End-end resources reserved for ldquocallrdquo
bull link bandwidth switch capacity
bull dedicated resources no sharing
bull circuit-like (guaranteed) performance
bull call setup required
Circuit Switching FDM and TDM
FDM
frequency
timeTDM
frequency
time
4 users
Example
Network Core Packet Switching
each end-end data stream divided into packets
bull user A B packets share network resources
bull each packet uses full link bandwidth
bull resources used as needed
resource contention bull aggregate resource
demand can exceed amount available
bull congestion packets queue wait for link use
bull store and forward packets move one hop at a timendash Node receives complete
packet before forwarding
Packet Switching Statistical Multiplexing
Sequence of A amp B packets does not have fixed pattern statistical multiplexing
In TDM each host gets same slot in revolving TDM frame
A
B
C10 MbsEthernet
15 Mbs
D E
statistical multiplexing
queue of packetswaiting for output
link
Packet switching versus circuit switching
bull Great for bursty datandash resource sharingndash simpler no call setup
bull Excessive congestion packet delay and lossndash protocols needed for reliable data transfer
congestion controlbull Q How to provide circuit-like behavior
ndash bandwidth guarantees needed for audiovideo appsndash still an unsolved problem (chapter 6)
Is packet switching a ldquoslam dunk winnerrdquo
Local Area Networks
bull Smaller scopendash Building or small campus
bull Usually owned by same organization as attached devices
bull Data rates much higherbull Usually broadcast systems
Protocolsbull Used for communications between entities in a systembull Must speak the same languagebull Entities
ndash User applicationsndash e-mail facilitiesndash terminals
bull Systemsndash Computerndash Terminalndash Remote sensor
Key Elements of a Protocol
bull Syntaxndash Data formatsndash Signal levels
bull Semanticsndash Control informationndash Error handling
bull Timingndash Speed matchingndash Sequencing
Whatrsquos a protocol
human protocolsbull ldquowhatrsquos the timerdquobull ldquoI have a questionrdquobull introductions
hellip specific msgs senthellip specific actions taken
when msgs received or other events
network protocolsbull machines rather than
humansbull all communication activity in
Internet governed by protocols
protocols define format order of msgs sent and received among network entities and actions taken on msg transmission receipt
Whatrsquos a protocola human protocol and a computer network protocol
Hi
HiGot thetime200
TCP connection reqTCP connectionreply
Get httpgaiacsumasseduindexhtm
ltfilegttime
In Summary a protocol is
bull An agreement about communication between two or more entities
bull It specifiesndash Format of messagesndash Meaning of messagesndash Rules for exchangendash Procedures for handling problems
Protocol Specification
bull As designers we can specify a protocol usingndash Event-Time Diagramsndash Transition Diagrams
bull We can implement a protocol with a Finite State Machine (FSM)
bull Internet Protocols are formalized by RFCs which are administered by IETF
bull You can find any RFC here
042223 20
Event -Time Diagrams
bull Define causal ordering
bull Define indicationrequestresponse actions
Transition Diagram
bull Illustratesndash Statesndash Input (the Event that causes transition)ndash Transitions (to new states)
Protocol ldquoLayersrdquo
Networks are complex
bull many ldquopiecesrdquondash hostsndash routersndash links of various
mediandash applicationsndash protocolsndash hardware software
Question Is there any hope of organizing
structure of network
Or at least our discussion of networks
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
A Communications Modelbull Source
ndash generates data to be transmittedbull Transmitter
ndash Converts data into transmittable signalsbull Transmission System
ndash Carries databull Receiver
ndash Converts received signal into databull Destination
ndash Takes incoming data
Simplified Communications Model - Diagram
Key Communications Tasksbull Transmission System Utilizationbull Interfacingbull Signal Generationbull Synchronizationbull Error detection and correctionbull Addressing and routingbull Recoverybull Message formattingbull Securitybull Network Management
Networking
bull Point to point communication not usually practicalndash Devices are too far apartndash Large set of devices would need impractical
number of connectionsbull Solution is a communications network
Simplified Network Model
The Network Core
bull mesh of interconnected routers
bull the fundamental question how is data transferred through netndash circuit switching dedicated
circuit per call telephone net
ndash packet-switching data sent thru net in discrete ldquochunksrdquo
Network Core Circuit Switching
End-end resources reserved for ldquocallrdquo
bull link bandwidth switch capacity
bull dedicated resources no sharing
bull circuit-like (guaranteed) performance
bull call setup required
Circuit Switching FDM and TDM
FDM
frequency
timeTDM
frequency
time
4 users
Example
Network Core Packet Switching
each end-end data stream divided into packets
bull user A B packets share network resources
bull each packet uses full link bandwidth
bull resources used as needed
resource contention bull aggregate resource
demand can exceed amount available
bull congestion packets queue wait for link use
bull store and forward packets move one hop at a timendash Node receives complete
packet before forwarding
Packet Switching Statistical Multiplexing
Sequence of A amp B packets does not have fixed pattern statistical multiplexing
In TDM each host gets same slot in revolving TDM frame
A
B
C10 MbsEthernet
15 Mbs
D E
statistical multiplexing
queue of packetswaiting for output
link
Packet switching versus circuit switching
bull Great for bursty datandash resource sharingndash simpler no call setup
bull Excessive congestion packet delay and lossndash protocols needed for reliable data transfer
congestion controlbull Q How to provide circuit-like behavior
ndash bandwidth guarantees needed for audiovideo appsndash still an unsolved problem (chapter 6)
Is packet switching a ldquoslam dunk winnerrdquo
Local Area Networks
bull Smaller scopendash Building or small campus
bull Usually owned by same organization as attached devices
bull Data rates much higherbull Usually broadcast systems
Protocolsbull Used for communications between entities in a systembull Must speak the same languagebull Entities
ndash User applicationsndash e-mail facilitiesndash terminals
bull Systemsndash Computerndash Terminalndash Remote sensor
Key Elements of a Protocol
bull Syntaxndash Data formatsndash Signal levels
bull Semanticsndash Control informationndash Error handling
bull Timingndash Speed matchingndash Sequencing
Whatrsquos a protocol
human protocolsbull ldquowhatrsquos the timerdquobull ldquoI have a questionrdquobull introductions
hellip specific msgs senthellip specific actions taken
when msgs received or other events
network protocolsbull machines rather than
humansbull all communication activity in
Internet governed by protocols
protocols define format order of msgs sent and received among network entities and actions taken on msg transmission receipt
Whatrsquos a protocola human protocol and a computer network protocol
Hi
HiGot thetime200
TCP connection reqTCP connectionreply
Get httpgaiacsumasseduindexhtm
ltfilegttime
In Summary a protocol is
bull An agreement about communication between two or more entities
bull It specifiesndash Format of messagesndash Meaning of messagesndash Rules for exchangendash Procedures for handling problems
Protocol Specification
bull As designers we can specify a protocol usingndash Event-Time Diagramsndash Transition Diagrams
bull We can implement a protocol with a Finite State Machine (FSM)
bull Internet Protocols are formalized by RFCs which are administered by IETF
bull You can find any RFC here
042223 20
Event -Time Diagrams
bull Define causal ordering
bull Define indicationrequestresponse actions
Transition Diagram
bull Illustratesndash Statesndash Input (the Event that causes transition)ndash Transitions (to new states)
Protocol ldquoLayersrdquo
Networks are complex
bull many ldquopiecesrdquondash hostsndash routersndash links of various
mediandash applicationsndash protocolsndash hardware software
Question Is there any hope of organizing
structure of network
Or at least our discussion of networks
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Simplified Communications Model - Diagram
Key Communications Tasksbull Transmission System Utilizationbull Interfacingbull Signal Generationbull Synchronizationbull Error detection and correctionbull Addressing and routingbull Recoverybull Message formattingbull Securitybull Network Management
Networking
bull Point to point communication not usually practicalndash Devices are too far apartndash Large set of devices would need impractical
number of connectionsbull Solution is a communications network
Simplified Network Model
The Network Core
bull mesh of interconnected routers
bull the fundamental question how is data transferred through netndash circuit switching dedicated
circuit per call telephone net
ndash packet-switching data sent thru net in discrete ldquochunksrdquo
Network Core Circuit Switching
End-end resources reserved for ldquocallrdquo
bull link bandwidth switch capacity
bull dedicated resources no sharing
bull circuit-like (guaranteed) performance
bull call setup required
Circuit Switching FDM and TDM
FDM
frequency
timeTDM
frequency
time
4 users
Example
Network Core Packet Switching
each end-end data stream divided into packets
bull user A B packets share network resources
bull each packet uses full link bandwidth
bull resources used as needed
resource contention bull aggregate resource
demand can exceed amount available
bull congestion packets queue wait for link use
bull store and forward packets move one hop at a timendash Node receives complete
packet before forwarding
Packet Switching Statistical Multiplexing
Sequence of A amp B packets does not have fixed pattern statistical multiplexing
In TDM each host gets same slot in revolving TDM frame
A
B
C10 MbsEthernet
15 Mbs
D E
statistical multiplexing
queue of packetswaiting for output
link
Packet switching versus circuit switching
bull Great for bursty datandash resource sharingndash simpler no call setup
bull Excessive congestion packet delay and lossndash protocols needed for reliable data transfer
congestion controlbull Q How to provide circuit-like behavior
ndash bandwidth guarantees needed for audiovideo appsndash still an unsolved problem (chapter 6)
Is packet switching a ldquoslam dunk winnerrdquo
Local Area Networks
bull Smaller scopendash Building or small campus
bull Usually owned by same organization as attached devices
bull Data rates much higherbull Usually broadcast systems
Protocolsbull Used for communications between entities in a systembull Must speak the same languagebull Entities
ndash User applicationsndash e-mail facilitiesndash terminals
bull Systemsndash Computerndash Terminalndash Remote sensor
Key Elements of a Protocol
bull Syntaxndash Data formatsndash Signal levels
bull Semanticsndash Control informationndash Error handling
bull Timingndash Speed matchingndash Sequencing
Whatrsquos a protocol
human protocolsbull ldquowhatrsquos the timerdquobull ldquoI have a questionrdquobull introductions
hellip specific msgs senthellip specific actions taken
when msgs received or other events
network protocolsbull machines rather than
humansbull all communication activity in
Internet governed by protocols
protocols define format order of msgs sent and received among network entities and actions taken on msg transmission receipt
Whatrsquos a protocola human protocol and a computer network protocol
Hi
HiGot thetime200
TCP connection reqTCP connectionreply
Get httpgaiacsumasseduindexhtm
ltfilegttime
In Summary a protocol is
bull An agreement about communication between two or more entities
bull It specifiesndash Format of messagesndash Meaning of messagesndash Rules for exchangendash Procedures for handling problems
Protocol Specification
bull As designers we can specify a protocol usingndash Event-Time Diagramsndash Transition Diagrams
bull We can implement a protocol with a Finite State Machine (FSM)
bull Internet Protocols are formalized by RFCs which are administered by IETF
bull You can find any RFC here
042223 20
Event -Time Diagrams
bull Define causal ordering
bull Define indicationrequestresponse actions
Transition Diagram
bull Illustratesndash Statesndash Input (the Event that causes transition)ndash Transitions (to new states)
Protocol ldquoLayersrdquo
Networks are complex
bull many ldquopiecesrdquondash hostsndash routersndash links of various
mediandash applicationsndash protocolsndash hardware software
Question Is there any hope of organizing
structure of network
Or at least our discussion of networks
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Key Communications Tasksbull Transmission System Utilizationbull Interfacingbull Signal Generationbull Synchronizationbull Error detection and correctionbull Addressing and routingbull Recoverybull Message formattingbull Securitybull Network Management
Networking
bull Point to point communication not usually practicalndash Devices are too far apartndash Large set of devices would need impractical
number of connectionsbull Solution is a communications network
Simplified Network Model
The Network Core
bull mesh of interconnected routers
bull the fundamental question how is data transferred through netndash circuit switching dedicated
circuit per call telephone net
ndash packet-switching data sent thru net in discrete ldquochunksrdquo
Network Core Circuit Switching
End-end resources reserved for ldquocallrdquo
bull link bandwidth switch capacity
bull dedicated resources no sharing
bull circuit-like (guaranteed) performance
bull call setup required
Circuit Switching FDM and TDM
FDM
frequency
timeTDM
frequency
time
4 users
Example
Network Core Packet Switching
each end-end data stream divided into packets
bull user A B packets share network resources
bull each packet uses full link bandwidth
bull resources used as needed
resource contention bull aggregate resource
demand can exceed amount available
bull congestion packets queue wait for link use
bull store and forward packets move one hop at a timendash Node receives complete
packet before forwarding
Packet Switching Statistical Multiplexing
Sequence of A amp B packets does not have fixed pattern statistical multiplexing
In TDM each host gets same slot in revolving TDM frame
A
B
C10 MbsEthernet
15 Mbs
D E
statistical multiplexing
queue of packetswaiting for output
link
Packet switching versus circuit switching
bull Great for bursty datandash resource sharingndash simpler no call setup
bull Excessive congestion packet delay and lossndash protocols needed for reliable data transfer
congestion controlbull Q How to provide circuit-like behavior
ndash bandwidth guarantees needed for audiovideo appsndash still an unsolved problem (chapter 6)
Is packet switching a ldquoslam dunk winnerrdquo
Local Area Networks
bull Smaller scopendash Building or small campus
bull Usually owned by same organization as attached devices
bull Data rates much higherbull Usually broadcast systems
Protocolsbull Used for communications between entities in a systembull Must speak the same languagebull Entities
ndash User applicationsndash e-mail facilitiesndash terminals
bull Systemsndash Computerndash Terminalndash Remote sensor
Key Elements of a Protocol
bull Syntaxndash Data formatsndash Signal levels
bull Semanticsndash Control informationndash Error handling
bull Timingndash Speed matchingndash Sequencing
Whatrsquos a protocol
human protocolsbull ldquowhatrsquos the timerdquobull ldquoI have a questionrdquobull introductions
hellip specific msgs senthellip specific actions taken
when msgs received or other events
network protocolsbull machines rather than
humansbull all communication activity in
Internet governed by protocols
protocols define format order of msgs sent and received among network entities and actions taken on msg transmission receipt
Whatrsquos a protocola human protocol and a computer network protocol
Hi
HiGot thetime200
TCP connection reqTCP connectionreply
Get httpgaiacsumasseduindexhtm
ltfilegttime
In Summary a protocol is
bull An agreement about communication between two or more entities
bull It specifiesndash Format of messagesndash Meaning of messagesndash Rules for exchangendash Procedures for handling problems
Protocol Specification
bull As designers we can specify a protocol usingndash Event-Time Diagramsndash Transition Diagrams
bull We can implement a protocol with a Finite State Machine (FSM)
bull Internet Protocols are formalized by RFCs which are administered by IETF
bull You can find any RFC here
042223 20
Event -Time Diagrams
bull Define causal ordering
bull Define indicationrequestresponse actions
Transition Diagram
bull Illustratesndash Statesndash Input (the Event that causes transition)ndash Transitions (to new states)
Protocol ldquoLayersrdquo
Networks are complex
bull many ldquopiecesrdquondash hostsndash routersndash links of various
mediandash applicationsndash protocolsndash hardware software
Question Is there any hope of organizing
structure of network
Or at least our discussion of networks
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Networking
bull Point to point communication not usually practicalndash Devices are too far apartndash Large set of devices would need impractical
number of connectionsbull Solution is a communications network
Simplified Network Model
The Network Core
bull mesh of interconnected routers
bull the fundamental question how is data transferred through netndash circuit switching dedicated
circuit per call telephone net
ndash packet-switching data sent thru net in discrete ldquochunksrdquo
Network Core Circuit Switching
End-end resources reserved for ldquocallrdquo
bull link bandwidth switch capacity
bull dedicated resources no sharing
bull circuit-like (guaranteed) performance
bull call setup required
Circuit Switching FDM and TDM
FDM
frequency
timeTDM
frequency
time
4 users
Example
Network Core Packet Switching
each end-end data stream divided into packets
bull user A B packets share network resources
bull each packet uses full link bandwidth
bull resources used as needed
resource contention bull aggregate resource
demand can exceed amount available
bull congestion packets queue wait for link use
bull store and forward packets move one hop at a timendash Node receives complete
packet before forwarding
Packet Switching Statistical Multiplexing
Sequence of A amp B packets does not have fixed pattern statistical multiplexing
In TDM each host gets same slot in revolving TDM frame
A
B
C10 MbsEthernet
15 Mbs
D E
statistical multiplexing
queue of packetswaiting for output
link
Packet switching versus circuit switching
bull Great for bursty datandash resource sharingndash simpler no call setup
bull Excessive congestion packet delay and lossndash protocols needed for reliable data transfer
congestion controlbull Q How to provide circuit-like behavior
ndash bandwidth guarantees needed for audiovideo appsndash still an unsolved problem (chapter 6)
Is packet switching a ldquoslam dunk winnerrdquo
Local Area Networks
bull Smaller scopendash Building or small campus
bull Usually owned by same organization as attached devices
bull Data rates much higherbull Usually broadcast systems
Protocolsbull Used for communications between entities in a systembull Must speak the same languagebull Entities
ndash User applicationsndash e-mail facilitiesndash terminals
bull Systemsndash Computerndash Terminalndash Remote sensor
Key Elements of a Protocol
bull Syntaxndash Data formatsndash Signal levels
bull Semanticsndash Control informationndash Error handling
bull Timingndash Speed matchingndash Sequencing
Whatrsquos a protocol
human protocolsbull ldquowhatrsquos the timerdquobull ldquoI have a questionrdquobull introductions
hellip specific msgs senthellip specific actions taken
when msgs received or other events
network protocolsbull machines rather than
humansbull all communication activity in
Internet governed by protocols
protocols define format order of msgs sent and received among network entities and actions taken on msg transmission receipt
Whatrsquos a protocola human protocol and a computer network protocol
Hi
HiGot thetime200
TCP connection reqTCP connectionreply
Get httpgaiacsumasseduindexhtm
ltfilegttime
In Summary a protocol is
bull An agreement about communication between two or more entities
bull It specifiesndash Format of messagesndash Meaning of messagesndash Rules for exchangendash Procedures for handling problems
Protocol Specification
bull As designers we can specify a protocol usingndash Event-Time Diagramsndash Transition Diagrams
bull We can implement a protocol with a Finite State Machine (FSM)
bull Internet Protocols are formalized by RFCs which are administered by IETF
bull You can find any RFC here
042223 20
Event -Time Diagrams
bull Define causal ordering
bull Define indicationrequestresponse actions
Transition Diagram
bull Illustratesndash Statesndash Input (the Event that causes transition)ndash Transitions (to new states)
Protocol ldquoLayersrdquo
Networks are complex
bull many ldquopiecesrdquondash hostsndash routersndash links of various
mediandash applicationsndash protocolsndash hardware software
Question Is there any hope of organizing
structure of network
Or at least our discussion of networks
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Simplified Network Model
The Network Core
bull mesh of interconnected routers
bull the fundamental question how is data transferred through netndash circuit switching dedicated
circuit per call telephone net
ndash packet-switching data sent thru net in discrete ldquochunksrdquo
Network Core Circuit Switching
End-end resources reserved for ldquocallrdquo
bull link bandwidth switch capacity
bull dedicated resources no sharing
bull circuit-like (guaranteed) performance
bull call setup required
Circuit Switching FDM and TDM
FDM
frequency
timeTDM
frequency
time
4 users
Example
Network Core Packet Switching
each end-end data stream divided into packets
bull user A B packets share network resources
bull each packet uses full link bandwidth
bull resources used as needed
resource contention bull aggregate resource
demand can exceed amount available
bull congestion packets queue wait for link use
bull store and forward packets move one hop at a timendash Node receives complete
packet before forwarding
Packet Switching Statistical Multiplexing
Sequence of A amp B packets does not have fixed pattern statistical multiplexing
In TDM each host gets same slot in revolving TDM frame
A
B
C10 MbsEthernet
15 Mbs
D E
statistical multiplexing
queue of packetswaiting for output
link
Packet switching versus circuit switching
bull Great for bursty datandash resource sharingndash simpler no call setup
bull Excessive congestion packet delay and lossndash protocols needed for reliable data transfer
congestion controlbull Q How to provide circuit-like behavior
ndash bandwidth guarantees needed for audiovideo appsndash still an unsolved problem (chapter 6)
Is packet switching a ldquoslam dunk winnerrdquo
Local Area Networks
bull Smaller scopendash Building or small campus
bull Usually owned by same organization as attached devices
bull Data rates much higherbull Usually broadcast systems
Protocolsbull Used for communications between entities in a systembull Must speak the same languagebull Entities
ndash User applicationsndash e-mail facilitiesndash terminals
bull Systemsndash Computerndash Terminalndash Remote sensor
Key Elements of a Protocol
bull Syntaxndash Data formatsndash Signal levels
bull Semanticsndash Control informationndash Error handling
bull Timingndash Speed matchingndash Sequencing
Whatrsquos a protocol
human protocolsbull ldquowhatrsquos the timerdquobull ldquoI have a questionrdquobull introductions
hellip specific msgs senthellip specific actions taken
when msgs received or other events
network protocolsbull machines rather than
humansbull all communication activity in
Internet governed by protocols
protocols define format order of msgs sent and received among network entities and actions taken on msg transmission receipt
Whatrsquos a protocola human protocol and a computer network protocol
Hi
HiGot thetime200
TCP connection reqTCP connectionreply
Get httpgaiacsumasseduindexhtm
ltfilegttime
In Summary a protocol is
bull An agreement about communication between two or more entities
bull It specifiesndash Format of messagesndash Meaning of messagesndash Rules for exchangendash Procedures for handling problems
Protocol Specification
bull As designers we can specify a protocol usingndash Event-Time Diagramsndash Transition Diagrams
bull We can implement a protocol with a Finite State Machine (FSM)
bull Internet Protocols are formalized by RFCs which are administered by IETF
bull You can find any RFC here
042223 20
Event -Time Diagrams
bull Define causal ordering
bull Define indicationrequestresponse actions
Transition Diagram
bull Illustratesndash Statesndash Input (the Event that causes transition)ndash Transitions (to new states)
Protocol ldquoLayersrdquo
Networks are complex
bull many ldquopiecesrdquondash hostsndash routersndash links of various
mediandash applicationsndash protocolsndash hardware software
Question Is there any hope of organizing
structure of network
Or at least our discussion of networks
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
The Network Core
bull mesh of interconnected routers
bull the fundamental question how is data transferred through netndash circuit switching dedicated
circuit per call telephone net
ndash packet-switching data sent thru net in discrete ldquochunksrdquo
Network Core Circuit Switching
End-end resources reserved for ldquocallrdquo
bull link bandwidth switch capacity
bull dedicated resources no sharing
bull circuit-like (guaranteed) performance
bull call setup required
Circuit Switching FDM and TDM
FDM
frequency
timeTDM
frequency
time
4 users
Example
Network Core Packet Switching
each end-end data stream divided into packets
bull user A B packets share network resources
bull each packet uses full link bandwidth
bull resources used as needed
resource contention bull aggregate resource
demand can exceed amount available
bull congestion packets queue wait for link use
bull store and forward packets move one hop at a timendash Node receives complete
packet before forwarding
Packet Switching Statistical Multiplexing
Sequence of A amp B packets does not have fixed pattern statistical multiplexing
In TDM each host gets same slot in revolving TDM frame
A
B
C10 MbsEthernet
15 Mbs
D E
statistical multiplexing
queue of packetswaiting for output
link
Packet switching versus circuit switching
bull Great for bursty datandash resource sharingndash simpler no call setup
bull Excessive congestion packet delay and lossndash protocols needed for reliable data transfer
congestion controlbull Q How to provide circuit-like behavior
ndash bandwidth guarantees needed for audiovideo appsndash still an unsolved problem (chapter 6)
Is packet switching a ldquoslam dunk winnerrdquo
Local Area Networks
bull Smaller scopendash Building or small campus
bull Usually owned by same organization as attached devices
bull Data rates much higherbull Usually broadcast systems
Protocolsbull Used for communications between entities in a systembull Must speak the same languagebull Entities
ndash User applicationsndash e-mail facilitiesndash terminals
bull Systemsndash Computerndash Terminalndash Remote sensor
Key Elements of a Protocol
bull Syntaxndash Data formatsndash Signal levels
bull Semanticsndash Control informationndash Error handling
bull Timingndash Speed matchingndash Sequencing
Whatrsquos a protocol
human protocolsbull ldquowhatrsquos the timerdquobull ldquoI have a questionrdquobull introductions
hellip specific msgs senthellip specific actions taken
when msgs received or other events
network protocolsbull machines rather than
humansbull all communication activity in
Internet governed by protocols
protocols define format order of msgs sent and received among network entities and actions taken on msg transmission receipt
Whatrsquos a protocola human protocol and a computer network protocol
Hi
HiGot thetime200
TCP connection reqTCP connectionreply
Get httpgaiacsumasseduindexhtm
ltfilegttime
In Summary a protocol is
bull An agreement about communication between two or more entities
bull It specifiesndash Format of messagesndash Meaning of messagesndash Rules for exchangendash Procedures for handling problems
Protocol Specification
bull As designers we can specify a protocol usingndash Event-Time Diagramsndash Transition Diagrams
bull We can implement a protocol with a Finite State Machine (FSM)
bull Internet Protocols are formalized by RFCs which are administered by IETF
bull You can find any RFC here
042223 20
Event -Time Diagrams
bull Define causal ordering
bull Define indicationrequestresponse actions
Transition Diagram
bull Illustratesndash Statesndash Input (the Event that causes transition)ndash Transitions (to new states)
Protocol ldquoLayersrdquo
Networks are complex
bull many ldquopiecesrdquondash hostsndash routersndash links of various
mediandash applicationsndash protocolsndash hardware software
Question Is there any hope of organizing
structure of network
Or at least our discussion of networks
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Network Core Circuit Switching
End-end resources reserved for ldquocallrdquo
bull link bandwidth switch capacity
bull dedicated resources no sharing
bull circuit-like (guaranteed) performance
bull call setup required
Circuit Switching FDM and TDM
FDM
frequency
timeTDM
frequency
time
4 users
Example
Network Core Packet Switching
each end-end data stream divided into packets
bull user A B packets share network resources
bull each packet uses full link bandwidth
bull resources used as needed
resource contention bull aggregate resource
demand can exceed amount available
bull congestion packets queue wait for link use
bull store and forward packets move one hop at a timendash Node receives complete
packet before forwarding
Packet Switching Statistical Multiplexing
Sequence of A amp B packets does not have fixed pattern statistical multiplexing
In TDM each host gets same slot in revolving TDM frame
A
B
C10 MbsEthernet
15 Mbs
D E
statistical multiplexing
queue of packetswaiting for output
link
Packet switching versus circuit switching
bull Great for bursty datandash resource sharingndash simpler no call setup
bull Excessive congestion packet delay and lossndash protocols needed for reliable data transfer
congestion controlbull Q How to provide circuit-like behavior
ndash bandwidth guarantees needed for audiovideo appsndash still an unsolved problem (chapter 6)
Is packet switching a ldquoslam dunk winnerrdquo
Local Area Networks
bull Smaller scopendash Building or small campus
bull Usually owned by same organization as attached devices
bull Data rates much higherbull Usually broadcast systems
Protocolsbull Used for communications between entities in a systembull Must speak the same languagebull Entities
ndash User applicationsndash e-mail facilitiesndash terminals
bull Systemsndash Computerndash Terminalndash Remote sensor
Key Elements of a Protocol
bull Syntaxndash Data formatsndash Signal levels
bull Semanticsndash Control informationndash Error handling
bull Timingndash Speed matchingndash Sequencing
Whatrsquos a protocol
human protocolsbull ldquowhatrsquos the timerdquobull ldquoI have a questionrdquobull introductions
hellip specific msgs senthellip specific actions taken
when msgs received or other events
network protocolsbull machines rather than
humansbull all communication activity in
Internet governed by protocols
protocols define format order of msgs sent and received among network entities and actions taken on msg transmission receipt
Whatrsquos a protocola human protocol and a computer network protocol
Hi
HiGot thetime200
TCP connection reqTCP connectionreply
Get httpgaiacsumasseduindexhtm
ltfilegttime
In Summary a protocol is
bull An agreement about communication between two or more entities
bull It specifiesndash Format of messagesndash Meaning of messagesndash Rules for exchangendash Procedures for handling problems
Protocol Specification
bull As designers we can specify a protocol usingndash Event-Time Diagramsndash Transition Diagrams
bull We can implement a protocol with a Finite State Machine (FSM)
bull Internet Protocols are formalized by RFCs which are administered by IETF
bull You can find any RFC here
042223 20
Event -Time Diagrams
bull Define causal ordering
bull Define indicationrequestresponse actions
Transition Diagram
bull Illustratesndash Statesndash Input (the Event that causes transition)ndash Transitions (to new states)
Protocol ldquoLayersrdquo
Networks are complex
bull many ldquopiecesrdquondash hostsndash routersndash links of various
mediandash applicationsndash protocolsndash hardware software
Question Is there any hope of organizing
structure of network
Or at least our discussion of networks
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Circuit Switching FDM and TDM
FDM
frequency
timeTDM
frequency
time
4 users
Example
Network Core Packet Switching
each end-end data stream divided into packets
bull user A B packets share network resources
bull each packet uses full link bandwidth
bull resources used as needed
resource contention bull aggregate resource
demand can exceed amount available
bull congestion packets queue wait for link use
bull store and forward packets move one hop at a timendash Node receives complete
packet before forwarding
Packet Switching Statistical Multiplexing
Sequence of A amp B packets does not have fixed pattern statistical multiplexing
In TDM each host gets same slot in revolving TDM frame
A
B
C10 MbsEthernet
15 Mbs
D E
statistical multiplexing
queue of packetswaiting for output
link
Packet switching versus circuit switching
bull Great for bursty datandash resource sharingndash simpler no call setup
bull Excessive congestion packet delay and lossndash protocols needed for reliable data transfer
congestion controlbull Q How to provide circuit-like behavior
ndash bandwidth guarantees needed for audiovideo appsndash still an unsolved problem (chapter 6)
Is packet switching a ldquoslam dunk winnerrdquo
Local Area Networks
bull Smaller scopendash Building or small campus
bull Usually owned by same organization as attached devices
bull Data rates much higherbull Usually broadcast systems
Protocolsbull Used for communications between entities in a systembull Must speak the same languagebull Entities
ndash User applicationsndash e-mail facilitiesndash terminals
bull Systemsndash Computerndash Terminalndash Remote sensor
Key Elements of a Protocol
bull Syntaxndash Data formatsndash Signal levels
bull Semanticsndash Control informationndash Error handling
bull Timingndash Speed matchingndash Sequencing
Whatrsquos a protocol
human protocolsbull ldquowhatrsquos the timerdquobull ldquoI have a questionrdquobull introductions
hellip specific msgs senthellip specific actions taken
when msgs received or other events
network protocolsbull machines rather than
humansbull all communication activity in
Internet governed by protocols
protocols define format order of msgs sent and received among network entities and actions taken on msg transmission receipt
Whatrsquos a protocola human protocol and a computer network protocol
Hi
HiGot thetime200
TCP connection reqTCP connectionreply
Get httpgaiacsumasseduindexhtm
ltfilegttime
In Summary a protocol is
bull An agreement about communication between two or more entities
bull It specifiesndash Format of messagesndash Meaning of messagesndash Rules for exchangendash Procedures for handling problems
Protocol Specification
bull As designers we can specify a protocol usingndash Event-Time Diagramsndash Transition Diagrams
bull We can implement a protocol with a Finite State Machine (FSM)
bull Internet Protocols are formalized by RFCs which are administered by IETF
bull You can find any RFC here
042223 20
Event -Time Diagrams
bull Define causal ordering
bull Define indicationrequestresponse actions
Transition Diagram
bull Illustratesndash Statesndash Input (the Event that causes transition)ndash Transitions (to new states)
Protocol ldquoLayersrdquo
Networks are complex
bull many ldquopiecesrdquondash hostsndash routersndash links of various
mediandash applicationsndash protocolsndash hardware software
Question Is there any hope of organizing
structure of network
Or at least our discussion of networks
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Network Core Packet Switching
each end-end data stream divided into packets
bull user A B packets share network resources
bull each packet uses full link bandwidth
bull resources used as needed
resource contention bull aggregate resource
demand can exceed amount available
bull congestion packets queue wait for link use
bull store and forward packets move one hop at a timendash Node receives complete
packet before forwarding
Packet Switching Statistical Multiplexing
Sequence of A amp B packets does not have fixed pattern statistical multiplexing
In TDM each host gets same slot in revolving TDM frame
A
B
C10 MbsEthernet
15 Mbs
D E
statistical multiplexing
queue of packetswaiting for output
link
Packet switching versus circuit switching
bull Great for bursty datandash resource sharingndash simpler no call setup
bull Excessive congestion packet delay and lossndash protocols needed for reliable data transfer
congestion controlbull Q How to provide circuit-like behavior
ndash bandwidth guarantees needed for audiovideo appsndash still an unsolved problem (chapter 6)
Is packet switching a ldquoslam dunk winnerrdquo
Local Area Networks
bull Smaller scopendash Building or small campus
bull Usually owned by same organization as attached devices
bull Data rates much higherbull Usually broadcast systems
Protocolsbull Used for communications between entities in a systembull Must speak the same languagebull Entities
ndash User applicationsndash e-mail facilitiesndash terminals
bull Systemsndash Computerndash Terminalndash Remote sensor
Key Elements of a Protocol
bull Syntaxndash Data formatsndash Signal levels
bull Semanticsndash Control informationndash Error handling
bull Timingndash Speed matchingndash Sequencing
Whatrsquos a protocol
human protocolsbull ldquowhatrsquos the timerdquobull ldquoI have a questionrdquobull introductions
hellip specific msgs senthellip specific actions taken
when msgs received or other events
network protocolsbull machines rather than
humansbull all communication activity in
Internet governed by protocols
protocols define format order of msgs sent and received among network entities and actions taken on msg transmission receipt
Whatrsquos a protocola human protocol and a computer network protocol
Hi
HiGot thetime200
TCP connection reqTCP connectionreply
Get httpgaiacsumasseduindexhtm
ltfilegttime
In Summary a protocol is
bull An agreement about communication between two or more entities
bull It specifiesndash Format of messagesndash Meaning of messagesndash Rules for exchangendash Procedures for handling problems
Protocol Specification
bull As designers we can specify a protocol usingndash Event-Time Diagramsndash Transition Diagrams
bull We can implement a protocol with a Finite State Machine (FSM)
bull Internet Protocols are formalized by RFCs which are administered by IETF
bull You can find any RFC here
042223 20
Event -Time Diagrams
bull Define causal ordering
bull Define indicationrequestresponse actions
Transition Diagram
bull Illustratesndash Statesndash Input (the Event that causes transition)ndash Transitions (to new states)
Protocol ldquoLayersrdquo
Networks are complex
bull many ldquopiecesrdquondash hostsndash routersndash links of various
mediandash applicationsndash protocolsndash hardware software
Question Is there any hope of organizing
structure of network
Or at least our discussion of networks
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Packet Switching Statistical Multiplexing
Sequence of A amp B packets does not have fixed pattern statistical multiplexing
In TDM each host gets same slot in revolving TDM frame
A
B
C10 MbsEthernet
15 Mbs
D E
statistical multiplexing
queue of packetswaiting for output
link
Packet switching versus circuit switching
bull Great for bursty datandash resource sharingndash simpler no call setup
bull Excessive congestion packet delay and lossndash protocols needed for reliable data transfer
congestion controlbull Q How to provide circuit-like behavior
ndash bandwidth guarantees needed for audiovideo appsndash still an unsolved problem (chapter 6)
Is packet switching a ldquoslam dunk winnerrdquo
Local Area Networks
bull Smaller scopendash Building or small campus
bull Usually owned by same organization as attached devices
bull Data rates much higherbull Usually broadcast systems
Protocolsbull Used for communications between entities in a systembull Must speak the same languagebull Entities
ndash User applicationsndash e-mail facilitiesndash terminals
bull Systemsndash Computerndash Terminalndash Remote sensor
Key Elements of a Protocol
bull Syntaxndash Data formatsndash Signal levels
bull Semanticsndash Control informationndash Error handling
bull Timingndash Speed matchingndash Sequencing
Whatrsquos a protocol
human protocolsbull ldquowhatrsquos the timerdquobull ldquoI have a questionrdquobull introductions
hellip specific msgs senthellip specific actions taken
when msgs received or other events
network protocolsbull machines rather than
humansbull all communication activity in
Internet governed by protocols
protocols define format order of msgs sent and received among network entities and actions taken on msg transmission receipt
Whatrsquos a protocola human protocol and a computer network protocol
Hi
HiGot thetime200
TCP connection reqTCP connectionreply
Get httpgaiacsumasseduindexhtm
ltfilegttime
In Summary a protocol is
bull An agreement about communication between two or more entities
bull It specifiesndash Format of messagesndash Meaning of messagesndash Rules for exchangendash Procedures for handling problems
Protocol Specification
bull As designers we can specify a protocol usingndash Event-Time Diagramsndash Transition Diagrams
bull We can implement a protocol with a Finite State Machine (FSM)
bull Internet Protocols are formalized by RFCs which are administered by IETF
bull You can find any RFC here
042223 20
Event -Time Diagrams
bull Define causal ordering
bull Define indicationrequestresponse actions
Transition Diagram
bull Illustratesndash Statesndash Input (the Event that causes transition)ndash Transitions (to new states)
Protocol ldquoLayersrdquo
Networks are complex
bull many ldquopiecesrdquondash hostsndash routersndash links of various
mediandash applicationsndash protocolsndash hardware software
Question Is there any hope of organizing
structure of network
Or at least our discussion of networks
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Packet switching versus circuit switching
bull Great for bursty datandash resource sharingndash simpler no call setup
bull Excessive congestion packet delay and lossndash protocols needed for reliable data transfer
congestion controlbull Q How to provide circuit-like behavior
ndash bandwidth guarantees needed for audiovideo appsndash still an unsolved problem (chapter 6)
Is packet switching a ldquoslam dunk winnerrdquo
Local Area Networks
bull Smaller scopendash Building or small campus
bull Usually owned by same organization as attached devices
bull Data rates much higherbull Usually broadcast systems
Protocolsbull Used for communications between entities in a systembull Must speak the same languagebull Entities
ndash User applicationsndash e-mail facilitiesndash terminals
bull Systemsndash Computerndash Terminalndash Remote sensor
Key Elements of a Protocol
bull Syntaxndash Data formatsndash Signal levels
bull Semanticsndash Control informationndash Error handling
bull Timingndash Speed matchingndash Sequencing
Whatrsquos a protocol
human protocolsbull ldquowhatrsquos the timerdquobull ldquoI have a questionrdquobull introductions
hellip specific msgs senthellip specific actions taken
when msgs received or other events
network protocolsbull machines rather than
humansbull all communication activity in
Internet governed by protocols
protocols define format order of msgs sent and received among network entities and actions taken on msg transmission receipt
Whatrsquos a protocola human protocol and a computer network protocol
Hi
HiGot thetime200
TCP connection reqTCP connectionreply
Get httpgaiacsumasseduindexhtm
ltfilegttime
In Summary a protocol is
bull An agreement about communication between two or more entities
bull It specifiesndash Format of messagesndash Meaning of messagesndash Rules for exchangendash Procedures for handling problems
Protocol Specification
bull As designers we can specify a protocol usingndash Event-Time Diagramsndash Transition Diagrams
bull We can implement a protocol with a Finite State Machine (FSM)
bull Internet Protocols are formalized by RFCs which are administered by IETF
bull You can find any RFC here
042223 20
Event -Time Diagrams
bull Define causal ordering
bull Define indicationrequestresponse actions
Transition Diagram
bull Illustratesndash Statesndash Input (the Event that causes transition)ndash Transitions (to new states)
Protocol ldquoLayersrdquo
Networks are complex
bull many ldquopiecesrdquondash hostsndash routersndash links of various
mediandash applicationsndash protocolsndash hardware software
Question Is there any hope of organizing
structure of network
Or at least our discussion of networks
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Local Area Networks
bull Smaller scopendash Building or small campus
bull Usually owned by same organization as attached devices
bull Data rates much higherbull Usually broadcast systems
Protocolsbull Used for communications between entities in a systembull Must speak the same languagebull Entities
ndash User applicationsndash e-mail facilitiesndash terminals
bull Systemsndash Computerndash Terminalndash Remote sensor
Key Elements of a Protocol
bull Syntaxndash Data formatsndash Signal levels
bull Semanticsndash Control informationndash Error handling
bull Timingndash Speed matchingndash Sequencing
Whatrsquos a protocol
human protocolsbull ldquowhatrsquos the timerdquobull ldquoI have a questionrdquobull introductions
hellip specific msgs senthellip specific actions taken
when msgs received or other events
network protocolsbull machines rather than
humansbull all communication activity in
Internet governed by protocols
protocols define format order of msgs sent and received among network entities and actions taken on msg transmission receipt
Whatrsquos a protocola human protocol and a computer network protocol
Hi
HiGot thetime200
TCP connection reqTCP connectionreply
Get httpgaiacsumasseduindexhtm
ltfilegttime
In Summary a protocol is
bull An agreement about communication between two or more entities
bull It specifiesndash Format of messagesndash Meaning of messagesndash Rules for exchangendash Procedures for handling problems
Protocol Specification
bull As designers we can specify a protocol usingndash Event-Time Diagramsndash Transition Diagrams
bull We can implement a protocol with a Finite State Machine (FSM)
bull Internet Protocols are formalized by RFCs which are administered by IETF
bull You can find any RFC here
042223 20
Event -Time Diagrams
bull Define causal ordering
bull Define indicationrequestresponse actions
Transition Diagram
bull Illustratesndash Statesndash Input (the Event that causes transition)ndash Transitions (to new states)
Protocol ldquoLayersrdquo
Networks are complex
bull many ldquopiecesrdquondash hostsndash routersndash links of various
mediandash applicationsndash protocolsndash hardware software
Question Is there any hope of organizing
structure of network
Or at least our discussion of networks
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Protocolsbull Used for communications between entities in a systembull Must speak the same languagebull Entities
ndash User applicationsndash e-mail facilitiesndash terminals
bull Systemsndash Computerndash Terminalndash Remote sensor
Key Elements of a Protocol
bull Syntaxndash Data formatsndash Signal levels
bull Semanticsndash Control informationndash Error handling
bull Timingndash Speed matchingndash Sequencing
Whatrsquos a protocol
human protocolsbull ldquowhatrsquos the timerdquobull ldquoI have a questionrdquobull introductions
hellip specific msgs senthellip specific actions taken
when msgs received or other events
network protocolsbull machines rather than
humansbull all communication activity in
Internet governed by protocols
protocols define format order of msgs sent and received among network entities and actions taken on msg transmission receipt
Whatrsquos a protocola human protocol and a computer network protocol
Hi
HiGot thetime200
TCP connection reqTCP connectionreply
Get httpgaiacsumasseduindexhtm
ltfilegttime
In Summary a protocol is
bull An agreement about communication between two or more entities
bull It specifiesndash Format of messagesndash Meaning of messagesndash Rules for exchangendash Procedures for handling problems
Protocol Specification
bull As designers we can specify a protocol usingndash Event-Time Diagramsndash Transition Diagrams
bull We can implement a protocol with a Finite State Machine (FSM)
bull Internet Protocols are formalized by RFCs which are administered by IETF
bull You can find any RFC here
042223 20
Event -Time Diagrams
bull Define causal ordering
bull Define indicationrequestresponse actions
Transition Diagram
bull Illustratesndash Statesndash Input (the Event that causes transition)ndash Transitions (to new states)
Protocol ldquoLayersrdquo
Networks are complex
bull many ldquopiecesrdquondash hostsndash routersndash links of various
mediandash applicationsndash protocolsndash hardware software
Question Is there any hope of organizing
structure of network
Or at least our discussion of networks
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Key Elements of a Protocol
bull Syntaxndash Data formatsndash Signal levels
bull Semanticsndash Control informationndash Error handling
bull Timingndash Speed matchingndash Sequencing
Whatrsquos a protocol
human protocolsbull ldquowhatrsquos the timerdquobull ldquoI have a questionrdquobull introductions
hellip specific msgs senthellip specific actions taken
when msgs received or other events
network protocolsbull machines rather than
humansbull all communication activity in
Internet governed by protocols
protocols define format order of msgs sent and received among network entities and actions taken on msg transmission receipt
Whatrsquos a protocola human protocol and a computer network protocol
Hi
HiGot thetime200
TCP connection reqTCP connectionreply
Get httpgaiacsumasseduindexhtm
ltfilegttime
In Summary a protocol is
bull An agreement about communication between two or more entities
bull It specifiesndash Format of messagesndash Meaning of messagesndash Rules for exchangendash Procedures for handling problems
Protocol Specification
bull As designers we can specify a protocol usingndash Event-Time Diagramsndash Transition Diagrams
bull We can implement a protocol with a Finite State Machine (FSM)
bull Internet Protocols are formalized by RFCs which are administered by IETF
bull You can find any RFC here
042223 20
Event -Time Diagrams
bull Define causal ordering
bull Define indicationrequestresponse actions
Transition Diagram
bull Illustratesndash Statesndash Input (the Event that causes transition)ndash Transitions (to new states)
Protocol ldquoLayersrdquo
Networks are complex
bull many ldquopiecesrdquondash hostsndash routersndash links of various
mediandash applicationsndash protocolsndash hardware software
Question Is there any hope of organizing
structure of network
Or at least our discussion of networks
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Whatrsquos a protocol
human protocolsbull ldquowhatrsquos the timerdquobull ldquoI have a questionrdquobull introductions
hellip specific msgs senthellip specific actions taken
when msgs received or other events
network protocolsbull machines rather than
humansbull all communication activity in
Internet governed by protocols
protocols define format order of msgs sent and received among network entities and actions taken on msg transmission receipt
Whatrsquos a protocola human protocol and a computer network protocol
Hi
HiGot thetime200
TCP connection reqTCP connectionreply
Get httpgaiacsumasseduindexhtm
ltfilegttime
In Summary a protocol is
bull An agreement about communication between two or more entities
bull It specifiesndash Format of messagesndash Meaning of messagesndash Rules for exchangendash Procedures for handling problems
Protocol Specification
bull As designers we can specify a protocol usingndash Event-Time Diagramsndash Transition Diagrams
bull We can implement a protocol with a Finite State Machine (FSM)
bull Internet Protocols are formalized by RFCs which are administered by IETF
bull You can find any RFC here
042223 20
Event -Time Diagrams
bull Define causal ordering
bull Define indicationrequestresponse actions
Transition Diagram
bull Illustratesndash Statesndash Input (the Event that causes transition)ndash Transitions (to new states)
Protocol ldquoLayersrdquo
Networks are complex
bull many ldquopiecesrdquondash hostsndash routersndash links of various
mediandash applicationsndash protocolsndash hardware software
Question Is there any hope of organizing
structure of network
Or at least our discussion of networks
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Whatrsquos a protocola human protocol and a computer network protocol
Hi
HiGot thetime200
TCP connection reqTCP connectionreply
Get httpgaiacsumasseduindexhtm
ltfilegttime
In Summary a protocol is
bull An agreement about communication between two or more entities
bull It specifiesndash Format of messagesndash Meaning of messagesndash Rules for exchangendash Procedures for handling problems
Protocol Specification
bull As designers we can specify a protocol usingndash Event-Time Diagramsndash Transition Diagrams
bull We can implement a protocol with a Finite State Machine (FSM)
bull Internet Protocols are formalized by RFCs which are administered by IETF
bull You can find any RFC here
042223 20
Event -Time Diagrams
bull Define causal ordering
bull Define indicationrequestresponse actions
Transition Diagram
bull Illustratesndash Statesndash Input (the Event that causes transition)ndash Transitions (to new states)
Protocol ldquoLayersrdquo
Networks are complex
bull many ldquopiecesrdquondash hostsndash routersndash links of various
mediandash applicationsndash protocolsndash hardware software
Question Is there any hope of organizing
structure of network
Or at least our discussion of networks
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
In Summary a protocol is
bull An agreement about communication between two or more entities
bull It specifiesndash Format of messagesndash Meaning of messagesndash Rules for exchangendash Procedures for handling problems
Protocol Specification
bull As designers we can specify a protocol usingndash Event-Time Diagramsndash Transition Diagrams
bull We can implement a protocol with a Finite State Machine (FSM)
bull Internet Protocols are formalized by RFCs which are administered by IETF
bull You can find any RFC here
042223 20
Event -Time Diagrams
bull Define causal ordering
bull Define indicationrequestresponse actions
Transition Diagram
bull Illustratesndash Statesndash Input (the Event that causes transition)ndash Transitions (to new states)
Protocol ldquoLayersrdquo
Networks are complex
bull many ldquopiecesrdquondash hostsndash routersndash links of various
mediandash applicationsndash protocolsndash hardware software
Question Is there any hope of organizing
structure of network
Or at least our discussion of networks
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Protocol Specification
bull As designers we can specify a protocol usingndash Event-Time Diagramsndash Transition Diagrams
bull We can implement a protocol with a Finite State Machine (FSM)
bull Internet Protocols are formalized by RFCs which are administered by IETF
bull You can find any RFC here
042223 20
Event -Time Diagrams
bull Define causal ordering
bull Define indicationrequestresponse actions
Transition Diagram
bull Illustratesndash Statesndash Input (the Event that causes transition)ndash Transitions (to new states)
Protocol ldquoLayersrdquo
Networks are complex
bull many ldquopiecesrdquondash hostsndash routersndash links of various
mediandash applicationsndash protocolsndash hardware software
Question Is there any hope of organizing
structure of network
Or at least our discussion of networks
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
042223 20
Event -Time Diagrams
bull Define causal ordering
bull Define indicationrequestresponse actions
Transition Diagram
bull Illustratesndash Statesndash Input (the Event that causes transition)ndash Transitions (to new states)
Protocol ldquoLayersrdquo
Networks are complex
bull many ldquopiecesrdquondash hostsndash routersndash links of various
mediandash applicationsndash protocolsndash hardware software
Question Is there any hope of organizing
structure of network
Or at least our discussion of networks
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Transition Diagram
bull Illustratesndash Statesndash Input (the Event that causes transition)ndash Transitions (to new states)
Protocol ldquoLayersrdquo
Networks are complex
bull many ldquopiecesrdquondash hostsndash routersndash links of various
mediandash applicationsndash protocolsndash hardware software
Question Is there any hope of organizing
structure of network
Or at least our discussion of networks
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Protocol ldquoLayersrdquo
Networks are complex
bull many ldquopiecesrdquondash hostsndash routersndash links of various
mediandash applicationsndash protocolsndash hardware software
Question Is there any hope of organizing
structure of network
Or at least our discussion of networks
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Why layeringDealing with complex systemsbull explicit structure allows identification relationship of
complex systemrsquos piecesndash layered reference model for discussion
bull modularization eases maintenance updating of systemndash change of implementation of layerrsquos service transparent to
rest of systemndash eg change in gate procedure doesnrsquot affect rest of
systembull Can layering sometimes be undesirable
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Internet protocol stackbull application supporting network
applicationsndash FTP SMTP HTTP
bull transport process-process data transferndash TCP UDP
bull network routing of datagrams from source to destinationndash IP routing protocols
bull link data transfer between neighboring network elementsndash PPP Ethernet
bull physical bits ldquoon the wirerdquo
application
transport
network
link
physical
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
sourceapplicatio
ntransportnetwork
linkphysical
HtHn Msegment Ht
datagram
destinationapplicatio
ntransportnetwork
linkphysical
HtHnHl MHtHn MHt M
Mnetwork
linkphysical
linkphysical
HtHnHl MHtHn M
HtHn M
HtHnHl M
router
switch
Encapsulationmessage MHt M
Hnframe
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
OSI
bull Open Systems Interconnectionbull Developed by the International
Organization for Standardization (ISO)bull Seven layersbull A theoretical system delivered too latebull TCPIP is the de facto standard
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
How do loss and delay occurpackets queue in router buffers bull packet arrival rate to link exceeds output link capacitybull packets queue wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)free (available) buffers arriving packets dropped (loss) if no free buffers
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Four sources of packet delay
bull 1 nodal processing ndash check bit errorsndash determine output link
A
B
propagationtransmission
nodalprocessing queueing
bull 2 queueingndash time waiting at output link
for transmission ndash depends on congestion
level of router
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Delay in packet-switched networks
3 Transmission delaybull R=link bandwidth (bps)bull L=packet length (bits)bull time to send bits into
link = LR
4 Propagation delaybull d = length of physical linkbull s = propagation speed in
medium (~2x108 msec)bull propagation delay = ds
A
B
propagationtransmission
nodalprocessing queueing
Note s and R are very different quantities
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Data Communication and Networks
Lecture 2
Data Transmission and Encoding Concepts
September 14 2006
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Simplified Data Communications Model
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
S(t) = A sin(2ft + Φ)
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Terminology (1)
bull Transmitterbull Receiverbull Medium
ndash Guided mediumbull eg twisted pair optical fiber
ndash Unguided mediumbull eg air water vacuum
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Terminology (2)
bull Direct linkndash No intermediate devices
bull Point-to-pointndash Direct link ndash Only 2 devices share link
bull Multi-pointndash More than two devices share the link
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Terminology (3)
bull Simplexndash One direction
bull eg Television
bull Half duplexndash Either direction but only one way at a time
bull eg police radio
bull Full duplexndash Both directions at the same time
bull eg telephone
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Analog and Digital Data Transmission
bull Data ndash Entities that convey meaning
bull Signalsndash Electric or electromagnetic representations of
databull Transmission
ndash Communication of data by propagation and processing of signals
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Data
bull Analogndash Continuous values within some intervalndash eg sound video
bull Digitalndash Discrete valuesndash eg text integers
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Signals
bull Means by which data are propagatedbull Analog
ndash Continuously variablendash Various media
bull wire fiber optic spacendash Speech bandwidth 100Hz to 7kHzndash Telephone bandwidth 300Hz to 3400Hzndash Video bandwidth 4MHz
bull Digitalndash Use two DC components
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Data and Signals
bull Usually use digital signals for digital data and analog signals for analog data
bull Can use analog signal to carry digital datandash Modem
bull Can use digital signal to carry analog data ndash Compact Disc audio
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Analog Transmission
bull Analog signal transmitted without regard to content
bull May be analog or digital databull Attenuated over distance bull Use amplifiers to boost signalbull Also amplifies noise
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Digital Transmission
bull Concerned with contentbull Integrity endangered by noise attenuation etcbull Repeaters usedbull Repeater receives signalbull Extracts bit patternbull Retransmitsbull Attenuation is overcomebull Noise is not amplified
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Advantages amp Disadvantages of Digital
bull Cheaperbull Less susceptible to noisebull Greater attenuation
ndash Pulses become rounded and smallerndash Leads to loss of information
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Attenuation of Digital Signals
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Interpreting Signals
bull Need to knowndash Timing of bits - when they start and endndash Signal levels
bull Factors affecting successful interpreting of signalsndash Signal to noise rationdash Data ratendash Bandwidth
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Encoding Schemes
bull Nonreturn to Zero-Level (NRZ-L)bull Nonreturn to Zero Inverted (NRZI)bull Bipolar -AMIbull Pseudoternarybull Manchesterbull Differential Manchesterbull B8ZSbull HDB3
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Nonreturn to Zero-Level (NRZ-L)
bull Two different voltages for 0 and 1 bitsbull Voltage constant during bit interval
ndash no transition Ie no return to zero voltagebull eg Absence of voltage for zero constant
positive voltage for onebull More often negative voltage for one value
and positive for the otherbull This is NRZ-L
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Nonreturn to Zero Inverted
bull Nonreturn to zero inverted on onesbull Constant voltage pulse for duration of bitbull Data encoded as presence or absence of
signal transition at beginning of bit timebull Transition (low to high or high to low)
denotes a binary 1bull No transition denotes binary 0bull An example of differential encoding
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
NRZ
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Differential Encoding
bull Data represented by changes rather than levels
bull More reliable detection of transition rather than level
bull However in complex transmission layouts it is easy to lose sense of polarity
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Biphasebull Manchester
ndash Transition in middle of each bit periodndash Transition serves as clock and datandash Low to high represents onendash High to low represents zerondash Used by IEEE 8023
bull Differential Manchesterndash Midbit transition is clocking onlyndash Transition at start of a bit period represents zerondash No transition at start of a bit period represents onendash Note this is a differential encoding schemendash Used by IEEE 8025
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Biphase Pros and Cons
bull Conndash At least one transition per bit time and
possibly twondash Maximum modulation rate is twice NRZndash Requires more bandwidth
bull Prosndash Synchronization on mid bit transition (self
clocking)
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Asynchronous
bull Data transmitted one character at a timendash 5 to 8 bits
bull Timing only needs maintaining within each character
bull Resync with each character
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Asynchronous (diagram)
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Asynchronous - Behaviorbull In a steady stream interval between
characters is uniform (length of stop element)bull In idle state receiver looks for transition 1 to 0bull Then samples next seven intervals (char
length)bull Then looks for next 1 to 0 for next charbull Overhead of 2 or 3 bits per char (~20)bull Good for data with large gaps (keyboard)
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Synchronous - Bit Level
bull Block of data transmitted without start or stop bits
bull Clocks must be synchronizedbull Can use separate clock line
ndash Good over short distancesndash Subject to impairments
bull Embed clock signal in datandash Manchester encodingndash Carrier frequency (analog)
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Synchronous - Block Level
bull Need to indicate start and end of blockbull Use preamble and postamble
ndash eg series of SYN (hex 16) charactersndash eg block of 11111111 patterns ending in
11111110
bull More efficient (lower overhead) than async
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Synchronous (diagram)
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Synchronous Bit Error Detection
bull Which kinds of errors can we handle if we have synchronous bit transfer
bull Reference to bit transmission protocol in distcompppt
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Data Link Layer
Excerpted from Joe Conronrsquos notes
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Link Layer Servicesbull Framing link access
ndash encapsulate datagram into frame adding header trailer
ndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Encoding Information FramesTypical Fields in a Frame
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl Data Check
sum
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Error Detectionbull Additional bits added by transmitter for
error detection codebull Parity
ndash Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones
ndash Even number of bit errors goes undetectedndash Checksum can be a simple XOR operation
of bits to be checked
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Parity CheckingSingle Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Internet checksum
Senderbull treat segment contents as
sequence of 16-bit integersbull checksum addition (1rsquos
complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
Receiverbull compute checksum of received
segmentbull check if computed checksum
equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Multiple Access protocolsbull single shared broadcast channel bull two or more simultaneous transmissions by
nodes interference ndash collision if node receives two or more signals at the
same timemultiple access protocolbull distributed algorithm that determines how nodes
share channel ie determine when node can transmit
bull communication about channel sharing must use channel itself
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send at
rate R2 When M nodes want to transmit each can send
at average rate RM3 Fully decentralized
ndash no special node to coordinate transmissionsndash no synchronization of clocks slots
4 Simple
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
MAC Protocols a taxonomyThree broad classesbull Channel Partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)
ndash allocate piece to node for exclusive usebull Random Access
ndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can
take longer turns
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access bull access to channel in rounds bull each station gets fixed length slot (length =
pkt trans time) in each round bull unused slots go idle bull example 6-station LAN 134 have pkt slots
256 idle
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have pkt frequency bands 256 idle
frequ
ency
ban
ds
time
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Random Access Protocolsbull When node has packet to send
ndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)
bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire framebull If channel sensed busy defer transmission
bull Human analogy donrsquot interrupt others
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
CSMA collisionscollisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
ndash collisions detected within short timendash colliding transmissions aborted reducing channel
wastage bull collision detection
ndash easy in wired LANs measure signal strengths compare transmitted received signals
ndash difficult in wireless LANs receiver shut off while transmitting
bull human analogy the polite conversationalist
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
CSMACD collision detection
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Ethernet uses CSMACD
bull No slotsbull adapter doesnrsquot transmit if
it senses that some other adapter is transmitting that is carrier sense
bull transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
bull Before attempting a retransmission adapter waits a random time that is random access
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Ethernet CSMACD algorithm1 Adaptor receives datagram
from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Ethernetrsquos CSMACD (more)Jam Signal make sure all
other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff bull Goal adapt retransmission
attempts to estimated current loadndash heavy load random wait will
be longer
bull first collision choose K from 01 delay is K 512 bit transmission times
bull after second collision choose K from 0123hellip
bull after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
CSMACD efficiency (probabilistic)
bull Tprop = max prop between 2 nodes in LAN
bull ttrans = time to transmit max-size frame
bull Efficiency goes to 1 as tprop goes to 0
bull Goes to 1 as ttrans goes to infinity
transprop tt 511efficiency
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
References to Performance
bull Simon Lam A Carrier Sense Multiple Access Protocol for Local Networks Computer Networks Vol 4 pp 21-32 1980
bull D Bertsekas and R Gallagher Data Networks 2nd Ed Prentice Hall 1991
bull Possible paper project show an elegant derivation and compare to performance of Lelann algorithm
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N
bandwidth allocated even if only 1 active node Random access MAC protocols
ndash efficient at low load single node can fully utilize channel
ndash high load collision overheadldquotaking turnsrdquo protocols
look for best of both worlds
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
ldquoTaking Turnsrdquo MAC protocolsPolling bull master node
ldquoinvitesrdquo slave nodes to transmit in turn
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
Token passingbull control token passed from
one node to next sequentially
bull token messagebull concerns
ndash token overhead ndash latencyndash single point of failure (token)
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Summary of MAC protocols
bull What do you do with a shared mediandash Channel Partitioning by time frequency or
codebull Time Division Frequency Division
ndash Achieving real time guarantees distnotespdf
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Wireless Networks
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Chapter 6 Wireless and Mobile Networks
Background bull wireless (mobile) phone subscribers now
exceeds wired phone subscribersbull computer nets laptops palmtops PDAs
Internet-enabled phone promise anytime untethered Internet access
bull two important (but different) challengesndash communication over wireless linkndash handling mobile user who changes point of
attachment to network
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Elements of a wireless network
network infrastructure
base stationbull typically connected to
wired networkbull relay - responsible for
sending packets between wired network and wireless host(s) in its ldquoareardquondash eg cell towers 80211
access points
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Elements of a wireless network
network infrastructure
wireless linkbull typically used to connect
mobile(s) to base stationbull also used as backbone
link bull multiple access protocol
coordinates link access bull various data rates
transmission distance
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps80215
80211b
80211ag
IS-95 CDMA GSM
UMTSWCDMA CDMA2000
11 p-to-p link
2G
3G
Indoor
10 ndash 30m
Outdoor
50 ndash 200m
Mid rangeoutdoor
200m ndash 4Km
Long rangeoutdoor
5Km ndash 20Km
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Elements of a wireless network
network infrastructure
infrastructure modebull base station connects
mobiles into wired network
bull handoff mobile changes base station providing connection into wired network
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Elements of a wireless networkAd hoc modebull no base stationsbull nodes can only transmit
to other nodes within link coverage
bull nodes organize themselves into a network route among themselves
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
IEEE 80211 MAC Protocol CSMACA80211 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK increase random backoff interval
repeat 2
80211 receiver- if frame received OK return ACK after SIFS (ACK needed due to
hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Avoiding collisions (more)idea allow sender to ldquoreserverdquo channel rather than random
access of data frames avoid collisions of long data framesbull sender first transmits small request-to-send (RTS) packets
to BS using CSMAndash RTSs may still collide with each other (but theyrsquore short)
bull BS broadcasts clear-to-send CTS in response to RTSbull RTS heard by all nodes
ndash sender transmits data framendash other stations defer transmissions
Avoid data frame collisions completely using small reservation packets
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Collision Avoidance RTS-CTS exchange
APA B
time
RTS(A) RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
How do you contact a mobile friend
bull search all phone books
bull call her parentsbull expect her to let you
know where heshe is
I wonder where Alice moved toConsider friend frequently changing
addresses how do you find her
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Basic idea SIM card vs phone number
bull When Alice turns on her phone she registers her SIM card to local operator who records location
bull On Bobrsquos call Bobrsquos operator maps phone number to SIM
bull Second mapping from SIM to location
I know her phone number
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Long Distance Mobility approachbull Let routing handle it routers advertise permanent
address of mobile-nodes-in-residence via routing table exchangendash routing tables indicate where each mobile locatedndash no changes to end-systems
bull Let end-systems handle it ndash indirect routing communication from correspondent to
mobile goes through home agent then forwarded to remote
ndash direct routing correspondent gets foreign address of mobile sends directly to mobile
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Mobility registration
End resultbull Foreign agent knows about mobilebull Home agent knows location of mobile
wide area network
home network
visited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home ldquothis mobile is resident in my networkrdquo
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
24
1correspondent addresses packets using home address of mobile
home agent intercepts packets forwards to foreign agent
foreign agent receives packets forwards to mobile
mobile replies directly to correspondent
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Indirect Routing moving between networks
bull suppose mobile user moves to another networkndash registers with new foreign agentndash new foreign agent registers with home agentndash home agent update care-of-address for mobilendash packets continue to be forwarded to mobile (but
with new care-of-address)bull mobility changing foreign networks
transparent ongoing connections can be maintained
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Reliable Transmission
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender = 008
30008 = 000027
microseconds
L R RTT + L R
=
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Pipelined protocolsPipelining sender allows multiple ldquoin-flightrdquo
yet-to-be-acknowledged pktsndash range of sequence numbers must be increasedndash buffering at sender andor receiver
bull Two generic forms of pipelined protocols go-Back-N selective repeat
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Pipelining (by factor of 3)first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender = 024
30008 = 00008
microseconds
3 L R RTT + L R
=
Increase utilizationby a factor of 3
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Go-Back-NSenderbull k-bit seq in pkt headerbull ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
bull ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquondash may receive duplicate ACKs (see receiver)
bull timer for each in-flight pktbull timeout(n) retransmit pkt n and all higher seq pkts in window
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Selective Repeatbull receiver individually acknowledges all
correctly received pktsndash buffers pkts as needed for eventual in-order
delivery to upper layerbull sender only resends pkts for which ACK not
receivedndash sender timer for each unACKed pkt
bull sender windowndash N consecutive seq rsquosndash again limits seq s of sent unACKed pkts
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Selective repeat sender receiver windows
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Look at global negative acknowledgment scheme
bull Negative acknowledgment protocol along with token passing in distcompppt
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Network Layer circuit switching vs packet
switching
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Switching Networks
bull Long distance transmission is typically done over a network of switched nodes
bull Nodes not concerned with content of databull End devices are stations
ndash Computer terminal phone etcbull A collection of nodes and connections is a
communications networkbull Data routed by being switched from node to
node
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Technology
bull Two different switching technologiesndash Circuit switchingndash Packet switching
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Simple Switched Network
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Circuit Switching
bull Dedicated communication path between two stations (during conversation)
bull Three phasesndash Establishndash Transferndash Disconnect
bull Must have switching capacity and channel capacity to establish connection
bull Must have intelligence to work out routing
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Circuit Switching - Issues
bull Circuit switching is inefficient (designed for voice)ndash Resources dedicated to a particular callndash Much of the time a data connection is idlendash Data rate is fixed
bull Both ends must operate at the same rate
bull Set up (connection) takes timebull Once connected transfer is transparent
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Packet Switching
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Basic Operation
bull Data transmitted in small packetsndash Typically 1000 octetsndash Longer messages split into series of packetsndash Each packet contains a portion of user data plus some
control infobull Control info
ndash Routing (addressing) infobull Packets are received stored briefly (buffered) and
passed on to the next nodendash Store and forward
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Use of Packets
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Network layerbull transport segment from sending to receiving host bull on sending side encapsulates segments into
datagramsbull on rcving side delivers segments to transport layerbull network layer protocols in every host routerbull Router examines header fields in all IP datagrams
passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Key Network-Layer Functions
bull forwarding move packets from routerrsquos input to appropriate router output
bull routing determine route taken by packets from source to dest
ndash Routing algorithms
analogy
bull routing process of planning trip from source to dest
bull forwarding process of getting through single interchange
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
1
23
0111
value in arrivingpacketrsquos header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Many different interconnection topologies
bull Pointer to interconnectionppt
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Virtual circuit vs datagram networks
(Shasha students read on your own)
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Network layer connection and connection-less service
bull Datagram network provides network-layer connectionless service
bull VC network provides network-layer connection service
bull Analogous to the transport-layer services butndash Service host-to-hostndash No choice network provides one or the otherndash Implementation in the core
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Virtual circuits
bull call setup teardown for each call before data can flowbull each packet carries VC identifier (not destination host address)bull every router on source-dest path maintains ldquostaterdquo for each
passing connectionbull link router resources (bandwidth buffers) may be allocated to
VC
ldquosource-to-dest path behaves much like telephone circuitrdquondash performance-wisendash network actions along source-to-dest path
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
VC implementation
A VC consists of1 Path from source to destination2 VC numbers one number for each link along path3 Entries in forwarding tables in routers along path
bull Packet belonging to VC carries a VC numberbull VC number must be changed on each link
ndash New VC number comes from forwarding table
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Forwarding table12 22 32
1 2 3
VC number
interfacenumber
Incoming interface Incoming VC Outgoing interface Outgoing VC
1 12 2 222 63 1 18 3 7 2 171 97 3 87hellip hellip hellip hellip
Forwarding table innorthwest router
Routers maintain connection state information
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Virtual circuits signaling protocols
bull used to setup maintain teardown VCbull used in ATM frame-relay X25bull not used in todayrsquos Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Initiate call 2 incoming call3 Accept call4 Call connected
5 Data flow begins 6 Receive data
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Datagram networksbull no call setup at network layerbull routers no state about end-to-end connections
ndash no network-level concept of ldquoconnectionrdquobull packets forwarded using destination host address
ndash packets between same source-dest pair may take different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1 Send data 2 Receive data
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Datagram or VC network why
Internetbull data exchange among
computersndash ldquoelasticrdquo service no strict
timing req bull ldquosmartrdquo end systems (computers)
ndash can adapt perform control error recovery
ndash simple inside network complexity at ldquoedgerdquo
bull many link types ndash different characteristicsndash uniform service difficult
ATMbull evolved from telephonybull human conversation
ndash strict timing reliability requirements
ndash need for guaranteed service
bull ldquodumbrdquo end systemsndash telephonesndash complexity inside
network
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
The Internet Network layer
forwardingtable
Host router network layer functions
Routing protocolsbullpath selectionbullRIP OSPF BGP
IP protocolbulladdressing conventionsbulldatagram formatbullpacket handling conventions
ICMP protocolbullerror reportingbullrouter ldquosignalingrdquo
Transport layer TCP UDP
Link layer
physical layer
Networklayer
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Routing
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Routing Strategies
bull Fixedbull Floodingbull Randombull Adaptive
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Fixed Routing
bull Single permanent route for each source to destination pair
bull Determine routes using a least cost algorithm
bull Route fixed at least until a change in network topology
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Fixed RoutingTables
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Floodingbull No network info requiredbull Packet sent by node to every neighborbull Incoming packets retransmitted on every link except
incoming linkbull Eventually a number of copies will arrive at destinationbull Each packet is uniquely numbered so duplicates can
be discardedbull Nodes can remember packets already forwarded to
keep network load in boundsbull Can include a hop count in packets
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Flooding Example
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Properties of Flooding
bull All possible routes are triedndash Very robust
bull At least one packet will have taken minimum hop count routendash Can be used to set up virtual circuit
bull All nodes are visitedndash Useful to distribute information (eg routing)
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Random Routing
bull Node selects one outgoing path for retransmission of incoming packet
bull Selection can be random or round robinbull Can select outgoing path based on
probability calculationbull No network info neededbull Route is typically not least cost nor
minimum hop
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Adaptive Routingbull Used by almost all packet switching networksbull Routing decisions change as conditions on the
network changendash Failurendash Congestion
bull Requires info about networkbull Decisions more complexbull Tradeoff between quality of network info and overheadbull Reacting too quickly can cause oscillation (bathtub)bull Reacting too slowly can create irrelevance
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Routing Algorithm classification
Global or decentralized information
Globalbull all routers have complete
topology link cost infobull ldquolink staterdquo algorithmsDecentralized bull router knows physically-
connected neighbors link costs to neighbors
bull iterative process of computation exchange of info with neighbors
bull ldquodistance vectorrdquo algorithms
Static or dynamicStatic bull routes change slowly
over timeDynamic bull routes change more
quicklyndash periodic updatendash in response to link
cost changes
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
A Link-State Routing Algorithm
Dijkstrarsquos algorithmbull net topology link costs known
to all nodesndash accomplished via ldquolink
state broadcastrdquo ndash all nodes have same info
bull computes least cost paths from one node (lsquosourcerdquo) to all other nodesndash gives forwarding table for
that nodebull iterative after k iterations
know least cost path to k destrsquos
Notationbull c(xy) link cost from node x to
y = infin if not direct neighborsbull D(v) current value of cost of
path from source to dest vbull p(v) predecessor node along
path from source to vbull N set of nodes whose least
cost path definitively known
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Dijsktrarsquos Algorithm1 Initialization 2 N = u 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(uv) 6 else D(v) = infin 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N 12 D(v) = min( D(v) D(w) + c(wv) ) 13 new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v 15 until all nodes in N
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Distance vector algorithm Basic idea bull Each node periodically sends its own distance vector
estimate to neighborsbull When node a node x receives new DV estimate from
neighbor it updates its own DV using B-F equation
Dx(y) larr minvc(xv) + Dv(y) for each node y ∊ N
bull Under minor natural conditions the estimate Dx(y) converge the actual least cost dx(y)
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Distance Vector Algorithm Properties
Iterative asynchronous each local iteration caused by
bull local link cost change bull DV update message from
neighbor
Distributedbull each node notifies
neighbors only when its DV changesndash neighbors then notify their
neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute estimates
if DV to any dest has changed notify neighbors
Each node
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Hierarchical OSPF
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Hierarchical OSPFbull Two-level hierarchy local area backbone
ndash Link-state advertisements only in area ndash each nodes has detailed area topology only know
direction (shortest path) to nets in other areasbull Area border routers ldquosummarizerdquo distances to nets
in own area advertise to other Area Border routersbull Backbone routers run OSPF routing limited to
backbonebull Boundary routers connect to other ASrsquos
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Internet inter-AS routing BGP
bull BGP (Border Gateway Protocol) the de facto standard
bull BGP provides each AS a means to1Obtain subnet reachability information from
neighboring ASs2Propagate the reachability information to all routers
internal to the AS3Determine ldquogoodrdquo routes to subnets based on
reachability information and policybull Allows a subnet to advertise its existence to
rest of the Internet ldquoI am hererdquo
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-
Why different Intra- and Inter-AS routing Policy bull Inter-AS admin wants control over how its traffic
routed who routes through its net bull Intra-AS single admin so no policy decisions needed
Scalebull hierarchical routing saves table size reduced update
trafficPerformance bull Intra-AS can focus on performancebull Inter-AS policy may dominate over performance
- Data Communication Essentials
- A Communications Model
- Simplified Communications Model - Diagram
- Key Communications Tasks
- Networking
- Simplified Network Model
- The Network Core
- Network Core Circuit Switching
- Circuit Switching FDM and TDM
- Network Core Packet Switching
- Packet Switching Statistical Multiplexing
- Packet switching versus circuit switching
- Local Area Networks
- Protocols
- Key Elements of a Protocol
- Whatrsquos a protocol
- Slide 17
- In Summary a protocol is
- Protocol Specification
- Event -Time Diagrams
- Transition Diagram
- Protocol ldquoLayersrdquo
- Why layering
- Internet protocol stack
- Encapsulation
- OSI
- How do loss and delay occur
- Four sources of packet delay
- Delay in packet-switched networks
- Data Communication and Networks
- Simplified Data Communications Model
- Slide 32
- Terminology (1)
- Terminology (2)
- Terminology (3)
- Analog and Digital Data Transmission
- Data
- Signals
- Data and Signals
- Analog Transmission
- Digital Transmission
- Advantages amp Disadvantages of Digital
- Attenuation of Digital Signals
- Interpreting Signals
- Encoding Schemes
- Nonreturn to Zero-Level (NRZ-L)
- Nonreturn to Zero Inverted
- NRZ
- Differential Encoding
- Biphase
- Biphase Pros and Cons
- Asynchronous
- Asynchronous (diagram)
- Asynchronous - Behavior
- Synchronous - Bit Level
- Synchronous - Block Level
- Synchronous (diagram)
- Synchronous Bit Error Detection
- Data Link Layer
- Link Layer Services
- Encoding Information Frames
- Error Detection
- Parity Checking
- Internet checksum
- Multiple Access protocols
- Ideal Multiple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- CSMACD efficiency (probabilistic)
- References to Performance
- ldquoTaking Turnsrdquo MAC protocols
- Slide 81
- Summary of MAC protocols
- Wireless Networks
- Chapter 6 Wireless and Mobile Networks
- Elements of a wireless network
- Slide 86
- Characteristics of selected wireless link standards
- Slide 88
- Slide 89
- IEEE 80211 MAC Protocol CSMACA
- Avoiding collisions (more)
- Collision Avoidance RTS-CTS exchange
- How do you contact a mobile friend
- Basic idea SIM card vs phone number
- Long Distance Mobility approach
- Mobility registration
- Mobility via Indirect Routing
- Indirect Routing moving between networks
- Reliable Transmission
- rdt30 stop-and-wait operation
- Pipelined protocols
- Pipelining (by factor of 3)
- Go-Back-N
- Selective Repeat
- Selective repeat sender receiver windows
- Look at global negative acknowledgment scheme
- Network Layer circuit switching vs packet switching
- Switching Networks
- Technology
- Simple Switched Network
- Circuit Switching
- Circuit Switching - Issues
- Packet Switching
- Basic Operation
- Use of Packets
- Network layer
- Key Network-Layer Functions
- Slide 118
- Many different interconnection topologies
- Virtual circuit vs datagram networks (Shasha students read on your own)
- Network layer connection and connection-less service
- Virtual circuits
- VC implementation
- Forwarding table
- Virtual circuits signaling protocols
- Datagram networks
- Datagram or VC network why
- The Internet Network layer
- Routing
- Routing Strategies
- Fixed Routing
- Fixed Routing Tables
- Flooding
- Flooding Example
- Properties of Flooding
- Random Routing
- Adaptive Routing
- Routing Algorithm classification
- A Link-State Routing Algorithm
- Dijsktrarsquos Algorithm
- Distance vector algorithm
- Distance Vector Algorithm Properties
- Hierarchical OSPF
- Slide 144
- Internet inter-AS routing BGP
- Why different Intra- and Inter-AS routing
-