link layer and lans ossi mokryn and irad ratmansky, based on slides from the top down approach book...
TRANSCRIPT
Link Layer and LANs
Ossi Mokryn and Irad Ratmansky Based on Slides from the Top Down Approach book
1
The Data Link Layer
Our goals Understand principles behind data link layer
services Sharing a broadcast channel multiple access Link layer addressing Interconnection of different LAN segments
Instantiation and implementation of various link layer technologies
2
Link Layer IntroductionSome terminology hosts and routers are nodes communication channels that
connect adjacent nodes along communication path are links wired links wireless links LANs
layer-2 packet is a frame encapsulates datagram
ldquolinkrdquo
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
3
Link layer context Datagram transferred by
different link protocols over different links eg Ethernet on first
link frame relay on intermediate links 80211 on last link
Each link protocol provides different services eg may or may not
provide rdt over link
transportation analogy trip from Princeton to
Lausanne limo Princeton to JFK plane JFK to Geneva train Geneva to Lausanne
tourist = datagram transport segment =
communication link transportation mode =
link layer protocol travel agent = routing
algorithm
4
Physical Media physical link
Transmitted data bit propagates across link guided media
signals propagate in solid media (eg copper fiber)
unguided media
signals propagate freely (eg radio bands)
The physical Layer defines the ldquorepresentationrdquo of bitsItrsquos also provides protection by both detecting and correcting corrupted bits (See how it works)
5
Physical Media (Examples)Twisted Pair (TP) Two insulated copper wires May be shielded or
not Category 3
Traditional phone wires Supports 10-Mbps Ethernet Category 5
Supports 100Mbps Ethernet (ldquoFast-Ethernetrdquo)
6
Physical Media (Examples)Coaxial cable Two concentric shielded wires
Basebandsingle channel on cable
broadbandmultiple channel on cable
common uses for 10-Mbps Ethernet and TV cables
7
Physical Media (Examples)
Fiber Glass fiber carrying light pulses
Single mode or multi mode High point-to-point speed Very low error rate (caused by low fiberrsquos attenuation) Secured
Common used for 100-Mbps Ethernet and 1000-Mbps Ethernet (ldquoGigabit Ethernetrdquo)
8
Physical Media (Examples) Many more
Radio bands (WiFi high bit error rate) Microwave (Requires hosts in light of sight) Satellite (very slow RTT)
9
Link Layer Services
Recall that were actually considering the MAC layer in the IEEE 802 model Framing (Frame structure)
encapsulate datagram into frame adding header trailer
Link Access (The protocol) Addressing
Introduces ldquoMACrdquo addresses used in frame headers to identify hosts (actually NICs) who are part of the network
Different for IP addresses Channel Access
Defines the set of rules which allows the hosts to use the (possibly shared) medium
10
Link Layer Services Flow Control
pacing between adjacent sending and receiving nodes Error Detection
errors caused by signal attenuation noise receiver detects presence of errors
signals sender for retransmission or drops frame Error Correction
receiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but
not at same time RDT
Offers some reliability between the hosts (Why is this redundant)
11
Link Layer Services
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI
card 80211 card sending side
encapsulates datagram in a frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to rcving node adapter is semi-
autonomous link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
12
Link TypesThree types of ldquolinksrdquo point-to-point (P2P)
PPP for dial-up access Point-to-point link
between Ethernet switch and host
Switched Networks ATM (used for WAN)
Broadcast Traditional Ethernet and
its predecessors Upstream HFC 80211 wireless LAN
Wersquoll focus on Broadcast media
13
Multiple Access protocols
Ossi Mokryn - Data link layer
single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the
same timeMultiple ACcess (MAC) Protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
14
Human Analogy
Ossi Mokryn - Data link layer15
Rules for lsquoparty conversationrsquo Give everyone a chance to speak Dont speak until you are spoken to Dont monopolize the conversation Raise your hand if you have a question Dont interrupt when someone is speaking Dont fall asleep when someone else is talking
MAC Protocols measures Assume a shared medium with a channel rate of R
[bpsec] Efficient
When one node wants to transmit it ca send at rate R Fair
When N users want to transmit each can send at average rate RN
DecentralizedNo special node uses to coordinate transmission (no ldquoleaderrdquo)No synchronization of clocks or slotsFault tolerant
SimpleShould be very fast and implemented in NICs firmware
16
MAC Protocol TypesThree broad classes Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands or by code)
allocate piece to a node for exclusive its use Random Access
Channel not divided allow collisions ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMATDMA (Time Division Multiple Access) Access the channel in roundsldquo Each station gets fixed length slot (length = packets
trans time) in each round Each slot called a Time-Slot Unused slots go idle Example 6-station LAN 134 have packtes slots 256
idle
18
Channel Partitioning MAC protocols FDMAFDMA (Frequency Division Multiple Access) Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle example 6-station LAN 134 have packets frequency
bands 256 idle
frequ
ency
bands
time
19
TDM FDM summary FDM Enables the division of a channel with
capacity C bits per seconds to N sub-channels each gets a different frequency range and capacity of CN
TDM The division of channel to N sub-channels each gets CN capacity by giving the entire channel to each of the N stations for 1N of the time
The division makes each sub channel less busy but the overall waiting time is bigger by a factor of N compared to having one channel (Little theorem)
20
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA) used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
21
Random Access MAC Protocols From here on wersquoll focus on Random Access MAC
protocols When node has packet to send
Transmit at full channel data rate R No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies
How to detect collisions How to recover from collisions
Examples of random access MAC protocols ALOHA slotted ALOHA CSMACD (Ethernet) CSMACA (Wireless)
22
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
The Data Link Layer
Our goals Understand principles behind data link layer
services Sharing a broadcast channel multiple access Link layer addressing Interconnection of different LAN segments
Instantiation and implementation of various link layer technologies
2
Link Layer IntroductionSome terminology hosts and routers are nodes communication channels that
connect adjacent nodes along communication path are links wired links wireless links LANs
layer-2 packet is a frame encapsulates datagram
ldquolinkrdquo
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
3
Link layer context Datagram transferred by
different link protocols over different links eg Ethernet on first
link frame relay on intermediate links 80211 on last link
Each link protocol provides different services eg may or may not
provide rdt over link
transportation analogy trip from Princeton to
Lausanne limo Princeton to JFK plane JFK to Geneva train Geneva to Lausanne
tourist = datagram transport segment =
communication link transportation mode =
link layer protocol travel agent = routing
algorithm
4
Physical Media physical link
Transmitted data bit propagates across link guided media
signals propagate in solid media (eg copper fiber)
unguided media
signals propagate freely (eg radio bands)
The physical Layer defines the ldquorepresentationrdquo of bitsItrsquos also provides protection by both detecting and correcting corrupted bits (See how it works)
5
Physical Media (Examples)Twisted Pair (TP) Two insulated copper wires May be shielded or
not Category 3
Traditional phone wires Supports 10-Mbps Ethernet Category 5
Supports 100Mbps Ethernet (ldquoFast-Ethernetrdquo)
6
Physical Media (Examples)Coaxial cable Two concentric shielded wires
Basebandsingle channel on cable
broadbandmultiple channel on cable
common uses for 10-Mbps Ethernet and TV cables
7
Physical Media (Examples)
Fiber Glass fiber carrying light pulses
Single mode or multi mode High point-to-point speed Very low error rate (caused by low fiberrsquos attenuation) Secured
Common used for 100-Mbps Ethernet and 1000-Mbps Ethernet (ldquoGigabit Ethernetrdquo)
8
Physical Media (Examples) Many more
Radio bands (WiFi high bit error rate) Microwave (Requires hosts in light of sight) Satellite (very slow RTT)
9
Link Layer Services
Recall that were actually considering the MAC layer in the IEEE 802 model Framing (Frame structure)
encapsulate datagram into frame adding header trailer
Link Access (The protocol) Addressing
Introduces ldquoMACrdquo addresses used in frame headers to identify hosts (actually NICs) who are part of the network
Different for IP addresses Channel Access
Defines the set of rules which allows the hosts to use the (possibly shared) medium
10
Link Layer Services Flow Control
pacing between adjacent sending and receiving nodes Error Detection
errors caused by signal attenuation noise receiver detects presence of errors
signals sender for retransmission or drops frame Error Correction
receiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but
not at same time RDT
Offers some reliability between the hosts (Why is this redundant)
11
Link Layer Services
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI
card 80211 card sending side
encapsulates datagram in a frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to rcving node adapter is semi-
autonomous link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
12
Link TypesThree types of ldquolinksrdquo point-to-point (P2P)
PPP for dial-up access Point-to-point link
between Ethernet switch and host
Switched Networks ATM (used for WAN)
Broadcast Traditional Ethernet and
its predecessors Upstream HFC 80211 wireless LAN
Wersquoll focus on Broadcast media
13
Multiple Access protocols
Ossi Mokryn - Data link layer
single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the
same timeMultiple ACcess (MAC) Protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
14
Human Analogy
Ossi Mokryn - Data link layer15
Rules for lsquoparty conversationrsquo Give everyone a chance to speak Dont speak until you are spoken to Dont monopolize the conversation Raise your hand if you have a question Dont interrupt when someone is speaking Dont fall asleep when someone else is talking
MAC Protocols measures Assume a shared medium with a channel rate of R
[bpsec] Efficient
When one node wants to transmit it ca send at rate R Fair
When N users want to transmit each can send at average rate RN
DecentralizedNo special node uses to coordinate transmission (no ldquoleaderrdquo)No synchronization of clocks or slotsFault tolerant
SimpleShould be very fast and implemented in NICs firmware
16
MAC Protocol TypesThree broad classes Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands or by code)
allocate piece to a node for exclusive its use Random Access
Channel not divided allow collisions ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMATDMA (Time Division Multiple Access) Access the channel in roundsldquo Each station gets fixed length slot (length = packets
trans time) in each round Each slot called a Time-Slot Unused slots go idle Example 6-station LAN 134 have packtes slots 256
idle
18
Channel Partitioning MAC protocols FDMAFDMA (Frequency Division Multiple Access) Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle example 6-station LAN 134 have packets frequency
bands 256 idle
frequ
ency
bands
time
19
TDM FDM summary FDM Enables the division of a channel with
capacity C bits per seconds to N sub-channels each gets a different frequency range and capacity of CN
TDM The division of channel to N sub-channels each gets CN capacity by giving the entire channel to each of the N stations for 1N of the time
The division makes each sub channel less busy but the overall waiting time is bigger by a factor of N compared to having one channel (Little theorem)
20
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA) used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
21
Random Access MAC Protocols From here on wersquoll focus on Random Access MAC
protocols When node has packet to send
Transmit at full channel data rate R No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies
How to detect collisions How to recover from collisions
Examples of random access MAC protocols ALOHA slotted ALOHA CSMACD (Ethernet) CSMACA (Wireless)
22
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Link Layer IntroductionSome terminology hosts and routers are nodes communication channels that
connect adjacent nodes along communication path are links wired links wireless links LANs
layer-2 packet is a frame encapsulates datagram
ldquolinkrdquo
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
3
Link layer context Datagram transferred by
different link protocols over different links eg Ethernet on first
link frame relay on intermediate links 80211 on last link
Each link protocol provides different services eg may or may not
provide rdt over link
transportation analogy trip from Princeton to
Lausanne limo Princeton to JFK plane JFK to Geneva train Geneva to Lausanne
tourist = datagram transport segment =
communication link transportation mode =
link layer protocol travel agent = routing
algorithm
4
Physical Media physical link
Transmitted data bit propagates across link guided media
signals propagate in solid media (eg copper fiber)
unguided media
signals propagate freely (eg radio bands)
The physical Layer defines the ldquorepresentationrdquo of bitsItrsquos also provides protection by both detecting and correcting corrupted bits (See how it works)
5
Physical Media (Examples)Twisted Pair (TP) Two insulated copper wires May be shielded or
not Category 3
Traditional phone wires Supports 10-Mbps Ethernet Category 5
Supports 100Mbps Ethernet (ldquoFast-Ethernetrdquo)
6
Physical Media (Examples)Coaxial cable Two concentric shielded wires
Basebandsingle channel on cable
broadbandmultiple channel on cable
common uses for 10-Mbps Ethernet and TV cables
7
Physical Media (Examples)
Fiber Glass fiber carrying light pulses
Single mode or multi mode High point-to-point speed Very low error rate (caused by low fiberrsquos attenuation) Secured
Common used for 100-Mbps Ethernet and 1000-Mbps Ethernet (ldquoGigabit Ethernetrdquo)
8
Physical Media (Examples) Many more
Radio bands (WiFi high bit error rate) Microwave (Requires hosts in light of sight) Satellite (very slow RTT)
9
Link Layer Services
Recall that were actually considering the MAC layer in the IEEE 802 model Framing (Frame structure)
encapsulate datagram into frame adding header trailer
Link Access (The protocol) Addressing
Introduces ldquoMACrdquo addresses used in frame headers to identify hosts (actually NICs) who are part of the network
Different for IP addresses Channel Access
Defines the set of rules which allows the hosts to use the (possibly shared) medium
10
Link Layer Services Flow Control
pacing between adjacent sending and receiving nodes Error Detection
errors caused by signal attenuation noise receiver detects presence of errors
signals sender for retransmission or drops frame Error Correction
receiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but
not at same time RDT
Offers some reliability between the hosts (Why is this redundant)
11
Link Layer Services
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI
card 80211 card sending side
encapsulates datagram in a frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to rcving node adapter is semi-
autonomous link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
12
Link TypesThree types of ldquolinksrdquo point-to-point (P2P)
PPP for dial-up access Point-to-point link
between Ethernet switch and host
Switched Networks ATM (used for WAN)
Broadcast Traditional Ethernet and
its predecessors Upstream HFC 80211 wireless LAN
Wersquoll focus on Broadcast media
13
Multiple Access protocols
Ossi Mokryn - Data link layer
single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the
same timeMultiple ACcess (MAC) Protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
14
Human Analogy
Ossi Mokryn - Data link layer15
Rules for lsquoparty conversationrsquo Give everyone a chance to speak Dont speak until you are spoken to Dont monopolize the conversation Raise your hand if you have a question Dont interrupt when someone is speaking Dont fall asleep when someone else is talking
MAC Protocols measures Assume a shared medium with a channel rate of R
[bpsec] Efficient
When one node wants to transmit it ca send at rate R Fair
When N users want to transmit each can send at average rate RN
DecentralizedNo special node uses to coordinate transmission (no ldquoleaderrdquo)No synchronization of clocks or slotsFault tolerant
SimpleShould be very fast and implemented in NICs firmware
16
MAC Protocol TypesThree broad classes Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands or by code)
allocate piece to a node for exclusive its use Random Access
Channel not divided allow collisions ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMATDMA (Time Division Multiple Access) Access the channel in roundsldquo Each station gets fixed length slot (length = packets
trans time) in each round Each slot called a Time-Slot Unused slots go idle Example 6-station LAN 134 have packtes slots 256
idle
18
Channel Partitioning MAC protocols FDMAFDMA (Frequency Division Multiple Access) Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle example 6-station LAN 134 have packets frequency
bands 256 idle
frequ
ency
bands
time
19
TDM FDM summary FDM Enables the division of a channel with
capacity C bits per seconds to N sub-channels each gets a different frequency range and capacity of CN
TDM The division of channel to N sub-channels each gets CN capacity by giving the entire channel to each of the N stations for 1N of the time
The division makes each sub channel less busy but the overall waiting time is bigger by a factor of N compared to having one channel (Little theorem)
20
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA) used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
21
Random Access MAC Protocols From here on wersquoll focus on Random Access MAC
protocols When node has packet to send
Transmit at full channel data rate R No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies
How to detect collisions How to recover from collisions
Examples of random access MAC protocols ALOHA slotted ALOHA CSMACD (Ethernet) CSMACA (Wireless)
22
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Link layer context Datagram transferred by
different link protocols over different links eg Ethernet on first
link frame relay on intermediate links 80211 on last link
Each link protocol provides different services eg may or may not
provide rdt over link
transportation analogy trip from Princeton to
Lausanne limo Princeton to JFK plane JFK to Geneva train Geneva to Lausanne
tourist = datagram transport segment =
communication link transportation mode =
link layer protocol travel agent = routing
algorithm
4
Physical Media physical link
Transmitted data bit propagates across link guided media
signals propagate in solid media (eg copper fiber)
unguided media
signals propagate freely (eg radio bands)
The physical Layer defines the ldquorepresentationrdquo of bitsItrsquos also provides protection by both detecting and correcting corrupted bits (See how it works)
5
Physical Media (Examples)Twisted Pair (TP) Two insulated copper wires May be shielded or
not Category 3
Traditional phone wires Supports 10-Mbps Ethernet Category 5
Supports 100Mbps Ethernet (ldquoFast-Ethernetrdquo)
6
Physical Media (Examples)Coaxial cable Two concentric shielded wires
Basebandsingle channel on cable
broadbandmultiple channel on cable
common uses for 10-Mbps Ethernet and TV cables
7
Physical Media (Examples)
Fiber Glass fiber carrying light pulses
Single mode or multi mode High point-to-point speed Very low error rate (caused by low fiberrsquos attenuation) Secured
Common used for 100-Mbps Ethernet and 1000-Mbps Ethernet (ldquoGigabit Ethernetrdquo)
8
Physical Media (Examples) Many more
Radio bands (WiFi high bit error rate) Microwave (Requires hosts in light of sight) Satellite (very slow RTT)
9
Link Layer Services
Recall that were actually considering the MAC layer in the IEEE 802 model Framing (Frame structure)
encapsulate datagram into frame adding header trailer
Link Access (The protocol) Addressing
Introduces ldquoMACrdquo addresses used in frame headers to identify hosts (actually NICs) who are part of the network
Different for IP addresses Channel Access
Defines the set of rules which allows the hosts to use the (possibly shared) medium
10
Link Layer Services Flow Control
pacing between adjacent sending and receiving nodes Error Detection
errors caused by signal attenuation noise receiver detects presence of errors
signals sender for retransmission or drops frame Error Correction
receiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but
not at same time RDT
Offers some reliability between the hosts (Why is this redundant)
11
Link Layer Services
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI
card 80211 card sending side
encapsulates datagram in a frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to rcving node adapter is semi-
autonomous link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
12
Link TypesThree types of ldquolinksrdquo point-to-point (P2P)
PPP for dial-up access Point-to-point link
between Ethernet switch and host
Switched Networks ATM (used for WAN)
Broadcast Traditional Ethernet and
its predecessors Upstream HFC 80211 wireless LAN
Wersquoll focus on Broadcast media
13
Multiple Access protocols
Ossi Mokryn - Data link layer
single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the
same timeMultiple ACcess (MAC) Protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
14
Human Analogy
Ossi Mokryn - Data link layer15
Rules for lsquoparty conversationrsquo Give everyone a chance to speak Dont speak until you are spoken to Dont monopolize the conversation Raise your hand if you have a question Dont interrupt when someone is speaking Dont fall asleep when someone else is talking
MAC Protocols measures Assume a shared medium with a channel rate of R
[bpsec] Efficient
When one node wants to transmit it ca send at rate R Fair
When N users want to transmit each can send at average rate RN
DecentralizedNo special node uses to coordinate transmission (no ldquoleaderrdquo)No synchronization of clocks or slotsFault tolerant
SimpleShould be very fast and implemented in NICs firmware
16
MAC Protocol TypesThree broad classes Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands or by code)
allocate piece to a node for exclusive its use Random Access
Channel not divided allow collisions ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMATDMA (Time Division Multiple Access) Access the channel in roundsldquo Each station gets fixed length slot (length = packets
trans time) in each round Each slot called a Time-Slot Unused slots go idle Example 6-station LAN 134 have packtes slots 256
idle
18
Channel Partitioning MAC protocols FDMAFDMA (Frequency Division Multiple Access) Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle example 6-station LAN 134 have packets frequency
bands 256 idle
frequ
ency
bands
time
19
TDM FDM summary FDM Enables the division of a channel with
capacity C bits per seconds to N sub-channels each gets a different frequency range and capacity of CN
TDM The division of channel to N sub-channels each gets CN capacity by giving the entire channel to each of the N stations for 1N of the time
The division makes each sub channel less busy but the overall waiting time is bigger by a factor of N compared to having one channel (Little theorem)
20
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA) used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
21
Random Access MAC Protocols From here on wersquoll focus on Random Access MAC
protocols When node has packet to send
Transmit at full channel data rate R No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies
How to detect collisions How to recover from collisions
Examples of random access MAC protocols ALOHA slotted ALOHA CSMACD (Ethernet) CSMACA (Wireless)
22
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Physical Media physical link
Transmitted data bit propagates across link guided media
signals propagate in solid media (eg copper fiber)
unguided media
signals propagate freely (eg radio bands)
The physical Layer defines the ldquorepresentationrdquo of bitsItrsquos also provides protection by both detecting and correcting corrupted bits (See how it works)
5
Physical Media (Examples)Twisted Pair (TP) Two insulated copper wires May be shielded or
not Category 3
Traditional phone wires Supports 10-Mbps Ethernet Category 5
Supports 100Mbps Ethernet (ldquoFast-Ethernetrdquo)
6
Physical Media (Examples)Coaxial cable Two concentric shielded wires
Basebandsingle channel on cable
broadbandmultiple channel on cable
common uses for 10-Mbps Ethernet and TV cables
7
Physical Media (Examples)
Fiber Glass fiber carrying light pulses
Single mode or multi mode High point-to-point speed Very low error rate (caused by low fiberrsquos attenuation) Secured
Common used for 100-Mbps Ethernet and 1000-Mbps Ethernet (ldquoGigabit Ethernetrdquo)
8
Physical Media (Examples) Many more
Radio bands (WiFi high bit error rate) Microwave (Requires hosts in light of sight) Satellite (very slow RTT)
9
Link Layer Services
Recall that were actually considering the MAC layer in the IEEE 802 model Framing (Frame structure)
encapsulate datagram into frame adding header trailer
Link Access (The protocol) Addressing
Introduces ldquoMACrdquo addresses used in frame headers to identify hosts (actually NICs) who are part of the network
Different for IP addresses Channel Access
Defines the set of rules which allows the hosts to use the (possibly shared) medium
10
Link Layer Services Flow Control
pacing between adjacent sending and receiving nodes Error Detection
errors caused by signal attenuation noise receiver detects presence of errors
signals sender for retransmission or drops frame Error Correction
receiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but
not at same time RDT
Offers some reliability between the hosts (Why is this redundant)
11
Link Layer Services
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI
card 80211 card sending side
encapsulates datagram in a frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to rcving node adapter is semi-
autonomous link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
12
Link TypesThree types of ldquolinksrdquo point-to-point (P2P)
PPP for dial-up access Point-to-point link
between Ethernet switch and host
Switched Networks ATM (used for WAN)
Broadcast Traditional Ethernet and
its predecessors Upstream HFC 80211 wireless LAN
Wersquoll focus on Broadcast media
13
Multiple Access protocols
Ossi Mokryn - Data link layer
single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the
same timeMultiple ACcess (MAC) Protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
14
Human Analogy
Ossi Mokryn - Data link layer15
Rules for lsquoparty conversationrsquo Give everyone a chance to speak Dont speak until you are spoken to Dont monopolize the conversation Raise your hand if you have a question Dont interrupt when someone is speaking Dont fall asleep when someone else is talking
MAC Protocols measures Assume a shared medium with a channel rate of R
[bpsec] Efficient
When one node wants to transmit it ca send at rate R Fair
When N users want to transmit each can send at average rate RN
DecentralizedNo special node uses to coordinate transmission (no ldquoleaderrdquo)No synchronization of clocks or slotsFault tolerant
SimpleShould be very fast and implemented in NICs firmware
16
MAC Protocol TypesThree broad classes Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands or by code)
allocate piece to a node for exclusive its use Random Access
Channel not divided allow collisions ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMATDMA (Time Division Multiple Access) Access the channel in roundsldquo Each station gets fixed length slot (length = packets
trans time) in each round Each slot called a Time-Slot Unused slots go idle Example 6-station LAN 134 have packtes slots 256
idle
18
Channel Partitioning MAC protocols FDMAFDMA (Frequency Division Multiple Access) Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle example 6-station LAN 134 have packets frequency
bands 256 idle
frequ
ency
bands
time
19
TDM FDM summary FDM Enables the division of a channel with
capacity C bits per seconds to N sub-channels each gets a different frequency range and capacity of CN
TDM The division of channel to N sub-channels each gets CN capacity by giving the entire channel to each of the N stations for 1N of the time
The division makes each sub channel less busy but the overall waiting time is bigger by a factor of N compared to having one channel (Little theorem)
20
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA) used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
21
Random Access MAC Protocols From here on wersquoll focus on Random Access MAC
protocols When node has packet to send
Transmit at full channel data rate R No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies
How to detect collisions How to recover from collisions
Examples of random access MAC protocols ALOHA slotted ALOHA CSMACD (Ethernet) CSMACA (Wireless)
22
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Physical Media (Examples)Twisted Pair (TP) Two insulated copper wires May be shielded or
not Category 3
Traditional phone wires Supports 10-Mbps Ethernet Category 5
Supports 100Mbps Ethernet (ldquoFast-Ethernetrdquo)
6
Physical Media (Examples)Coaxial cable Two concentric shielded wires
Basebandsingle channel on cable
broadbandmultiple channel on cable
common uses for 10-Mbps Ethernet and TV cables
7
Physical Media (Examples)
Fiber Glass fiber carrying light pulses
Single mode or multi mode High point-to-point speed Very low error rate (caused by low fiberrsquos attenuation) Secured
Common used for 100-Mbps Ethernet and 1000-Mbps Ethernet (ldquoGigabit Ethernetrdquo)
8
Physical Media (Examples) Many more
Radio bands (WiFi high bit error rate) Microwave (Requires hosts in light of sight) Satellite (very slow RTT)
9
Link Layer Services
Recall that were actually considering the MAC layer in the IEEE 802 model Framing (Frame structure)
encapsulate datagram into frame adding header trailer
Link Access (The protocol) Addressing
Introduces ldquoMACrdquo addresses used in frame headers to identify hosts (actually NICs) who are part of the network
Different for IP addresses Channel Access
Defines the set of rules which allows the hosts to use the (possibly shared) medium
10
Link Layer Services Flow Control
pacing between adjacent sending and receiving nodes Error Detection
errors caused by signal attenuation noise receiver detects presence of errors
signals sender for retransmission or drops frame Error Correction
receiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but
not at same time RDT
Offers some reliability between the hosts (Why is this redundant)
11
Link Layer Services
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI
card 80211 card sending side
encapsulates datagram in a frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to rcving node adapter is semi-
autonomous link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
12
Link TypesThree types of ldquolinksrdquo point-to-point (P2P)
PPP for dial-up access Point-to-point link
between Ethernet switch and host
Switched Networks ATM (used for WAN)
Broadcast Traditional Ethernet and
its predecessors Upstream HFC 80211 wireless LAN
Wersquoll focus on Broadcast media
13
Multiple Access protocols
Ossi Mokryn - Data link layer
single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the
same timeMultiple ACcess (MAC) Protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
14
Human Analogy
Ossi Mokryn - Data link layer15
Rules for lsquoparty conversationrsquo Give everyone a chance to speak Dont speak until you are spoken to Dont monopolize the conversation Raise your hand if you have a question Dont interrupt when someone is speaking Dont fall asleep when someone else is talking
MAC Protocols measures Assume a shared medium with a channel rate of R
[bpsec] Efficient
When one node wants to transmit it ca send at rate R Fair
When N users want to transmit each can send at average rate RN
DecentralizedNo special node uses to coordinate transmission (no ldquoleaderrdquo)No synchronization of clocks or slotsFault tolerant
SimpleShould be very fast and implemented in NICs firmware
16
MAC Protocol TypesThree broad classes Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands or by code)
allocate piece to a node for exclusive its use Random Access
Channel not divided allow collisions ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMATDMA (Time Division Multiple Access) Access the channel in roundsldquo Each station gets fixed length slot (length = packets
trans time) in each round Each slot called a Time-Slot Unused slots go idle Example 6-station LAN 134 have packtes slots 256
idle
18
Channel Partitioning MAC protocols FDMAFDMA (Frequency Division Multiple Access) Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle example 6-station LAN 134 have packets frequency
bands 256 idle
frequ
ency
bands
time
19
TDM FDM summary FDM Enables the division of a channel with
capacity C bits per seconds to N sub-channels each gets a different frequency range and capacity of CN
TDM The division of channel to N sub-channels each gets CN capacity by giving the entire channel to each of the N stations for 1N of the time
The division makes each sub channel less busy but the overall waiting time is bigger by a factor of N compared to having one channel (Little theorem)
20
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA) used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
21
Random Access MAC Protocols From here on wersquoll focus on Random Access MAC
protocols When node has packet to send
Transmit at full channel data rate R No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies
How to detect collisions How to recover from collisions
Examples of random access MAC protocols ALOHA slotted ALOHA CSMACD (Ethernet) CSMACA (Wireless)
22
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Physical Media (Examples)Coaxial cable Two concentric shielded wires
Basebandsingle channel on cable
broadbandmultiple channel on cable
common uses for 10-Mbps Ethernet and TV cables
7
Physical Media (Examples)
Fiber Glass fiber carrying light pulses
Single mode or multi mode High point-to-point speed Very low error rate (caused by low fiberrsquos attenuation) Secured
Common used for 100-Mbps Ethernet and 1000-Mbps Ethernet (ldquoGigabit Ethernetrdquo)
8
Physical Media (Examples) Many more
Radio bands (WiFi high bit error rate) Microwave (Requires hosts in light of sight) Satellite (very slow RTT)
9
Link Layer Services
Recall that were actually considering the MAC layer in the IEEE 802 model Framing (Frame structure)
encapsulate datagram into frame adding header trailer
Link Access (The protocol) Addressing
Introduces ldquoMACrdquo addresses used in frame headers to identify hosts (actually NICs) who are part of the network
Different for IP addresses Channel Access
Defines the set of rules which allows the hosts to use the (possibly shared) medium
10
Link Layer Services Flow Control
pacing between adjacent sending and receiving nodes Error Detection
errors caused by signal attenuation noise receiver detects presence of errors
signals sender for retransmission or drops frame Error Correction
receiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but
not at same time RDT
Offers some reliability between the hosts (Why is this redundant)
11
Link Layer Services
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI
card 80211 card sending side
encapsulates datagram in a frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to rcving node adapter is semi-
autonomous link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
12
Link TypesThree types of ldquolinksrdquo point-to-point (P2P)
PPP for dial-up access Point-to-point link
between Ethernet switch and host
Switched Networks ATM (used for WAN)
Broadcast Traditional Ethernet and
its predecessors Upstream HFC 80211 wireless LAN
Wersquoll focus on Broadcast media
13
Multiple Access protocols
Ossi Mokryn - Data link layer
single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the
same timeMultiple ACcess (MAC) Protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
14
Human Analogy
Ossi Mokryn - Data link layer15
Rules for lsquoparty conversationrsquo Give everyone a chance to speak Dont speak until you are spoken to Dont monopolize the conversation Raise your hand if you have a question Dont interrupt when someone is speaking Dont fall asleep when someone else is talking
MAC Protocols measures Assume a shared medium with a channel rate of R
[bpsec] Efficient
When one node wants to transmit it ca send at rate R Fair
When N users want to transmit each can send at average rate RN
DecentralizedNo special node uses to coordinate transmission (no ldquoleaderrdquo)No synchronization of clocks or slotsFault tolerant
SimpleShould be very fast and implemented in NICs firmware
16
MAC Protocol TypesThree broad classes Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands or by code)
allocate piece to a node for exclusive its use Random Access
Channel not divided allow collisions ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMATDMA (Time Division Multiple Access) Access the channel in roundsldquo Each station gets fixed length slot (length = packets
trans time) in each round Each slot called a Time-Slot Unused slots go idle Example 6-station LAN 134 have packtes slots 256
idle
18
Channel Partitioning MAC protocols FDMAFDMA (Frequency Division Multiple Access) Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle example 6-station LAN 134 have packets frequency
bands 256 idle
frequ
ency
bands
time
19
TDM FDM summary FDM Enables the division of a channel with
capacity C bits per seconds to N sub-channels each gets a different frequency range and capacity of CN
TDM The division of channel to N sub-channels each gets CN capacity by giving the entire channel to each of the N stations for 1N of the time
The division makes each sub channel less busy but the overall waiting time is bigger by a factor of N compared to having one channel (Little theorem)
20
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA) used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
21
Random Access MAC Protocols From here on wersquoll focus on Random Access MAC
protocols When node has packet to send
Transmit at full channel data rate R No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies
How to detect collisions How to recover from collisions
Examples of random access MAC protocols ALOHA slotted ALOHA CSMACD (Ethernet) CSMACA (Wireless)
22
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Physical Media (Examples)
Fiber Glass fiber carrying light pulses
Single mode or multi mode High point-to-point speed Very low error rate (caused by low fiberrsquos attenuation) Secured
Common used for 100-Mbps Ethernet and 1000-Mbps Ethernet (ldquoGigabit Ethernetrdquo)
8
Physical Media (Examples) Many more
Radio bands (WiFi high bit error rate) Microwave (Requires hosts in light of sight) Satellite (very slow RTT)
9
Link Layer Services
Recall that were actually considering the MAC layer in the IEEE 802 model Framing (Frame structure)
encapsulate datagram into frame adding header trailer
Link Access (The protocol) Addressing
Introduces ldquoMACrdquo addresses used in frame headers to identify hosts (actually NICs) who are part of the network
Different for IP addresses Channel Access
Defines the set of rules which allows the hosts to use the (possibly shared) medium
10
Link Layer Services Flow Control
pacing between adjacent sending and receiving nodes Error Detection
errors caused by signal attenuation noise receiver detects presence of errors
signals sender for retransmission or drops frame Error Correction
receiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but
not at same time RDT
Offers some reliability between the hosts (Why is this redundant)
11
Link Layer Services
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI
card 80211 card sending side
encapsulates datagram in a frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to rcving node adapter is semi-
autonomous link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
12
Link TypesThree types of ldquolinksrdquo point-to-point (P2P)
PPP for dial-up access Point-to-point link
between Ethernet switch and host
Switched Networks ATM (used for WAN)
Broadcast Traditional Ethernet and
its predecessors Upstream HFC 80211 wireless LAN
Wersquoll focus on Broadcast media
13
Multiple Access protocols
Ossi Mokryn - Data link layer
single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the
same timeMultiple ACcess (MAC) Protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
14
Human Analogy
Ossi Mokryn - Data link layer15
Rules for lsquoparty conversationrsquo Give everyone a chance to speak Dont speak until you are spoken to Dont monopolize the conversation Raise your hand if you have a question Dont interrupt when someone is speaking Dont fall asleep when someone else is talking
MAC Protocols measures Assume a shared medium with a channel rate of R
[bpsec] Efficient
When one node wants to transmit it ca send at rate R Fair
When N users want to transmit each can send at average rate RN
DecentralizedNo special node uses to coordinate transmission (no ldquoleaderrdquo)No synchronization of clocks or slotsFault tolerant
SimpleShould be very fast and implemented in NICs firmware
16
MAC Protocol TypesThree broad classes Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands or by code)
allocate piece to a node for exclusive its use Random Access
Channel not divided allow collisions ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMATDMA (Time Division Multiple Access) Access the channel in roundsldquo Each station gets fixed length slot (length = packets
trans time) in each round Each slot called a Time-Slot Unused slots go idle Example 6-station LAN 134 have packtes slots 256
idle
18
Channel Partitioning MAC protocols FDMAFDMA (Frequency Division Multiple Access) Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle example 6-station LAN 134 have packets frequency
bands 256 idle
frequ
ency
bands
time
19
TDM FDM summary FDM Enables the division of a channel with
capacity C bits per seconds to N sub-channels each gets a different frequency range and capacity of CN
TDM The division of channel to N sub-channels each gets CN capacity by giving the entire channel to each of the N stations for 1N of the time
The division makes each sub channel less busy but the overall waiting time is bigger by a factor of N compared to having one channel (Little theorem)
20
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA) used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
21
Random Access MAC Protocols From here on wersquoll focus on Random Access MAC
protocols When node has packet to send
Transmit at full channel data rate R No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies
How to detect collisions How to recover from collisions
Examples of random access MAC protocols ALOHA slotted ALOHA CSMACD (Ethernet) CSMACA (Wireless)
22
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Physical Media (Examples) Many more
Radio bands (WiFi high bit error rate) Microwave (Requires hosts in light of sight) Satellite (very slow RTT)
9
Link Layer Services
Recall that were actually considering the MAC layer in the IEEE 802 model Framing (Frame structure)
encapsulate datagram into frame adding header trailer
Link Access (The protocol) Addressing
Introduces ldquoMACrdquo addresses used in frame headers to identify hosts (actually NICs) who are part of the network
Different for IP addresses Channel Access
Defines the set of rules which allows the hosts to use the (possibly shared) medium
10
Link Layer Services Flow Control
pacing between adjacent sending and receiving nodes Error Detection
errors caused by signal attenuation noise receiver detects presence of errors
signals sender for retransmission or drops frame Error Correction
receiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but
not at same time RDT
Offers some reliability between the hosts (Why is this redundant)
11
Link Layer Services
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI
card 80211 card sending side
encapsulates datagram in a frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to rcving node adapter is semi-
autonomous link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
12
Link TypesThree types of ldquolinksrdquo point-to-point (P2P)
PPP for dial-up access Point-to-point link
between Ethernet switch and host
Switched Networks ATM (used for WAN)
Broadcast Traditional Ethernet and
its predecessors Upstream HFC 80211 wireless LAN
Wersquoll focus on Broadcast media
13
Multiple Access protocols
Ossi Mokryn - Data link layer
single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the
same timeMultiple ACcess (MAC) Protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
14
Human Analogy
Ossi Mokryn - Data link layer15
Rules for lsquoparty conversationrsquo Give everyone a chance to speak Dont speak until you are spoken to Dont monopolize the conversation Raise your hand if you have a question Dont interrupt when someone is speaking Dont fall asleep when someone else is talking
MAC Protocols measures Assume a shared medium with a channel rate of R
[bpsec] Efficient
When one node wants to transmit it ca send at rate R Fair
When N users want to transmit each can send at average rate RN
DecentralizedNo special node uses to coordinate transmission (no ldquoleaderrdquo)No synchronization of clocks or slotsFault tolerant
SimpleShould be very fast and implemented in NICs firmware
16
MAC Protocol TypesThree broad classes Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands or by code)
allocate piece to a node for exclusive its use Random Access
Channel not divided allow collisions ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMATDMA (Time Division Multiple Access) Access the channel in roundsldquo Each station gets fixed length slot (length = packets
trans time) in each round Each slot called a Time-Slot Unused slots go idle Example 6-station LAN 134 have packtes slots 256
idle
18
Channel Partitioning MAC protocols FDMAFDMA (Frequency Division Multiple Access) Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle example 6-station LAN 134 have packets frequency
bands 256 idle
frequ
ency
bands
time
19
TDM FDM summary FDM Enables the division of a channel with
capacity C bits per seconds to N sub-channels each gets a different frequency range and capacity of CN
TDM The division of channel to N sub-channels each gets CN capacity by giving the entire channel to each of the N stations for 1N of the time
The division makes each sub channel less busy but the overall waiting time is bigger by a factor of N compared to having one channel (Little theorem)
20
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA) used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
21
Random Access MAC Protocols From here on wersquoll focus on Random Access MAC
protocols When node has packet to send
Transmit at full channel data rate R No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies
How to detect collisions How to recover from collisions
Examples of random access MAC protocols ALOHA slotted ALOHA CSMACD (Ethernet) CSMACA (Wireless)
22
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Link Layer Services
Recall that were actually considering the MAC layer in the IEEE 802 model Framing (Frame structure)
encapsulate datagram into frame adding header trailer
Link Access (The protocol) Addressing
Introduces ldquoMACrdquo addresses used in frame headers to identify hosts (actually NICs) who are part of the network
Different for IP addresses Channel Access
Defines the set of rules which allows the hosts to use the (possibly shared) medium
10
Link Layer Services Flow Control
pacing between adjacent sending and receiving nodes Error Detection
errors caused by signal attenuation noise receiver detects presence of errors
signals sender for retransmission or drops frame Error Correction
receiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but
not at same time RDT
Offers some reliability between the hosts (Why is this redundant)
11
Link Layer Services
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI
card 80211 card sending side
encapsulates datagram in a frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to rcving node adapter is semi-
autonomous link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
12
Link TypesThree types of ldquolinksrdquo point-to-point (P2P)
PPP for dial-up access Point-to-point link
between Ethernet switch and host
Switched Networks ATM (used for WAN)
Broadcast Traditional Ethernet and
its predecessors Upstream HFC 80211 wireless LAN
Wersquoll focus on Broadcast media
13
Multiple Access protocols
Ossi Mokryn - Data link layer
single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the
same timeMultiple ACcess (MAC) Protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
14
Human Analogy
Ossi Mokryn - Data link layer15
Rules for lsquoparty conversationrsquo Give everyone a chance to speak Dont speak until you are spoken to Dont monopolize the conversation Raise your hand if you have a question Dont interrupt when someone is speaking Dont fall asleep when someone else is talking
MAC Protocols measures Assume a shared medium with a channel rate of R
[bpsec] Efficient
When one node wants to transmit it ca send at rate R Fair
When N users want to transmit each can send at average rate RN
DecentralizedNo special node uses to coordinate transmission (no ldquoleaderrdquo)No synchronization of clocks or slotsFault tolerant
SimpleShould be very fast and implemented in NICs firmware
16
MAC Protocol TypesThree broad classes Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands or by code)
allocate piece to a node for exclusive its use Random Access
Channel not divided allow collisions ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMATDMA (Time Division Multiple Access) Access the channel in roundsldquo Each station gets fixed length slot (length = packets
trans time) in each round Each slot called a Time-Slot Unused slots go idle Example 6-station LAN 134 have packtes slots 256
idle
18
Channel Partitioning MAC protocols FDMAFDMA (Frequency Division Multiple Access) Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle example 6-station LAN 134 have packets frequency
bands 256 idle
frequ
ency
bands
time
19
TDM FDM summary FDM Enables the division of a channel with
capacity C bits per seconds to N sub-channels each gets a different frequency range and capacity of CN
TDM The division of channel to N sub-channels each gets CN capacity by giving the entire channel to each of the N stations for 1N of the time
The division makes each sub channel less busy but the overall waiting time is bigger by a factor of N compared to having one channel (Little theorem)
20
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA) used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
21
Random Access MAC Protocols From here on wersquoll focus on Random Access MAC
protocols When node has packet to send
Transmit at full channel data rate R No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies
How to detect collisions How to recover from collisions
Examples of random access MAC protocols ALOHA slotted ALOHA CSMACD (Ethernet) CSMACA (Wireless)
22
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Link Layer Services Flow Control
pacing between adjacent sending and receiving nodes Error Detection
errors caused by signal attenuation noise receiver detects presence of errors
signals sender for retransmission or drops frame Error Correction
receiver identifies and corrects bit error(s) without resorting to retransmission
Half-duplex and full-duplex with half duplex nodes at both ends of link can transmit but
not at same time RDT
Offers some reliability between the hosts (Why is this redundant)
11
Link Layer Services
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI
card 80211 card sending side
encapsulates datagram in a frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to rcving node adapter is semi-
autonomous link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
12
Link TypesThree types of ldquolinksrdquo point-to-point (P2P)
PPP for dial-up access Point-to-point link
between Ethernet switch and host
Switched Networks ATM (used for WAN)
Broadcast Traditional Ethernet and
its predecessors Upstream HFC 80211 wireless LAN
Wersquoll focus on Broadcast media
13
Multiple Access protocols
Ossi Mokryn - Data link layer
single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the
same timeMultiple ACcess (MAC) Protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
14
Human Analogy
Ossi Mokryn - Data link layer15
Rules for lsquoparty conversationrsquo Give everyone a chance to speak Dont speak until you are spoken to Dont monopolize the conversation Raise your hand if you have a question Dont interrupt when someone is speaking Dont fall asleep when someone else is talking
MAC Protocols measures Assume a shared medium with a channel rate of R
[bpsec] Efficient
When one node wants to transmit it ca send at rate R Fair
When N users want to transmit each can send at average rate RN
DecentralizedNo special node uses to coordinate transmission (no ldquoleaderrdquo)No synchronization of clocks or slotsFault tolerant
SimpleShould be very fast and implemented in NICs firmware
16
MAC Protocol TypesThree broad classes Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands or by code)
allocate piece to a node for exclusive its use Random Access
Channel not divided allow collisions ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMATDMA (Time Division Multiple Access) Access the channel in roundsldquo Each station gets fixed length slot (length = packets
trans time) in each round Each slot called a Time-Slot Unused slots go idle Example 6-station LAN 134 have packtes slots 256
idle
18
Channel Partitioning MAC protocols FDMAFDMA (Frequency Division Multiple Access) Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle example 6-station LAN 134 have packets frequency
bands 256 idle
frequ
ency
bands
time
19
TDM FDM summary FDM Enables the division of a channel with
capacity C bits per seconds to N sub-channels each gets a different frequency range and capacity of CN
TDM The division of channel to N sub-channels each gets CN capacity by giving the entire channel to each of the N stations for 1N of the time
The division makes each sub channel less busy but the overall waiting time is bigger by a factor of N compared to having one channel (Little theorem)
20
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA) used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
21
Random Access MAC Protocols From here on wersquoll focus on Random Access MAC
protocols When node has packet to send
Transmit at full channel data rate R No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies
How to detect collisions How to recover from collisions
Examples of random access MAC protocols ALOHA slotted ALOHA CSMACD (Ethernet) CSMACA (Wireless)
22
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Link Layer Services
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI
card 80211 card sending side
encapsulates datagram in a frame
adds error checking bits rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to rcving node adapter is semi-
autonomous link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
12
Link TypesThree types of ldquolinksrdquo point-to-point (P2P)
PPP for dial-up access Point-to-point link
between Ethernet switch and host
Switched Networks ATM (used for WAN)
Broadcast Traditional Ethernet and
its predecessors Upstream HFC 80211 wireless LAN
Wersquoll focus on Broadcast media
13
Multiple Access protocols
Ossi Mokryn - Data link layer
single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the
same timeMultiple ACcess (MAC) Protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
14
Human Analogy
Ossi Mokryn - Data link layer15
Rules for lsquoparty conversationrsquo Give everyone a chance to speak Dont speak until you are spoken to Dont monopolize the conversation Raise your hand if you have a question Dont interrupt when someone is speaking Dont fall asleep when someone else is talking
MAC Protocols measures Assume a shared medium with a channel rate of R
[bpsec] Efficient
When one node wants to transmit it ca send at rate R Fair
When N users want to transmit each can send at average rate RN
DecentralizedNo special node uses to coordinate transmission (no ldquoleaderrdquo)No synchronization of clocks or slotsFault tolerant
SimpleShould be very fast and implemented in NICs firmware
16
MAC Protocol TypesThree broad classes Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands or by code)
allocate piece to a node for exclusive its use Random Access
Channel not divided allow collisions ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMATDMA (Time Division Multiple Access) Access the channel in roundsldquo Each station gets fixed length slot (length = packets
trans time) in each round Each slot called a Time-Slot Unused slots go idle Example 6-station LAN 134 have packtes slots 256
idle
18
Channel Partitioning MAC protocols FDMAFDMA (Frequency Division Multiple Access) Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle example 6-station LAN 134 have packets frequency
bands 256 idle
frequ
ency
bands
time
19
TDM FDM summary FDM Enables the division of a channel with
capacity C bits per seconds to N sub-channels each gets a different frequency range and capacity of CN
TDM The division of channel to N sub-channels each gets CN capacity by giving the entire channel to each of the N stations for 1N of the time
The division makes each sub channel less busy but the overall waiting time is bigger by a factor of N compared to having one channel (Little theorem)
20
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA) used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
21
Random Access MAC Protocols From here on wersquoll focus on Random Access MAC
protocols When node has packet to send
Transmit at full channel data rate R No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies
How to detect collisions How to recover from collisions
Examples of random access MAC protocols ALOHA slotted ALOHA CSMACD (Ethernet) CSMACA (Wireless)
22
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Link TypesThree types of ldquolinksrdquo point-to-point (P2P)
PPP for dial-up access Point-to-point link
between Ethernet switch and host
Switched Networks ATM (used for WAN)
Broadcast Traditional Ethernet and
its predecessors Upstream HFC 80211 wireless LAN
Wersquoll focus on Broadcast media
13
Multiple Access protocols
Ossi Mokryn - Data link layer
single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the
same timeMultiple ACcess (MAC) Protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
14
Human Analogy
Ossi Mokryn - Data link layer15
Rules for lsquoparty conversationrsquo Give everyone a chance to speak Dont speak until you are spoken to Dont monopolize the conversation Raise your hand if you have a question Dont interrupt when someone is speaking Dont fall asleep when someone else is talking
MAC Protocols measures Assume a shared medium with a channel rate of R
[bpsec] Efficient
When one node wants to transmit it ca send at rate R Fair
When N users want to transmit each can send at average rate RN
DecentralizedNo special node uses to coordinate transmission (no ldquoleaderrdquo)No synchronization of clocks or slotsFault tolerant
SimpleShould be very fast and implemented in NICs firmware
16
MAC Protocol TypesThree broad classes Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands or by code)
allocate piece to a node for exclusive its use Random Access
Channel not divided allow collisions ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMATDMA (Time Division Multiple Access) Access the channel in roundsldquo Each station gets fixed length slot (length = packets
trans time) in each round Each slot called a Time-Slot Unused slots go idle Example 6-station LAN 134 have packtes slots 256
idle
18
Channel Partitioning MAC protocols FDMAFDMA (Frequency Division Multiple Access) Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle example 6-station LAN 134 have packets frequency
bands 256 idle
frequ
ency
bands
time
19
TDM FDM summary FDM Enables the division of a channel with
capacity C bits per seconds to N sub-channels each gets a different frequency range and capacity of CN
TDM The division of channel to N sub-channels each gets CN capacity by giving the entire channel to each of the N stations for 1N of the time
The division makes each sub channel less busy but the overall waiting time is bigger by a factor of N compared to having one channel (Little theorem)
20
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA) used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
21
Random Access MAC Protocols From here on wersquoll focus on Random Access MAC
protocols When node has packet to send
Transmit at full channel data rate R No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies
How to detect collisions How to recover from collisions
Examples of random access MAC protocols ALOHA slotted ALOHA CSMACD (Ethernet) CSMACA (Wireless)
22
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Multiple Access protocols
Ossi Mokryn - Data link layer
single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the
same timeMultiple ACcess (MAC) Protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
14
Human Analogy
Ossi Mokryn - Data link layer15
Rules for lsquoparty conversationrsquo Give everyone a chance to speak Dont speak until you are spoken to Dont monopolize the conversation Raise your hand if you have a question Dont interrupt when someone is speaking Dont fall asleep when someone else is talking
MAC Protocols measures Assume a shared medium with a channel rate of R
[bpsec] Efficient
When one node wants to transmit it ca send at rate R Fair
When N users want to transmit each can send at average rate RN
DecentralizedNo special node uses to coordinate transmission (no ldquoleaderrdquo)No synchronization of clocks or slotsFault tolerant
SimpleShould be very fast and implemented in NICs firmware
16
MAC Protocol TypesThree broad classes Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands or by code)
allocate piece to a node for exclusive its use Random Access
Channel not divided allow collisions ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMATDMA (Time Division Multiple Access) Access the channel in roundsldquo Each station gets fixed length slot (length = packets
trans time) in each round Each slot called a Time-Slot Unused slots go idle Example 6-station LAN 134 have packtes slots 256
idle
18
Channel Partitioning MAC protocols FDMAFDMA (Frequency Division Multiple Access) Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle example 6-station LAN 134 have packets frequency
bands 256 idle
frequ
ency
bands
time
19
TDM FDM summary FDM Enables the division of a channel with
capacity C bits per seconds to N sub-channels each gets a different frequency range and capacity of CN
TDM The division of channel to N sub-channels each gets CN capacity by giving the entire channel to each of the N stations for 1N of the time
The division makes each sub channel less busy but the overall waiting time is bigger by a factor of N compared to having one channel (Little theorem)
20
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA) used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
21
Random Access MAC Protocols From here on wersquoll focus on Random Access MAC
protocols When node has packet to send
Transmit at full channel data rate R No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies
How to detect collisions How to recover from collisions
Examples of random access MAC protocols ALOHA slotted ALOHA CSMACD (Ethernet) CSMACA (Wireless)
22
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Human Analogy
Ossi Mokryn - Data link layer15
Rules for lsquoparty conversationrsquo Give everyone a chance to speak Dont speak until you are spoken to Dont monopolize the conversation Raise your hand if you have a question Dont interrupt when someone is speaking Dont fall asleep when someone else is talking
MAC Protocols measures Assume a shared medium with a channel rate of R
[bpsec] Efficient
When one node wants to transmit it ca send at rate R Fair
When N users want to transmit each can send at average rate RN
DecentralizedNo special node uses to coordinate transmission (no ldquoleaderrdquo)No synchronization of clocks or slotsFault tolerant
SimpleShould be very fast and implemented in NICs firmware
16
MAC Protocol TypesThree broad classes Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands or by code)
allocate piece to a node for exclusive its use Random Access
Channel not divided allow collisions ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMATDMA (Time Division Multiple Access) Access the channel in roundsldquo Each station gets fixed length slot (length = packets
trans time) in each round Each slot called a Time-Slot Unused slots go idle Example 6-station LAN 134 have packtes slots 256
idle
18
Channel Partitioning MAC protocols FDMAFDMA (Frequency Division Multiple Access) Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle example 6-station LAN 134 have packets frequency
bands 256 idle
frequ
ency
bands
time
19
TDM FDM summary FDM Enables the division of a channel with
capacity C bits per seconds to N sub-channels each gets a different frequency range and capacity of CN
TDM The division of channel to N sub-channels each gets CN capacity by giving the entire channel to each of the N stations for 1N of the time
The division makes each sub channel less busy but the overall waiting time is bigger by a factor of N compared to having one channel (Little theorem)
20
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA) used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
21
Random Access MAC Protocols From here on wersquoll focus on Random Access MAC
protocols When node has packet to send
Transmit at full channel data rate R No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies
How to detect collisions How to recover from collisions
Examples of random access MAC protocols ALOHA slotted ALOHA CSMACD (Ethernet) CSMACA (Wireless)
22
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
MAC Protocols measures Assume a shared medium with a channel rate of R
[bpsec] Efficient
When one node wants to transmit it ca send at rate R Fair
When N users want to transmit each can send at average rate RN
DecentralizedNo special node uses to coordinate transmission (no ldquoleaderrdquo)No synchronization of clocks or slotsFault tolerant
SimpleShould be very fast and implemented in NICs firmware
16
MAC Protocol TypesThree broad classes Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands or by code)
allocate piece to a node for exclusive its use Random Access
Channel not divided allow collisions ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMATDMA (Time Division Multiple Access) Access the channel in roundsldquo Each station gets fixed length slot (length = packets
trans time) in each round Each slot called a Time-Slot Unused slots go idle Example 6-station LAN 134 have packtes slots 256
idle
18
Channel Partitioning MAC protocols FDMAFDMA (Frequency Division Multiple Access) Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle example 6-station LAN 134 have packets frequency
bands 256 idle
frequ
ency
bands
time
19
TDM FDM summary FDM Enables the division of a channel with
capacity C bits per seconds to N sub-channels each gets a different frequency range and capacity of CN
TDM The division of channel to N sub-channels each gets CN capacity by giving the entire channel to each of the N stations for 1N of the time
The division makes each sub channel less busy but the overall waiting time is bigger by a factor of N compared to having one channel (Little theorem)
20
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA) used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
21
Random Access MAC Protocols From here on wersquoll focus on Random Access MAC
protocols When node has packet to send
Transmit at full channel data rate R No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies
How to detect collisions How to recover from collisions
Examples of random access MAC protocols ALOHA slotted ALOHA CSMACD (Ethernet) CSMACA (Wireless)
22
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
MAC Protocol TypesThree broad classes Channel Partitioning
Divide channel into smaller ldquopiecesrdquo (Time-Slots Frequancy-Bands or by code)
allocate piece to a node for exclusive its use Random Access
Channel not divided allow collisions ldquoRecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns Might uses a leader to coordinate the turns
17
Channel Partitioning MAC protocols TDMATDMA (Time Division Multiple Access) Access the channel in roundsldquo Each station gets fixed length slot (length = packets
trans time) in each round Each slot called a Time-Slot Unused slots go idle Example 6-station LAN 134 have packtes slots 256
idle
18
Channel Partitioning MAC protocols FDMAFDMA (Frequency Division Multiple Access) Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle example 6-station LAN 134 have packets frequency
bands 256 idle
frequ
ency
bands
time
19
TDM FDM summary FDM Enables the division of a channel with
capacity C bits per seconds to N sub-channels each gets a different frequency range and capacity of CN
TDM The division of channel to N sub-channels each gets CN capacity by giving the entire channel to each of the N stations for 1N of the time
The division makes each sub channel less busy but the overall waiting time is bigger by a factor of N compared to having one channel (Little theorem)
20
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA) used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
21
Random Access MAC Protocols From here on wersquoll focus on Random Access MAC
protocols When node has packet to send
Transmit at full channel data rate R No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies
How to detect collisions How to recover from collisions
Examples of random access MAC protocols ALOHA slotted ALOHA CSMACD (Ethernet) CSMACA (Wireless)
22
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Channel Partitioning MAC protocols TDMATDMA (Time Division Multiple Access) Access the channel in roundsldquo Each station gets fixed length slot (length = packets
trans time) in each round Each slot called a Time-Slot Unused slots go idle Example 6-station LAN 134 have packtes slots 256
idle
18
Channel Partitioning MAC protocols FDMAFDMA (Frequency Division Multiple Access) Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle example 6-station LAN 134 have packets frequency
bands 256 idle
frequ
ency
bands
time
19
TDM FDM summary FDM Enables the division of a channel with
capacity C bits per seconds to N sub-channels each gets a different frequency range and capacity of CN
TDM The division of channel to N sub-channels each gets CN capacity by giving the entire channel to each of the N stations for 1N of the time
The division makes each sub channel less busy but the overall waiting time is bigger by a factor of N compared to having one channel (Little theorem)
20
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA) used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
21
Random Access MAC Protocols From here on wersquoll focus on Random Access MAC
protocols When node has packet to send
Transmit at full channel data rate R No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies
How to detect collisions How to recover from collisions
Examples of random access MAC protocols ALOHA slotted ALOHA CSMACD (Ethernet) CSMACA (Wireless)
22
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Channel Partitioning MAC protocols FDMAFDMA (Frequency Division Multiple Access) Channel spectrum divided into frequency bands Each station assigned fixed frequency band Unused transmission time in frequency bands go idle example 6-station LAN 134 have packets frequency
bands 256 idle
frequ
ency
bands
time
19
TDM FDM summary FDM Enables the division of a channel with
capacity C bits per seconds to N sub-channels each gets a different frequency range and capacity of CN
TDM The division of channel to N sub-channels each gets CN capacity by giving the entire channel to each of the N stations for 1N of the time
The division makes each sub channel less busy but the overall waiting time is bigger by a factor of N compared to having one channel (Little theorem)
20
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA) used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
21
Random Access MAC Protocols From here on wersquoll focus on Random Access MAC
protocols When node has packet to send
Transmit at full channel data rate R No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies
How to detect collisions How to recover from collisions
Examples of random access MAC protocols ALOHA slotted ALOHA CSMACD (Ethernet) CSMACA (Wireless)
22
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
TDM FDM summary FDM Enables the division of a channel with
capacity C bits per seconds to N sub-channels each gets a different frequency range and capacity of CN
TDM The division of channel to N sub-channels each gets CN capacity by giving the entire channel to each of the N stations for 1N of the time
The division makes each sub channel less busy but the overall waiting time is bigger by a factor of N compared to having one channel (Little theorem)
20
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA) used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
21
Random Access MAC Protocols From here on wersquoll focus on Random Access MAC
protocols When node has packet to send
Transmit at full channel data rate R No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies
How to detect collisions How to recover from collisions
Examples of random access MAC protocols ALOHA slotted ALOHA CSMACD (Ethernet) CSMACA (Wireless)
22
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Channel Partitioning MAC protocols CDMA
Code Division Multiple Access (CDMA) used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence
allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
21
Random Access MAC Protocols From here on wersquoll focus on Random Access MAC
protocols When node has packet to send
Transmit at full channel data rate R No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies
How to detect collisions How to recover from collisions
Examples of random access MAC protocols ALOHA slotted ALOHA CSMACD (Ethernet) CSMACA (Wireless)
22
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Random Access MAC Protocols From here on wersquoll focus on Random Access MAC
protocols When node has packet to send
Transmit at full channel data rate R No a priori coordination among nodes
Two or more transmitting nodes ldquocollisionrdquo Random access MAC protocol specifies
How to detect collisions How to recover from collisions
Examples of random access MAC protocols ALOHA slotted ALOHA CSMACD (Ethernet) CSMACA (Wireless)
22
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Aloha Protocol Invented at the rsquo70s in Hawaii Intended for Radio networks but suitable for
every network where the station can listen to the channel while broadcasting and determine whether others also transmit
Basic idea every station may transmit when it wants to If collision is detected between frames back off and try again later If two frames are broadcast at the same time on
the channel a collision occurs and the both need to retransmit
Ossi Mokryn - Data link layer23
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Aloha Protocol All hosts
Transmit on one frequency (fT) Receive on other frequency (fR)
There is a central node which repeats whatever it receives from fT frequency on the other fR frequency
The central node used as a repeater Collisions are detected by the hosts
Receiving corrupted data (host knows what should be received)
Ossi Mokryn - Data link layer24
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Aloha Protocol
1 Accept a new frame arrives2 Transmit immediately and listen
If a collision occurred wait a random time and repeat to stage 2 Otherwise go back to stage 1 to handle a new frame
Ossi Mokryn - Data link layer25
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Aloha Protocol Simple Robust against failure of a host Distributed (excluding the central node which
uses as a repeater)
High load implicates low utilization of the channel and high delays
26
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Aloha - efficiency
Only 18
Suppose there are n stations and the probability that a station starts transmitting in a time unit is p
Then The probability that exactly one node transmits in a time unit is
27
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Aloha - efficiency
Only 18
Maximize the utilization function by differentiation yields maximum point at with utilization of
Trivial improvement Why be vulnerable for 2 time unitsSynchronize and use slotted time May transmit only at integer times
28
Efficiency is the long-run fraction of successful transmissions when there are many nodes each with many frames to send
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Slotted Aloha
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
29
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Slotted Aloha
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization30
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Slotted Aloha - efficiency
Suppose N nodes with many frames to send each transmits in slot with probability p
prob that node 1 has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
For max efficiency with N nodes find p that maximizes Np(1-p)N-1
For many nodes take limit of Np(1-p)N-1 as N goes to infinity gives 1e = 37
Efficiency is the long-run fraction of successful slots when there are many nodes each with many frames to send
At best channelused for useful transmissions 37of time
31
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Aloha - Summary Very popular at the beginning of time (ie
70s to 80s) Very simple to handle Lots and lots of basic probabilities calculations
for students Major problem Nodes donrsquot check whatrsquos
going on in the channel each acting on its own No manners
32
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit Protocol
1 Listen to the channel2 If channel sensed idle transmit entire frame3 If channel sensed busy defer transmission by
1-Persistent CSMAWait until channel is quiet and transmit immediately If collision occurs wait a random time and listen again (go to 1)
Non-Persistent CSMAWait a random time and listen again (go to 1)
They differ only by the treatment of 1st transmission CSMA human analogy donrsquot interrupt others33
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
CSMACD (Collision Detection)
CSMACD carrier sensing deferral as in CSMA When transmitting try to sense if there is a
collision collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
34
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
CSMACD Minimum Packet Size
35
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
36
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Unreliable connectionless service Connectionless No handshaking between sending
and receiving adapter Unreliable receiving adapter doesnrsquot send acks or
nacks to sending adapter stream of datagrams passed to network layer can have
gaps gaps will be filled if app is using TCP otherwise app will see the gaps
37
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
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 Goal adapt retransmission
attempts to estimated current load heavy load random wait will
be longer first collision choose K from
01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
38
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
8023 CSMACD (Ethernet) Algorithm
39
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Ethernet CSMACD algorithm
1 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
40
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Ethernet Minimum Packet Size
41
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Summary of MAC protocols What do you do with a shared media
Channel Partitioning by time frequency or code Time Division Frequency Division Code
Division Random partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies
(wire) hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211 (Wireless)
42
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
LAN technologies
Data link layer so far MAC protocols The random protocol approach
Next LAN technologies Addressing Ethernet Hubs bridges and switches
43
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM but can be modified
44
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
45
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) Analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another IP hierarchical address NOT portable
depends on IP subnet to which node is attached
46
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt TTL (Time To Live) time
after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
47
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
48
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110
In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
49
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Ossi Mokryn - Data link layer
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
50
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Ethernet
ldquodominantrdquo wired LAN technology developed at the 70s
cheap $20 for 100Mbs first widely used LAN technology Simple cheap Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
51
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Ethernet topology Through the Years
Classic Ethernet Now star topology Now star topology prevailsprevails
Connection choices hub or switch (more later)
Fast Ethernet 100 Mbs Gigabit Ethernet 1Gbps
52
hub orswitch
bull Shared Bus with CSMACDbull Bus maximal length 500 mbullTransmission rate 10Mbs
Through the years the only common is The Frame
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates what is the length in clock ticks of one bit
53
Type length length or type of frame
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Ethernet Frame Structure (more)
Ossi Mokryn - Data link layer
Addresses 6 bytes if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame MAC addresses also called Physical addresses
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped Before CRC there is a padding field for the CRC to pad to 64 bytes
54
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
55
Ethernet Technology 10Base2 10 10Mbps 2 under 200 meters max cable length thin coaxial cable in a bus topology
repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces physical layer device only
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Ethernet technology 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
56
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Ethernet technology 100BaseT Problem must keep minimal packet size when bandwidth increases With fixed cable length and propagation speed must increase minimal size proportionally to bandwidth increase Eg 100Mbs 1500m of cable prop remains 6μs minimal
size becomes 1200 bits Solutions
Cable length limited to 100m Prevent collisions by ldquoEthernet Switchesrdquo (later)
Max distance from node to Hub is 100 meters
57
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
58
Gbit Ethernet use standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short distances
between nodes to be efficiency uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbpsec now
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
HubsQ Why not just one big LAN Limited amount of supportable traffic on
single LAN all stations must share bandwidth
limited length 8023 (Ethernet) specifies maximum cable length
large ldquocollision domainrdquo (can collide with many stations)
limited number of stations 8025 (token ring) have token passing delays at each station
Ossi Mokryn - Data link layer59
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 360
Hubs (Multiport repeaters Bus in a box) Physical Layer devices essentially repeaters operating
at bit levels repeat received bits on one interface to all other interfaces
Canrsquot interconnect 10BaseT amp 100BaseT (because segments donrsquot share the same rate)
Hubs can be arranged in a hierarchy (or multi-tier design) with backbone hub at its top
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 361
Hubs (Multiport repeaters Bus in a box)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains node may collide
with any node residing at any segment in LAN Extends max distance between nodes but all the
segments become one large collision domain
Hub Advantages simple inexpensive device Multi-tier provides graceful degradation portions of the
LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m
per Hub)
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 362
Bridges Link Layer devices operate on Ethernet frames
examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment bridge uses
CSMACD to access segment and transmit Store and forward element So different types of
Ethernet types can be connected Transparent no need for any change to hosts LAN
adapters Forwarding is selective do not always flood All
connected segments can work independently in parallel
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 363
Bridge Filtering
bridges learn which hosts can be reached through which interfaces maintain filtering tables when frame received bridge ldquolearnsrdquo location of
sender incoming LAN segment records sender location in filtering table
filtering table entry (Node LAN Address Bridge Interface Time Stamp) stale entries in Filtering Table dropped (TTL can be
60 minutes)
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 364
Bridge Operation
bridge procedure(in_MAC in_portout_MAC)lookup in filtering table (out_MAC) receive out_portif (out_port not valid) no entry found for destination
then flood forward on all but the interface on which the frame arrived
if (in_port = out_port) destination is on LAN on which frame was received
then drop the frame
Otherwise (out_port is valid) entry found for destination then forward the frame on interface indicated
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 365
Bridge Learning exampleSuppose C sends frame to D and D replies back
with frame to C
C sends frame bridge has no info about D so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 366
Bridge Learning example
D generates reply to C sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1 so selectively
forwards frame out via interface 1
C 1
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 367
What will happen with loopsIncorrect learning
A
B
1 1
22
A 1 A 122
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 368
What will happen with loopsFrame looping
A
C
1 1
22
C C
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 369
What will happen with loopsFrame looping
A
B
1 1
22
B2 B1
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 370
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 8021D
Note redundant paths are good active redundant paths are bad (they cause loops)
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
How to Construct a Spanning Tree Bridges run a distributed spanning tree Algorithm Select what ports (and bridges) should actively
forward frames Finding the root flooding Building a tree Bellman-Ford Algorithm Can combine efficiently Standardized in IEEE 8021 specification
71
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 372
Spanning Tree Requirements Each bridge is assigned a unique identifier A broadcast address for bridges on a LAN A unique port identifier for all ports on all
bridges MAC address Bridge id + port number
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 373
Spanning Tree ConceptsRoot Bridge The bridge with the lowest bridge ID value is
elected the root bridge One root bridge chosen among all bridges Every other bridge calculates a path to the
root bridge
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 374
Spanning Tree ConceptsPath Cost A cost associated with each port on each
bridge default is 1
The cost associated with transmission onto the LAN connected to the port
Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 375
Spanning Tree ConceptsRoot Port The port on each bridge that is on the path
towards the root bridge The root port is part of the lowest cost path
towards the root bridge If port costs are equal on a bridge the port
with the lowest ID becomes root port
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 376
Spanning Tree ConceptsRoot Path Cost The minimum cost path to the root bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 377
Spanning Tree Concepts Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost path
to the root bridge for the LAN Only the designated bridge passes frames
towards the root bridge
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 378
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation1 Picks a root2 For each LAN
picks a designated bridgethat is closest to the root
3 All bridges on a LANsend packets towards the root via the designated bridge
B8
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 379
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree
Designated Bridge
root port
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 380
Spanning Tree AlgorithmAn Overview 1 Determine the root bridge among all bridges 2 Each bridge determines its root port
The port in the direction of the root bridge 3 Determine the designated bridge on each LAN
The bridge which accepts frames to forward towards the root bridge
The frames are sent on the root port of the designated bridge
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 381
Spanning Tree AlgorithmSelecting Root Bridge Initially each bridge considers itself to be the
root bridge Bridges send BDPU frames to its attached
LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root IDcostpriority)
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 382
Spanning Tree AlgorithmSelecting Root Ports Each bridge selects one of its ports which has
the minimal cost to the root bridge In case of a tie the lowest uplink (transmitter)
bridge ID is used In case of another tie the lowest port ID is
used
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 383
Spanning Tree AlgorithmSelect Designated Bridges
Initially each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3 Best one wins (lowest IDcostpriority)
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 384
ForwardingBlocking State Root and designated bridges will forward
frames to and from their attached LANs All other ports are in the blocking state
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 385
Ethernet Switches
layer 2 (frame) forwarding filtering using LAN addresses
Switching A-to-B and Arsquo-to-Brsquo simultaneously no collisions
large number of interfaces often individual hosts star-
connected into switch Ethernet but no collisions
Confused with Ethernet bridgeshellip
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 386
Ethernet Switches cut-through switching frame forwarded from
input to output port without awaiting for assembly of entire frame slight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
Offers VLANS (Virtual LANs) Nowadays routers are actually combined
with Ethernet switches
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Lecture 387
Ethernet Switches (more)
Dedicated
Shared
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Summary comparison
hubs routers Bridges
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5-88
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
Road-Map and Keywords IEEE 802 Model compared to the OSI
LLC MAC Physical Media
Coax Twisted Pairs Fibers Link Types
Point-to-point Broadcast Switched Different MAC protocol approaches
Channel Partitioning Random Access ldquoTaking Turnsrdquo Portioning MAC protocols
TDMA FDMA CDMA Random Access MAC protocols
Aloha Slotted Aloha LAN technology ndash Ethernet Protocol
MAC Addresses Frame Structure ARP LAN interconnect
Hubs Bridges and Ethernet Switches
89
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
IEEE 802 Model Compared to the OSI The Data-Link and Physical layers in the OSI model are
divided to other layers according to the IEEE 802 model
90
Higher Layers
Data-Link Layer
Physical Layer
IEEE 8021Higher Levels Interface
IEEE 8022Logical Link Control (LLC)
IEEE 8023CSMACDMedium AccessControl
IEEE 80211WirelessMedium AccessControl
IEEE 8025Token Ring
Medium AccessControl
CSMACDMedium
WirelessMedium
Token RingMedium
OSI IEEE 802
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91
IEEE 802 Model Compared to the OSI The LLC provides common interface for common LAN
functionality There are various media which offer different methods for
communication (OSI so called Physical layer) Each LAN technology uses different MAC (Medium Access
Control) method to use its corresponding medias
What kind of medias do we have What kind of corresponding MAC protocols do we have
91