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Welcome
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DATA COMMUNICATIONAND
COMPUTER
NETWORKS
INTRODUCTION
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Data comn are the exchange of data b/w twodevices via some form of txn medium such aswire cable.
These comn devices must be part of a comn sys(combination of HW & SW)
Effectiveness of data comn depends upon Delivery
Accuracy
Timeliness
Jitter
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Source
Generates data to be transmitted
Transmitter
Converts data into transmittable signals Transmission System
Carries data
Receiver
Converts received signal into data Destination
Takes incoming data
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Message
Sender
Receiver Txn Media
Protocol
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Simplex
Half - Duplex
Full - Duplex
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Simplex
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Half-Duplex
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Full-Duplex
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data flows move in one direction only, (radio or
cable television broadcasts)
data flows both ways, but only one direction at
a time (e.g., CB radio) (requires control info)
data flows in both directions at
the same time
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NETWORKS
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A network is a set of devices(referred as
nodes) connected by comn ch.
Node can be any device capable of sending/
receiving data generated by other nodes on
the network.
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Computer Network
Computer Network is a collection of autonomous
computers interconnected by a single technology. eg
Ethernet LAN
Is Interneta computer network?
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Distributed System is a collection of
independent computers that appear to its
users as a single coherent system
Existence of multiple autonomous computers is
transparent
It is like a virtual uniprocessor
Implemented in SW and build over computernetworks
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Performance
Throughput (High)
Delay (Low)
Reliability
Security
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Computers connected over a network canmake that information exchange easier andfaster.
The information moves directly fromcomputer to computer rather than through ahuman intermediary.
People can concentrate on getting their workdone rather than on moving informationaround the company.
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Resource Sharing
Robustness
Load Balancing Location Independence
Productivity
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Based on transmission technology
Broadcast networks
Pt-to-Pt networks
Based on scale Personal Area Networks (PAN)
Local Area Networks (LAN)
Metropolitan Area Networks (MAN)
Wide Area Networks (WAN)
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Pt to Pt
Broadcast
Topology Based Cct SW Vs Message SW Vs Packet Sw
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Broadcast Networks
Single comn channel shared by all cmptrs
Packets send by one cmptr received by all others.
Address in packet- specifies for whom intended.
Packet can also be addressed to all cmptrs (broadcast)
or a gp of cmptrs(Multicasting)
Smaller networks broadcast
Eg Ethernet(802.3),IBM Token Ring(802.5)
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Single comn channel shared by all cmptrs on
NW.
Packets sent by one cmptr received by all others. Address fd in packet- specifies for whom
intended.
Packet can also be addressed to all cmptrs(broadcast).
Some networks also support multicasting
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Point To Point Networks
Many connections between indl pairs of cmptrs.
Packets visit one or more intermediate machines.
Multiple routes .
Routing algorithm To determine the best route
Also called as unicasting
Larger networks pt to pt
Eg. WAN
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Many connections between indl pairs ofcmptrs.
Packets visit one or more intermediate
machines. Multiple routes.
Routing algorithm.
Smaller networks broadcast larger networks pt to pt
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LAN
WAN
MAN
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A local area network (LAN) is a number ofcomputers connected to each other bycable in a single location by a common
medium i.e switch, hub and etc, usually asingle floor of a building or all thecomputers in a small company within thegeographical area.
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Local Area Networks(LAN)
Within same building/ campus - upto a few kms.
Speed 4/16/10/100/1000 Mbps.
Normally broadcast type
Topology (bus,ring, star) Restricted in size- worst case txn time is bounded and
known in advance.eg 10 Mbps Ethernet LAN(10
BaseT) has max dia of 500m Simple network management
Eg- Ethernet(802.3),IBM Token Ring(802.5),Wi-
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Operates within the limited geographical area.
Allow access through high bandwidth up to 1000
mbps.
Controls the network under local administration
Provides the full time connectivity to local system.
Connects physical adjacent devices.
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Metropolitan Area Networks(MAN)
Bigger version of LAN.
Uses similar technology as LAN.
Can cover a city.
Eg- Wi-Max(802.16,Wireless MAN)
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While local area networks are perfect for sharingresources within a building or campus, they cannot beused to connect distant sites.
Wide area networks (WANs) fill this need. Stated
simply, wide area networks are the set of connectinglinks between local area networks.
These links are made over telephone lines leasedfrom the various telephone companies. In rareinstances, WANs can be created with satellite links,
packet radio, or microwave transceivers. These options are generally far more expensive than
leased telephone lines, but they can operate in areaswhere leased lines are not available.
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A wide area network links computers in differentlocations.
M C E M ESecunderabad
ARMY HQ
New Delhi
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Operates over a large geographical area.
Allow access over serial interface
Works at a local speed 2mbps is maximum speed
in internet.
Connects devices separates wide even global area.
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Wide Area Networks(WAN)
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Wide Area Networks(WAN)
Spans larger geographical area.
Collection of cmptrs (hosts) connected by a comn
subnet
Subnet consists of :-
Transmission lines.
Switching elements (or routers).
Packet sent from router to router. (Store and fwd /
packet switching)
Gen Pt to Pt Topologies
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Star
Mesh
Ring
Bus
Hierarchical
To reduce complexity of NW
Better comd & contrl Better mgmt
Tiered Architecture
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Point-to-Point Line Configuration
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Point-to-Point Line Configuration
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Point-to-Point Line Configuration
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Multipoint Line Configuration
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Mesh Topology
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Star Topology
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Tree Topology
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Bus Topology
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Ring Topology
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Hybrid Topology
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Baseband
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Broadband
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Bandwidth is the capacity of a medium to conveydata.
One example of bandwidth is automobile traffic. Atwo-lane road with a speed limit can accommodate
only so many cars before there are too many and atraffic jam results.
You can increase the bandwidth of a road by
making the cars travel more quickly (which corresponds
to using a faster transmission method in networks)or
by making the road wider (which corresponds to usingmore wires in networks).
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Means sending a digital signal over channel w/ochanging digital signal to an analog signal.
The cable connecting the computer can carry onesignal at a time, and all the system take turn using it.
This type of network is called Base band network. In the base band network, when a computer transmits
data it might be broken into many packet andtransmits separately.
The receiving system reassembles them back intooriginal. This is called packet switching network.
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The alternative to a packet switching network is cct
switching. In CSN two system established a cct before
communication and broken cct only after the finish the
communication between them.
HELLO HOW ARE YOU
YOUAREHOWHELLO
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VOIP FTP Telnet
Co-ax Wireless OFC
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Single comn channel shared by all cmptrs onNW.
Packets sent by one cmptr received by allothers.
Address fd in packet- specifies for whomintended.
Packet can also be addressed to all cmptrs
(broadcast). Some networks also support multicasting
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Computer NWs are often described using
layered architecture
Layered architecture specifies functionality at
each layer (modularity)
Higher layer protocols can operate without
knowing details of the lower layers .
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Fundamental idea is to provide a service but
keep details of implementation i.e. internal
state and algorithms, at each level hidden.
Info hiding.
Abstraction
Data encapsulation.
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Layering hides information
If it did not (layering violation) then changes to
one layer could require changes everywhere
Sometimes hiding information can degrade
performance
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Example. flow control protocol at an upper layer may think
packet loss is always because of network congestion
But if it is due to a lossy link then performancedegrades.
So hiding information about reason of packet loss
from flow control protocol leads to degraded
performance
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Conflict between information-hiding andachieving good performance
Leak enough information to allow good
performance but not so much that smallchanges in one layer need changes to other
layers
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Aprotocolis a set of rules and formats thatgovern the communication between peers
set of valid messages
meaning of each message
A protocol is necessary for any function that
requires cooperation between peers
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Problem: Exchange a file over a network that
may corrupt packets but doesnt lose or reorderthem
A simple protocol
send file as a series of packets send a checksum
receiver sends OK or not-OK message
sender waits for OK message if no response, resends entire file
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Switching
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Packet
Switch
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A network is a set of connected devices.
Prob how to connect them for one to one
comn
Pt to Pt
Different topology
Switching
SW NW consists of a series of interlinked nodes,
called switches.
SW are capable of creating temp conn b/w two
or more devices linked to SW.
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The end devices are called stations. The switching devices are called nodes.
Key features of a switched communicationnetwork Network Topology is not regular. Uses FDM or TDM for node-to-node communication.
There exist multiple paths between a source-destination pair for better network reliability.
The switching nodes are not concerned with thecontents of data.
Their purpose is to provide a switching facility thatwill move data from node to node until they reachthe destination.
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SwitchedNetworks
CircuitSwitched NW
PacketSwitched NW
DatagramNW
VirtualCircuit NW
MessageSwitched NW
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circuit switching implies that there is adedicated communication path between the
two stations.
The path is a connected through a sequenceof links between network nodes.
On each physical link, a logical channel is
dedicated to the connection.
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Circuit SW takes place at the physical layer Before starting comn, the stn must make a reservation of
resources. BW in FDM / time slots in TDM
SW Buffers
SW processing time SW input/output port
Data transfer are not packetized (Physical layer transfer),data are continuous flow
No addressing involved during data transfer SWs route the data based on their occupied band(FDM) ortime slot (TDM)
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Efficiency Low
Delay Minimal
No waiting at SWs Total delay = Time to est conn + transfer Data +
Disconnect the circuit
Data propagation delay 5msec per 1000km
No congestion
No Busy signal
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OSI Reference Model
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International Organisation for
Standardisation (ISO) -international organisation
responsible for a wide range
of standards.
1984 - Open Systems Interconnect ion (OSI)
Reference Modelapproved as international
std for comn architecture to aid NWinterconnection without necessarily requiring
complete redesign.
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7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical
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7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical
NIC Card
Hub
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Provides network services to
application processes (such as
electronic mail, file transfer, and
terminal emulation)
7 Application
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7 Application
6 Presentation
5Session
Transport4
Network3
Inter-host communication
Network services to applications
Data representation
End-to-end connection reliability
Addresses and best path Identifying source and destination
Path selection between two
systems (routing)
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7 Application
6 Presentation
5 Session
Transport4
Network3
Data Link2
Inter-host communication
Network services to applications
Data representation
End-to-end connection reliability
Addresses and best path
Access to media
Provides reliable transfer of dataacross media
Physical addressing, network
topology, error notification, flow
control
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7 Application
6 Presentation
5 Session
Transport4
Network3
Data Link2
Physical1
Inter-host communication
Network services to applications
Data representation
End-to-end connection reliability
Addresses and best path
Access to media
Binary transmission
Wires, connectors, voltages,
data rates
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Application
Presentation
Session
Transport
Network
Physical
Data Link
Application
Presentation
Session
Transport
Network
Physical
Data Link
Host A Host B
Data} {
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message
segmentdatagram
frame
sourceapplication
transportnetwork
linkphysical
HtHnHl M
HtHn MHt M
M
destinationapplicationtransportnetwork
link
physical
HtHnHl M
HtHn M
Ht M
Mnetwork
linkphysical
link
physical
HtHnHl MHtHn M
HtHnHl MHtHn M
HtHnHl M HtHnHl M
router
switch
Data Encapsulation
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Layer 2
Data Link Layer
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Reliable communication over a single link.
Introduces the notion of a frame
set of bits that belong together
Begin and end markers delimit a frame
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On a broadcast link (such as Ethernet) end-system must receive only bits meant for it
need datalink-layer address
also need to decide who gets to speak next these functions are provided by Medium
Access Sublayer
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DLL protocols are the first layer of SW &heavily dependent on underlying physical
link properties.
Hence, both physical and data link layers
are usually bundled together on Network
Interface Card
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24 bits
ROM
RAM
24 bits
2b5f.0c12. 3a56
Serial NumberVendor Code
MAC address is burned into ROM on a networkinterface card
Can it be changed? (OTW)
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Layer 3
Network Layer
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Breaks Transport Layer PDUs into
packets and ensures their delivery
Defines logical addressing
Responsible for routing on the NW
(Routers operate at this layer)
Found both in end-systems and in
intermediate systems
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Network Node
1 1GeneralExample
Network Host
10. 8.2.48TCP/IPExample
(Mask 255.0.0.0)
Network Node
1aceb0b. 0000.0c00.6e25Novell IPXExample
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BB
X Y
AA
CC
Presentation
Data Link
Physical
Data Link
Physical
Router A Router B Router C
Data Link
Physical
Data Link
Network
Transport
Session
Presentation
Application
Physical
Host X Host Y
Data LinkNetwork
Transport
Session
Application
Physical
Network Network Network
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AA BB
CCDD
XX
Can an alternate route substitute for a failed route?
Yes but With dynamic routing enabled
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Transport Layer
Layer 4
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Transport layer Multiplexes multiple applications to the same end to end
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Multiplexes multiple applications to the same end-to-end
connection
adds an application-specific identifier (port number) so that receivingend-system can hand in incoming packet to the correct application
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Transmit
Buffer Full
Not ReadyStopProcess
Segments
Buffer OKReadyGo
Resume Transmission
ReceiverSender
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Not common
Establishes, manages, and terminatessessions between applications
Provides full-duplex service, expedited
data delivery, and session synchronization
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Duplex
if transport layer is simplex, concatenates twotransport endpoints together
Expedited data delivery
allows some messages to skip ahead in end-system queues, by using a separate low-delay transport layer endpoint
Synchronization
allows users to place marks in data streamand to roll back to a pre-specified mark
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Presentation Layer
Layer 6
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U lik th l hi h d l ith h d
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Unlike other layers which deal with headers,
presentation layer also touches theapplication data
Hides data representation differences
between applications. For example: Endian-ness Characters (ASCII, unicode, EBCDIC.)
Can also encrypt data
Compression
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Application Layer
Layer 7
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Closest to the user
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Closest to the user.
Provides network services to the users applications. Application layer establishes the availability of intended
communication partners, synchronizes and establishesagreement on procedures control of data integrity.
Differs from other layers in that it does not provide
services to any other OSI layer, but rather, only toapplications outside the OSI model.
Examples of such applications are Spreadsheetprograms, Network virtual terminal, Mail services,Directory services, File transfer, access, and
management.
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TCP/ IP REFERENCEMODEL
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Research sponsored by United States DoD Major design goals
Ability to connect multiple NWs seamlessly
Survivability
Flexible architecture to cater for applications with divergentrequirements
DoD wanted connections to remain intact as long as source
and destination machines were intact.
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Connectionless packet switched service Uses Internet Protocol (IP) which defines packets
Permit injection of packets into any NW and have them
travel independently to the destination
Packet routing and congestion control are major issues.
Similar to OSI Network Layer .
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Designed to allow conversation between peer entities on
source and destination hosts Similar to OSI Transport Layer.
Two protocols defined for this layer:
Transmission Control Protocol (TCP) - handles flow control and
sequencing. Reliable connection oriented protocol
User Datagram Protocol (UDP) prompt delivery more importantthan accurate delivery. Unreliable connectionless protocol
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No session or presentation layer.
Application layer serves as the communication interfacefor users by providing specific application services to theuser
Protocols - Virtual terminal (telnet), FTP, SMTP, DNS,HTTP, etc.
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TCP/IP f d l d l d d h OSI
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TCP/IP reference models development preceded the OSI
model by several years. Unlike OSI, TCP/IP was never intended to be an
international standard.
TCP/IP was not designed with layers and does not fit neatly
into OSI models 7 layers.
OSI introduces the concept of Services, Interfaces and
Protocols. TCP/IP model originally did not distinguish
between these.
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P t l b tt hidd i OSI d l d th il
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Protocols better hidden in OSI model and thus easily
replaced with change in technology
In OSI model, protocols were invented after model was
devised. In TCP/IP model, protocols came first and model
was merely description of protocols
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ADDRESSES
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MAC SUB LAYER
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Static allocation The channel BW is divided equally among users.
Unused bandwidth will be lost.
Dynamic allocation
Users able to access unused bandwidth from
others.
There is no dedicated bandwidth.
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Traditional method example Frequency Division
Multiplexing (FDM). Involves splitting up the usable frequency into
smaller channels.
Unused bandwidth will be wasted, but the allocatedbandwidth is guaranteed.
Simple and efficient for known workloads.
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Larger the number of users, smaller are the FDM
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channels.
In computer systems, data is generally bursty, thus,
FDM results in poor bandwidth utilization as stn is
idle for most of the time.
Data bursts from one computer will take a long time
to travel over the bandwidth, while other computers
may not be using their resources.
Similar is the case for Time Division Multiplexing
(TDM)
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Key assumptions:
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Key assumptions:
Station Model
The network consists of independent stations (orcomputers).
All stations are considered to be equal.
They produce frames to be transmitted.
Once a frame is generated the station is blocked
and does nothing until the frame has beensuccessfully transmitted.
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Single Channel Assumption There will be a single channel for all of the
terminals to communicate on.
All stations can transmit and receive on it.
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C i S
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Carrier Sense
The terminal can sense if the channel is busy No terminal will transmit until the channel is idle
No Carrier Sense
The terminal cannot sense if the channel is busy
Terminals will transmit and check for collisions later.
LANs generally use Carrier Sense but not satellitenetworks. Why?
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If more than 2 users send at the same time - collision
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All collided packets are lost -> waste of bandwidth
Ideally, the MAC protocol for a broadcast channel with
the bit-rate R bpsshould satisfy:
if only 1 node is sending then the throughput is R
when M nodes have data to send than the
throughput is R/M
decentralized protocol no master
simple & inexpensive to implement
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Carrier Sense Multiple Access Protocols (CSMA) Collision-Free Protocols
Wireless LAN Protocols
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Stn monitors the channel until it is idle.
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Once it is idle, the Stn transmits.
If a collision occurs the stn waits for a random amount of timeand starts all over again.
Known as 1-persistent because the probability that it willtransmit on an idle channel is 1.
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Longer is the propagation delay, worse is theperformance of the protocol
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Limitations of 1 persistent protocol
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Propagation Delay.
Simultaneous Txn will result in collisions.
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Before sending Node senses the channel. If idle it
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transmits its frame. If channel is busy, the node waits for random time
and then repeats the algorithm.
Less greedy than 1-persistent protocol which
continues to sense the channel to try to seize itimmediately upon detecting the end of previous
transmission.
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Comparison of the channel utilization versus
load for various random access protocols.Lt Col Chandan Tiwari
Stations abort transmission on detecting collision asopposed to continuing to send out data that will be
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opposed to continuing to send out data that will be
lost anyway. In CSMA, each collision wastes at least one complete
frame time, because both stations transmit theircomplete frames even though they are garbled theinstant they collide.
Terminating the transmission on detecting collisionsaves both time and bandwidth.
The transmitting stns will then wait a random
amount of time and try again.
Used in Ethernet LAN.
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Collision detection can take as long as 2. A stn cannot be sure
that it has seized the channel until it has transmitted for 2
without hearing a collision Lt Col Chandan Tiwari
802.1: This standard gives an introduction to the set ofstandards
802 2: This standard describes the upper part of the DLL
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802.2: This standard describes the upper part of the DLL,
which uses Logical Link Control Protocol. 802.3: Describes the LAN Standard for Ethernet 802.4: Describes the LAN Standard Token Bus 802.5: Describes the LAN Standard Token Ring 802.6: Describes the LAN Standard Distributed Queue Dual Bus
(DQDB) 802.11: Wireless LAN 802.15: Bluetooth 802.16: Wireless MAN
Each standard covers the physical layer and MAC sublayerprotocol
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Ethernet refers to cable (the ether)
d i t i d LAN t h l
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dominant wired LAN technology
first widely used LAN technology Simpler, cheap
Kept up with speed race: 10 Mbps 10 Gbps
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LLC
MAC
Network layer
Physical layer
PKT
LLC PKT
MAC LLC PKT MAC
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WIRELESS 802.11
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Modes of operation
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Point Coordination Function (PCF) - In the presenceof a base station all communication must go through
the base station, called an access point
Distribution Coordination Function(DCF) - In the
absence of a base station the computers would just
send to one another directly. This mode is now
sometimes called adhoc networking
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Upon correct receipt of the data frame, B responds with anACK frame, terminating the exchange. If A's ACK timerexpires before the ACK gets back to it, the whole protocol isrun again.
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C is within range of A, so it may receive the RTS frame. If itdoes, it realizes that someone is going to send data soon,so for the good of all it desists from transmitting anythinguntil the exchange is completed. From the informationprovided in the RTS request, it can estimate how long thesequence will take, including the final ACK, so it asserts a
kind of virtual channel busy for itself, indicated by NAV(Network Allocation Vector). D does not hear the RTS, butit does hear the CTS, so it also asserts the NAV signal foritself.
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Duration field tells how long the frame and itsacknowledgement will occupy the channel. This field is alsopresent in the control frames and is how other stations managethe NAV mechanism
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the NAV mechanism.
The frame header contains four addresses. The source anddestination are obviously needed. The other two addresses areused for the source and destination base stations for intercelltraffic.
Sequence field allows fragments to be numbered. Of the 16 bitsavailable, 12 identify the frame and 4 identify the fragment.
Data field contains the payload, up to 2312 bytes
Checksum
Management frames have a format similar to that of data
frames, except without one of the base station addressesbecause management frames are restricted to a single cell.
Control frames are shorter still, having only one or twoaddresses, no Data field, and no Sequence field. The keyinformation here is in the Subtype field, usually RTS, CTS, or ACK.
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Link Layer: Introduction
Some terminology: hosts and routers are nodes
link
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5: DataLink Layer 5-226
hosts and routers are nodes
communication channels thatconnect adjacent nodes alongcommunication path are links wired links
wireless links LANs
layer-2 packet is a frame,encapsulates datagram
data-link layer has responsibility oftransferring datagram from one nodeto adjacent node over a link
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Link Layer Services (more)
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5: DataLink Layer 5-229
Flow Control: pacing between adjacent sending and receiving nodes important if large propagation delays
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 correctsbit error(s) withoutresorting to retransmission
Half-duplex and full-duplex with half duplex, nodes at both ends of link can transmit,
but not at same time Lt Col Chandan Tiwari
Adaptors Communicating
sending
rcvingdatagram
link layer protocol
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5: DataLink Layer 5-230
link layer implemented in
adaptor (aka NIC) Ethernet card, PCMCI
card, 802.11 card
sending side:
encapsulates datagram ina frame
adds error checking bits,rdt, flow control, etc.
receiving side looks for errors, rdt, flow
control, etc
extracts datagram, passesto rcving node
adapter is semi-autonomous
link & physical layers
g
nodeframe
nodeframe
adapter adapter
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Error Detection
EDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking, may include header fields
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5: DataLink Layer 5-232
p y g y
Error detection not 100% reliable! protocol may miss some errors, but rarely larger EDC field yields better detection and correction
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Parity Checking
Single Bit Parity: Two Dimensional Bit Parity:and correct
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5: DataLink Layer 5-233
Detect single bit errors Detectand correct
single bit errors
0 0
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Internet checksum
Goal:detect errors (e.g., flipped bits) in transmitted
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5: DataLink Layer 5-234
Sender: treat segment contents
as sequence of 16-bitintegers
checksum: addition (1scomplement sum) ofsegment contents
sender puts checksumvalue into UDP checksumfield
Receiver: compute checksum of received
segment
check if computed checksumequals checksum field value:
NO - error detected
YES - no error detected. Butmaybe errors nonetheless?
segment (note: used at transport layer only)
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Cyclic Redundancy Check
view data bits, D, as a binary number
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5: DataLink Layer 5-235
choose r+1 bit pattern (generator), G goal: choose r CRC bits, R, such that
exactly divisible by G (modulo 2)
receiver knows G, divides by G. If non-zero remainder:error detected!
can detect all burst errors less than r+1 bits widely used in practice (ATM, HDLC)
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CRC Example
Want:r
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5: DataLink Layer 5-236
D.2r
XOR R = nGequivalently:
D.2r = nG XOR Requivalently:
if we divide D.2r byG, want remainder R
R = remainder[ ]D.2rG
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Part 3: Link Layer
3.1 Introduction and 3.6 Hubs and switches
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5: DataLink Layer 5-237
services 3.2 Error detection
and correction
3.3Multiple accessprotocols
3.4 Link layeraddressing
3.5 Ethernet
3.7 PPP
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Multiple Access Links and Protocols
Two types of links: point-to-point
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5: DataLink Layer 5-238
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) Old-fashioned Ethernet
upstream HFC 802.11 wireless LAN
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Multiple Access protocols
single shared broadcast channel
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5: DataLink Layer 5-239
two or more simultaneous transmissions by nodes:interference collision if node receives two or more signals at the same time
multiple access protocol
distributed algorithm that determines how nodes sharechannel, i.e., determine when node can transmit
communication about channel sharing must use channelitself!
no out-of-band channel for coordination
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802.3 MAC Sub Layer Protocol
Preamble Length Data PadCheck-SourceDestination
SO
Bytes 7 1 2/ 6 2/ 6 2 0-1500 0-46 4
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Preamble of 7 bytes each with bit pattern 10101010 Manchester Encoding of this pattern produces 10 MHzsquare wave for 5.6 sec to allow the receiver clock to
synchronise with the senders. 1 byte Start of Frame 10101011 delimiter
6 byte source and destination addresses.
Length indicates number of bytes in data field.
Pad field ensures min frame size is 64 bytes (lessPreamble+SOF)
Checksum field 32 bit CRC
sumAddressAddressF
48 bit address field provide a unique MAC address
Higher order bit(47th bit) 0 for ordinary addresses and
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1 for group/multicast address. 46th bit to distinguishbetween local and global address-2^(48-2).
All bits are 1 for a broadcast address
The MAC ADDRESS is burned into ROM on a network
interface card
24 bits
ROMRAM
24 bits
00-90-F5 - 0C-99-6ASerial NumberVendor Code
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Ideal Multiple Access Protocol
Broadcast channel of rate R bps
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5: DataLink Layer 5-242
1. When one node wants to transmit, it can send atrate R.
2. When M nodes want to transmit, each can send ataverage rate R/M
3. Fully decentralized: No special node to coordinatetransmissions
4. Simple
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MAC Protocols: a taxonomy
Three broad classes:
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5: DataLink Layer 5-243
Channel Partitioning divide channel into smaller pieces (time slots,
frequency, code)
allocate piece to node for exclusive use
Random Access channel not divided, allow collisions
recover from collisions
Taking turns Nodes take turns, but nodes with more to send can take
longer turns
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Channel Partitioning MAC protocols: TDMA
TDMA: time division multiple access
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5: DataLink Layer 5-244
access to channel in "rounds" each station gets fixed length slot (length = pkt
trans time) in each round
unused slots go idle
example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6idle
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Channel Partitioning MAC protocols: FDMA
FDMA: frequency division multiple access
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5: DataLink Layer 5-245
channel spectrum divided into frequency bands each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example: 6-station LAN, 1,3,4 have pkt, frequencybands 2,5,6 idle
frequenc
ybands
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Random Access Protocols
When node has packet to send
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5: DataLink Layer 5-246
transmit at full channel data rate R.
no a prioricoordination among nodes
two or more transmitting nodes collision,
random access MAC protocol specifies: how to detect collisions
how to recover from collisions (e.g., via delayedretransmissions)
Examples of random access MAC protocols: slotted ALOHA ALOHA
CSMA, CSMA/CD, CSMA/CA
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Pure (unslotted) ALOHA
unslotted Aloha: simpler, no synchronization, no
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5: DataLink Layer 5-247
carrier sensing. when frame first arrives
transmit immediately
collision probability increases:
frame sent at t0 collides with other frames sent in [t0-1,t0+1]
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Pure Aloha efficiency
P(success by given node) = P(node transmits) .
P(no other node transmits in [p -1,p ] .
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5: DataLink Layer 5-248
0 0P(no other node transmits in [p0-1,p0]
= p . (1-p)N-1 . (1-p)N-1
= p . (1-p)2(N-1)
choosing optimum p and then letting n -> infty ...
= 1/(2e) = .18
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Slotted ALOHA
Assumptions
all frames same sizeOperation
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5: DataLink Layer 5-249
time is divided intoequal size slots, time totransmit 1 frame
nodes start to transmitframes only atbeginning of slots
nodes are synchronized
if 2 or more nodestransmit in slot, allnodes detect collision
when node obtains freshframe, it transmits in nextslot
no collision, node can sendnew frame in next slot
if collision, noderetransmits frame in eachsubsequent slot with prob.p until success
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Slotted ALOHA
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5: DataLink Layer 5-250
Pros
single active node cancontinuously transmitat full rate of channel
highly decentralized:only slots in nodesneed 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 synchronization (used
in a hub-spoke manner)Lt Col Chandan Tiwari
Slotted Aloha efficiency
For max efficiencywith N nodes, find p*
h
Efficiency is the long-runfraction of successful slots
h h d
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5: DataLink Layer 5-251
Suppose N nodes with
many frames to send,each transmits in slotwith probabilityp
prob that node 1 hassuccess in a slot= p(1-p)N-1
prob that any node hasa success = Np(1-p)N-1
that maximizesNp(1-p)N-1
For many nodes, takelimit of Np*(1-p*)N-1
as N goes to infinity,gives 1/e = .37
when there are many nodes,each with many frames to send
At best:channel
used for usefultransmissions 37%of time!
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CSMA (Carrier Sense Multiple Access)
CSMA l b f
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5: DataLink Layer 5-252
CSMA: listen before transmit:If channel sensed idle: transmit entire frame
If channel sensed busy, defer transmission
Human analogy: dont interrupt others!
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All frames must take more than 2t to send so that
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the txn is still taking place when the noise burst gets
back to the sender, otherwise the sender will
incorrectly conclude that the frame was successfully
sent For 10Mbps LAN with max length of 2500m and 04
repeaters 2t=50 micro sec which corresponds to
min frame size of 500 bits. With safety margin it is
chosen to be 512 bits(64 bytes)
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Collisions are caused when two adaptors transmit at the
same time (adaptors sense collision based on voltage
diff )
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CS 640 254
A B
A B
differences) Both found line to be idle
Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost
there at time T when
B starts collision!
How can we be sure A knows about the collision?
How can A know that a collision has taken place? There must be a mechanism to insure retransmission on collision
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CS 640 255
As message reaches B at time T
Bs message reaches A at time 2T
So, A must still be transmitting at 2T
IEEE 802.3 specifies max value of 2T to be 51.2us
This relates to maximum distance of2500m between hosts At 10Mbps it takes 0.1us to transmit one bit so 512 bits (64B) take
51.2us to send
So, Ethernet frames must be at least 64B long
14B header, 46B data, 4B CRC
Padding is used if data is less than 46B Send jamming signal after collision is detected to insure all hosts
see collision
48 bit signal
A B
time 0
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CS 640 256
A B
A B
time = 0
time = T
time = 2T
Ethernet uses CSMA/CD protocol
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Nodes are required to wait for random time for
retransmission after collision.
Need to have the randomization interval growexponentially as more and more consecutive collisions
take place.
Lt Col Chandan Tiwari
After collision, time divided into discrete time slots equal
to worst case RTT.
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After first collision, each stn waits 0 or 1 slot times
before trying again.
After second collision, each stn waits for 0,1,2 or 3 slottimes before trying again.
After i collisions, a random number between 0 and 2i1
is chosen.
Lt Col Chandan Tiwari
After 10 collisions, randomization interval frozen at
1023
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1023. Nodes give up after 16 collisions. Left to upper layers
to resolve the issue
The algorithm ensures
low delay when few nodes collide.
collision is resolved in a reasonable interval when
many nodes collide.
Lt Col Chandan Tiwari
CSMA collisions
collisions can still occur:
propagation delay means
spatial layout of nodes
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5: DataLink Layer 5-260
propagation delay meanstwo nodes may not hear
each others transmission
collision:
entire packet transmissiontime wasted
note:role of distance & propagation delay
in determining collision probability
Lt Col Chandan Tiwari
CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA llisi s detected ithi sh t tim
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5: DataLink Layer 5-261
collisions detectedwithin 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 whiletransmitting
human analogy: the polite conversationalist
Lt Col Chandan Tiwari
CSMA/CD collision detection
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5: DataLink Layer 5-262Lt Col Chandan Tiwari
Taking Turns MAC protocols
channel partitioning MAC protocols:
share channel efficiently and fairly at high load
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5: DataLink Layer 5-263
share channel efficiently and fairly at high load
inefficient at low load: delay in channel access,1/N bandwidth allocated even if only 1 activenode!
Random access MAC protocols
efficient at low load: single node can fullyutilize channel
high load: collision overhead
taking turns protocols
look for best of both worlds!Lt Col Chandan Tiwari
Taking Turns MAC protocols
Polling:
master node
Token passing:
control token passed from one
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5: DataLink Layer 5-264
master nodeinvites slave nodesto transmit in turn
concerns:
polling overhead latency
single point offailure (master)
control token passed from onenode to next sequentially.
token message
concerns:
token overhead latency
single point of failure (token)
Lt Col Chandan Tiwari
Summary of MAC protocols
What do you do with a shared media?Ch l P titi i b ti f d
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5: DataLink Layer 5-265
Channel Partitioning, by time, frequency or code Time Division, Frequency Division
Random partitioning (dynamic), ALOHA, S-ALOHA, CSMA, CSMA/CD
carrier sensing: easy in some technologies (wire), hardin others (wireless)
CSMA/CD used in Ethernet
CSMA/CA used in 802.11
Taking Turns polling from a central site, token passing
Lt Col Chandan Tiwari
Part 3: Link Layer
3.1 Introduction and
services
3.6 Hubs and switches
3 7 PPP
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5: DataLink Layer 5-266
services 3.2 Error detection
and correction
3.3Multiple access
protocols 3.4 Link layer
addressing
3.5 Ethernet
3.7 PPP
Lt Col Chandan Tiwari
MAC Addresses and ARP
32 bit IP dd ss:k l dd
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5: DataLink Layer 5-267
32-bit IP address: network-layeraddress used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address: used to get frame from one interface to another
physically-connected interface (same network)
48 bit MAC address (for most LANs)burned in the adapter ROM
Lt Col Chandan Tiwari
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
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5: DataLink Layer 5-268
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)
Lt Col Chandan Tiwari
LAN Address (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space
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5: DataLink Layer 5-269
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
Lt Col Chandan Tiwari
ARP: Address Resolution Protocol
Each IP node (Host,
Router) on LAN has
Question: how to determine
MAC address of B
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5: DataLink Layer 5-270
Router) on LAN hasARP table
ARP Table: IP/MACaddress mappings for
some LAN nodes< IP address; MAC address; TTL> TTL (Time To Live): time
after which addressmapping will be forgotten
(typically 20 min)
MAC address of Bknowing Bs 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
137.196.7.23
137.196.7.78
137.196.7.14
137.196.7.88
Lt Col Chandan Tiwari
ARP protocol: Same LAN (network)
A wants to send datagramto B, and Bs MAC address
not in As ARP table
A caches (saves) IP-to-
MAC address pair in itsP bl l f
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5: DataLink Layer 5-271
not in A s ARP table. A broadcasts ARP query
packet, containing B's IPaddress
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 (B's)
MAC address frame sent to As MAC
address (unicast)
MAC address pair in itsARP table until informationbecomes old (times out)
soft state: informationthat times out (goes
away) unless refreshed ARP is plug-and-play:
nodes create their ARPtables withoutintervention from net
administrator
Lt Col Chandan Tiwari
DHCP: Dynamic Host Configuration Protocol
Goal: allow host to dynamicallyobtain its IP addressfrom network server when it joins network
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5: DataLink Layer 5-272
from network server when it joins networkCan renew its lease on address in use
Allows reuse of addresses (only hold address while connected anon
Support for mobile users who want to join network (more shortly)DHCP overview:
host broadcasts DHCP discover msg
DHCP server responds with DHCP offer msg
host requests IP address: DHCP request msgDHCP server sends address: DHCP ack msg
Lt Col Chandan Tiwari
DHCP client-server scenario
223 1 2 1A DHCP
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5: DataLink Layer 5-273
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
A
B E
DHCPserver
arriving DHCPclient needs
address in this
network
Lt Col Chandan Tiwari
DHCP client-server scenarioDHCP server: 223.1.2.5 arriving
client
DHCP discover
src : 0.0.0.0, 68
dest.: 255.255.255.255,67
yiaddr: 0.0.0.0
transaction ID: 654
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5: DataLink Layer 5-274
time
DHCP offer
src: 223.1.2.5, 67
dest: 255.255.255.255, 68
yiaddrr: 223.1.2.4
transaction ID: 654
Lifetime: 3600 secs
DHCP requestsrc: 0.0.0.0, 68
dest:: 255.255.255.255, 67
yiaddrr: 223.1.2.4
transaction ID: 655
Lifetime: 3600 secs
DHCP ACK
src: 223.1.2.5, 67dest: 255.255.255.255, 68
yiaddrr: 223.1.2.4
transaction ID: 655
Lifetime: 3600 secs
Lt Col Chandan Tiwari
Routing to another LAN
walkthrough: send datagram from A to B via R
assume A knows B IP address
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5: DataLink Layer 5-275
Two ARP tables in router R, one for each IPnetwork (LAN). R works as a gateway into anothernetwork
A
RB
Lt Col Chandan Tiwari
A creates datagram with source A, destination B A uses ARP to get Rs MAC address for 111.111.111.110
A creates link-layer frame with R's MAC address as dest,frame contains A-to-B IP datagram
As adapter sends frame
Rs adapter receives frame
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5: DataLink Layer 5-276
R s adapter receives frame
R removes IP datagram from Ethernet frame, sees itsdestined to B
R uses ARP to get Bs MAC address
R creates frame containing A-to-B IP datagram sends to B
A
RB
Lt Col Chandan Tiwari
Part 3: Link Layer
3.1 Introduction and
services
3.6 Hubs and switches
3 7 PPP
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5: DataLink Layer 5-277
services 3.2 Error detection
and correction
3.3Multiple access
protocols 3.4 Link layer
addressing
3.5 Ethernet
3.7 PPP
Lt Col Chandan Tiwari
Ethernet
dominant wired LAN technology:
cheap $20 for 100Mbs!
first widely used LAN technology
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5: DataLink Layer 5-278
first widely used LAN technology
Simpler, cheaper than token LANs and ATM
Kept up with speed race: 10 Mbps 10 Gbps
Metcalfes Ethernetsketch
Lt Col Chandan Tiwari
Star topology
Bus topology popular through mid 90s
Now star topology prevails
Connection choices: hub or switch (more later)
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5: DataLink Layer 5-279
Connection choices: hub or switch (more later)
hub or
switch
Lt Col Chandan Tiwari
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or othernetwork layer protocol packet) in Ethernet frame
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5: DataLink Layer 5-280
Preamble: 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011
used to synchronize receiver, sender clock rates
Lt Col Chandan Tiwari
Ethernet Frame Structure (more)
Addresses: 6 bytes if adapter receives frame with matching destination
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5: DataLink Layer 5-281
f p f m m gaddress, or with broadcast address (eg ARP packet), itpasses data in frame to net-layer protocol
otherwise, adapter discards frame
Type: indicates the higher layer protocol (mostlyIP but others may be supported such as NovellIPX and AppleTalk)
CRC: checked at receiver, if error is detected, theframe is simply dropped
Lt Col Chandan Tiwari
Unreliable, connectionless service
Connectionless: No handshaking between sending
and receiving adapter
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5: DataLink Layer 5-282
and receiving adapter. Unreliable:receiving adapter doesnt 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
Lt Col Chandan Tiwari
Ethernet uses CSMA/CD
No slots
adapter doesnt transmit
Before attempting a
retransmission,
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5: DataLink Layer 5-283
adapter doesn t transmitif it senses that someother adapter istransmitting, that is,
carrier sense transmitting adapter
aborts when it sensesthat another adapter is
transmitting, that is,collision detection
retransmission,adapter waits arandom time, that is,random access
Minimum Ethernetframe size related tothe physical size ofthe network
Lt Col Chandan Tiwari
Ethernet CSMA/CD algorithm
1. Adaptor receivesdatagram from net layer &
creates frame
4. If adapter detectsanother transmission while
transmitting, aborts and
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5: DataLink Layer 5-284
creates frame2. If adapter senses channelidle (96 bit times), it startsto transmit frame. If it
senses channel busy, waitsuntil channel idle and thentransmits
3. If adapter transmits
entire frame withoutdetecting anothertransmission, the adapter isdone with frame !
transmitting, aborts andsends 48 bit jam signal
5. After aborting, adapterenters exponential backoff:
after the mth collision,adapter chooses a K atrandom from{0,1,2,,2m-1}. Adapter waits
K512 bit times and returnsto Step 2
Lt Col Chandan Tiwari
Ethernets CSMA/CD (more)
Jam Signal: make sure allother transmitters are aware
of collision; 48 bits
Exponential Backoff:
Goal: adapt retransmissionattempts to estimated
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5: DataLink Layer 5-285
Bit time: .1 microsec for 10Mbps Ethernet ;for K=1023, wait time is about50 msec
pcurrent load heavy load: random wait
will be longer
first collision: choose K
from {0,1}; delay is K 512bit transmission times
after second collision:choose K from {0,1,2,3}
after ten collisions, chooseK from {0,1,2,3,4,,1023}
Lt Col Chandan Tiwari
CSMA/CD efficiency
Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
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5: DataLink Layer 5-286
ttrans t me to transm t max s ze frame
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity
Much better than ALOHA, but still decentralized,simple, and cheap
transpr op tt /51
1efficiency
Lt Col Chandan Tiwari
Manchester encoding
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5: DataLink Layer 5-287
Used in 10BaseT
Each bit has a transition
Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized, global clock among nodes!
Hey, this is physical-layer stuff!Lt Col Chandan Tiwari
Chapter 4
Network Layer
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Lt Col Chandan Tiwari
Part 5: Network Layer
5. 1 Introduction
5.2 Datagram networks
5.5 Routing algorithms
Link state
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Lt Col Chandan Tiwari
5.3 Whats inside a router
5.4 IP: Internet Protocol
Datagram format
IPv4 addressing
ICMP
IPv6
Distance Vector
Hierarchical routing
5.6 Routing in the Internet
RIP OSPF
BGP
5.7 Broadcast and multicast
routing
Network layer transport segment from sending to receiving host
on sending side encapsulates segments intodatagrams
on rcving side, delivers segments to transport layer
network layer protocols in everyhost, router
Router examines header fields in all IP datagramspassing through it networkdata linkph si l
networknetworkd t li k
applicationtransportnetwork
data linkphysical
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Lt Col Chandan Tiwari
networkdata linkphysical
networkdata linkphysical
physical
networkdata linkphysical
networkdata linkphysical
data linkphysical
networkdata linkphysical
data linkphysical
applicationtransportnetworkdata linkphysical
forwarding:move packets from routers input
to appropriate router output
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Lt Col Chandan Tiwari
to appropriate router output
routing: determine route taken by packets
from source to dest.
routing algorithms
routing algorithm
local forwarding table
h d l t t li k
Interplay between routing and forwarding
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Lt Col Chandan Tiwari
1
23
0111
value in arriving
packets header
header value output link
0100
0101
0111
1001
3
2
2
1
Part 5: Network Layer
5. 1 Introduction
5.2 Datagram networks
5.5 Routing algorithms
Link state
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Lt Col Chandan Tiwari
5.3 Whats inside a router
5.4 IP: Internet Protocol
Datagram format
IPv4 addressing
ICMP
IPv6
Distance Vector
Hierarchical routing
5.6 Routing in the Internet
RIP OSPF
BGP
5.7 Broadcast and multicast
routing
datagram network provides network-layer
connectionless service
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Lt Col Chandan Tiwari
connectionless service
analogous to the transport-layer services,
but:
service: host-to-host
implementation: in network core
Datagram networks
routers: no state about end-to-end connections
no network-level concept of connection
packets forwarded using destination host address packets between same source-dest pair may take different paths
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Lt Col Chandan Tiwari
packets between same source dest pair may take different paths
applicationtransportnetworkdata linkphysical
applicationtransportnetwork
data linkphysical
1. Send data 2. Receive data
Destination Address Range Link Interface
11001000 00010111 00010000 00000000
4 billionpossible entries
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Lt Col Chandan Tiwari
11001000 00010111 00010000 00000000
through 0
11001000 00010111 00010111 11111111
11001000 00010111 00011000 00000000through 1
11001000 00010111 00011000 11111111
11001000 00010111 00011001 00000000
through 2
11001000 00010111 00011111 11111111
otherwise 3
Prefix Match Link Interface
11001000 00010111 00010 0
11001000 00010111 00011000 1
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Lt Col Chandan Tiwari
11001000 00010111 00011000 1
11001000 00010111 00011 2
otherwise 3
DA: 11001000 00010111 00011000 10101010
Examples
DA: 11001000 00010111 00010110 10100001 Which interface?
Which interface?
Part 5: Network Layer
5. 1 Introduction
5.2 Datagram networks
5.5 Routing algorithms
Link stateDi t V t
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Lt Col Chandan Tiwari
5.3 Whats inside a router
5.4 IP: Internet Protocol
Datagram format
IPv4 addressing
ICMP
IPv6
Distance Vector
Hierarchical routing
5.6 Routing in the Internet
RIP OSPF
BGP
5.7 Broadcast and multicast
routing
The Internet Network layer
Host, router network layer functions:
Transport layer: TCP, UDP
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Lt Col Chandan Tiwari
forwardingtable
Routing protocolspath selectionRIP, OSPF, BGP
IP protocoladdressing conventionsdatagram formatpacket handling conventions
ICMP protocolerror reportingrouter signaling
Link layer
physical layer
Network
layer
Part 5: Network Layer
5. 1 Introduction
5.2 Datagram networks
5.5 Routing algorithms
Link stateDi t V t
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Lt Col Chandan Tiwari
5.3 Whats inside a router
5.4 IP: Internet Protocol
Datagram format
IPv4 addressing
ICMP
IPv6
Distance Vector
Hierarchical routing
5.6 Routing in the Internet
RIP OSPF
BGP
5.7 Broadcast and multicast
routing
ver length
32 bits
16-bit identifierheadertime to
IP protocol versionnumber
header length(bytes)
max number
for
fragmentation/reassembly
total datagramlength (bytes)
head.len
type ofservice
type of data
flgsfragment
offsetupper
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Lt Col Chandan Tiwari
data(variable length,
typically a TCPor UDP segment)
headerchecksum
time tolive
32 bit source IP address
max numberremaining hops
(decremented ateach router)
upper layer protocolto deliver payload to
upperlayer
32 bit destination IP address
Options (if any) E.g. timestamp,record routetaken, specifylist of routers
to visit.
how much overhead
with TCP?
20 bytes of TCP
20 bytes of IP
= 40 bytes + app
layer overhead
IP Fragmentation & Reassembly
network links have MTU
(max.transfer size) - largest
possible link-level frame.
different link types, different
MTUfragmentation:
l
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Lt Col Chandan Tiwari
MTUs
large IP datagram divided
(fragmented) within net
one datagram becomes
several datagrams
reassembled only at final
destination
IP header bits used to identify,
order related fragments
in: one large datagramout: 3 smaller datagrams
reassembly
IP Fragmentation and Reassembly
ID=x
offset=0
fragflag=0
length=4000
One large datagram becomes
Example
4000 byte datagram
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Lt Col Chandan Tiwari
ID
=x
offset
=0
fragflag
=1
length
=1500ID=x
offset=185
fragflag=1
length=1500
ID
=x
offset
=370
fragflag
=0
length
=1040
One large datagram becomesseveral smaller datagrams MTU = 1500 bytes
1480 bytes indata field
offset =1480/8
Part 5: Network Layer
5. 1 Introduction
5.2 Datagram networks
5.5 Routing algorithms
Link stateDistance Vector
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Lt Col Chandan Tiwari
5.3 Whats inside a router
5.4 IP: Internet Protocol
Datagram format
IPv4 addressing
ICMP
IPv6
Distance Vector
Hierarchical routing
5.6 Routing in the Internet
RIP OSPF
BGP
5.7 Broadcast and multicast
routing
IP Addressing: introduction
IP address: 32-bit
identifier for host, router
interface
223.1.1.1
223.1.1.2223.1.1.4 223.1.2.9
223.1.2.1
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Lt Col Chandan Tiwari
interface: connection
between host/router and
physical link
routers typically havemultiple interfaces
host typically has one
interface
IP addresses associated
with each interface
223.1.1.3223.1.2.2
223.1.3.2223.1.3.1
223.1.3.27
223.1.1.1 = 11011111 00000001 00000001 00000001
223 1 11
Subnets
IP address:
subnet part (high order
bits)
host part (low order bits)
223.1.1.1
223.1.1.2223.1.1.4 223.1.2.9
223.1.2.1
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Lt Col Chandan Tiwari
host part (low order bits)
Whats a subnet ?
device interfaces with
same subnet part of IPaddress
can physically reach each
other without intervening
router
223.1.1.3223.1.2.2
223.1.3.2223.1.3.1
223.1.3.27
network consisting of 3 subnets
subnet
Subnets 223.1.1.0/24 223.1.2.0/24
Recipe
To determine the subnets,detach each interface
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Lt Col Chandan Tiwari
223.1.3.0/24
detach each interface
from its host or router,
creating islands of isolated
networks. Each isolated
network is called a
subnet.
Subnet mask: /24
Subnets
How many? 223.1.1.1
223.1.1.3
223.1.1.4
223.1.1.2
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Lt Col Chandan Tiwari
223.1.2.2223.1.2.1
223.1.2.6
223.1.3.2223.1.3.1
223.1.3.27
223.1.7.0
223.1.7.1223.1.8.0223.1.8.1
223.1.9.1
223.1.9.2
CIDR:Classless InterDomain Routing
subnet portion of address of arbitrary lengthaddress format a b c d/x where x is # bits in
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Lt Col Chandan Tiwari
address format: a.b.c.d/x, where x is # bits in
subnet portion of address
11001000 00010111 00010000 00000000
subnetpart
hostpart
200.23.16.0/23
Q: How does networkget subnet part of IP addr?
A:gets allocated portion of its provider ISPsaddress space
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address space
ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20
Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23
Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23
Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23
... .. . .
Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23
Q: How does hostget IP address?
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hard-coded by system admin in a file
/etc/network/interfaces
DHCP:Dynamic Host Configuration Protocol: dynamically getaddress from as server
plug-and-play
Organization 0
Hierarchical addressing allows efficient advertisement of routinginformation:
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Send me anythingwith addresses
beginning200.23.16.0/20
200.23.16.0/23
200.23.18.0/23
200.23.30.0/23
Fly-By-Night-ISP
Organization 0
Organization 7Internet
Organization 1
ISPs-R-UsSend me anythingwith addressesbeginning199.31.0.0/16
200.23.20.0/23Organization 2
...
...
ISPs-R-Us has a more specific route to Organization 1
200 23 16 0/23
Organization 0
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Send me anythingwith addressesbeginning
200.23.16.0/20
200.23.16.0/23
200.23.18.0/23
200.23.30.0/23
Fly-By-Night-ISP
Organiza