lecture 7 Switching

8/7/2019 lecture 7 Switching

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CS65-Computer Networks Department of EEE

By Dr.S.Muralidharan 1

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•• A circuit A circuit--switched network consists of a set of switched network consists of a set of

switches connected by physical links. A switches connected by physical links. A

connection between two stations is aconnection between two stations is a

dedicated path made of one or more links.dedicated path made of one or more links.

However, each connection uses only one However, each connection uses only one

dedicated channel on each link. Each link isdedicated channel on each link. Each link is

normally divided into n channels by using normally divided into n channels by using

FDM or TDM. FDM or TDM.

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•• In circuit switching, the resources need to be In circuit switching, the resources need to be

reserved during the setup phase;reserved during the setup phase;

the resources remain dedicated for the entirethe resources remain dedicated for the entire

duration of data transfer until the teardownduration of data transfer until the teardown

phase. phase.

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Information

Host 1 Host 2Switch 1 Switch 2

propagationdelay

between Host 1

and Host 2

Transmission delay

time

Timing in Circuit SwitchingTiming in Circuit Switching5 6

•• In a packet In a packet--switched network, there is no switched network, there is no

resource reservation; resources are allocated resource reservation; resources are allocated

on demand.on demand.

••

Data to be transmitted needs to be divided Data to be transmitted needs to be divided into packets of fixed or variable size. The sizeinto packets of fixed or variable size. The size

of the packet is determined by the network of the packet is determined by the network

and the governing protocol and the governing protocol

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– – Datagram Datagram

•• Each packet is treated independently Each packet is treated independently ieie..

Routed Routed

•• The destination may receive the packets inThe destination may receive the packets in

different sequencedifferent sequence

– – Virtual circuit Virtual circuit

•• A logical connection is established before any A logical connection is established before any

packets are sent. packets are sent.

•• The route between the stations is set up prior The route between the stations is set up prior

to data transfer. It doesn’t mean it is circuit to data transfer. It doesn’t mean it is circuit

switching. switching.

– – A packet is still buffered at each node and A packet is still buffered at each node and

queued for output over a line.queued for output over a line.8





Host A

Host BHost E

Host D

Host C

Node 1 Node 2

Node 3

Node 4

Node 5

Node 6 Node 7

•• Each packet is independently switched Each packet is independently switched

– – Each packet header contains destination Each packet header contains destination

addressaddress9

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Packet 1

Packet 2

Packet 3

Packet 1

Packet 2

Packet 3

Packet 1

Packet 2

Packet 3

Host 1 Host 2

processing

delay of Packet 1 at

Switch 2

propagationdelay between

Host 1 and

Switch 2transmissiontime of Packet 1

at Host 1

Timing of Datagram Packet SwitchingTiming of Datagram Packet Switching11 12





A

datagram

network

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••

A switch in a datagram network uses a A switch in a datagram network uses arouting table that is based on the destinationrouting table that is based on the destination

address.address.

•• The destination address in the header of aThe destination address in the header of a

packet in a datagram network remains the packet in a datagram network remains the

same during the entire journey of the packet. same during the entire journey of the packet.

•• Switching in the Internet is done by using theSwitching in the Internet is done by using the

datagram approach to packet switching at thedatagram approach to packet switching at thenetwork layer.network layer.

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Routing table in a datagram network

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•• A virtual A virtual--circuit network is a cross between acircuit network is a cross between a

circuit circuit--switched network and a datagram switched network and a datagram

network. It has some characteristics of both.network. It has some characteristics of both.

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Packet 1

Packet 2

Packet 3

Packet 1

Packet 2

Packet 3

Host 1 Host 2

VC

establishment

VC

termination

Data

transfer

Packet 1

Packet 2

Packet 3

Timing of Timing of VirtualVirtual--Circuit Packet SwitchingCircuit Packet Switching17

•• In virtual In virtual--circuit switching, all packetscircuit switching, all packets

belonging to the same source and belonging to the same source and destination travel the same path; but thedestination travel the same path; but the

packets may arrive at the destination with packets may arrive at the destination with

different delays if resource allocation is ondifferent delays if resource allocation is on

demand.demand.

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Host A

Host BHost E

Host D

Host C

Node 1 Node 2

Node 3

Node 4

Node 5

Node 6 Node 7

Virtual Circuit SwitchingVirtual Circuit Switching

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Virtual Virtual

Packet Packet

Switched Switched

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Virtual Virtual--circuit identifier circuit identifier

Switch and tables in a virtual Switch and tables in a virtual--circuit network circuit network

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Setup request in a virtual Setup request in a virtual--circuit network circuit network

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Setup acknowledgment in a virtual Setup acknowledgment in a virtual--circuit circuit

network network

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Source Source--toto--destination data transfer in a virtual destination data transfer in a virtual--circuit circuit

network network

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Circuit

Switching

Message

Switching

Datagram

Packet

Switching

Virtual Circuit

Packet

Switching

User

responsible for

message lossprotection

Network

responsible for

messages

Network may be

responsible for

individualpackets

Network may be

responsible foe

packetsequence

Usually no

speed or code

conversion

Speed and code

conversion

Speed and code

conversion

Speed and code

conversion

Fixed bandwidth

transmission

Dynamic use of

bandwidth

Dynamic use of

bandwidth

Dynamic use of

bandwidth

No overhead

hits after callsetup

Overhead hits in

each message

Overhead hits in

each packet

Overhead hits in

each packet

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lecture 7 Switching