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Contents

• Circuit Switching v/s Packet switching• Logical Channel v/s Physical channel• Statistical Time Division Multiplexing• Connectionless and Connection Oriented

Data Communications

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Switching MethodsSwitching

Circuit Switching

Message Switching

Packet SwitchingDatagram Approach

Virtual Circuit Approach

SVC PVC

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Circuit Switching

ABCD

SAS

Direction of transmission

Physical Channels (Time Slots)

BCDABCD ABCDC ABD

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-- - - -

Circuit Switching

ABCD

SAS

Direction of transmission

Inefficient Utilization of media

---A--- AA-

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All Communications are bursty

Time

DATA

Request for some website home page

Data transfer from the web site

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Message Switching

• Store and Forward technique• Send the message to next node• Next node stores it in memory• It takes a decision about the next hop closer to

destination• Forwards it to next hop when link to the next hop

is available and next node is ready to receive it• Next hop repeats the same process• Message finally reaches its destination node

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Message Switching

A

B

D

E

Store & Forward

Store & Forward

Store & Forward

Store & Forward

M1

M1

M1

M1

M2

M2

M2

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Message Switching

• Messages are bigger in size• Storage and processing requires more resources• Sometimes nodes may not have sufficient

resources• Messages remain stored in memory of a node for

longer period• Entire process becomes slow• Error will require full message to be retransmitted• Suitable for services like Telegraphs etc• Not suitable for fast modern networks

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Evolution of Packet switching

• Break the message into smaller packets• Transmit the packets hop by hop to destination• Destination reassembles packets into original

message• Requires less resources at nodes• Process becomes faster compared to message

switching• Error requires only retransmission of errored

packet not the full message

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Packet Switching: Statistical Multiplexing

ABCD

A

Direction of transmission

Virtual ChannelsNo Physical channel like a Time Slot

BD ABD

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Packet Switching

• Allot Bandwidth on Demand• Buffer Data and allow bandwidth to only

those hosts which have data to transmit.• To the data, add some delimiters to indicate

end of data transmitted by a particular host.• Add some tags (addresses or channel

identifiers) to indicate the sender.

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Packet Switching

ABCD

ABD

Direction of transmission

FFFFFF101010CF001010101110BF101111011AF

F: A flag to delimit the data transmitted by one host

F F FFFFFFF

A, B, C: Identifier for the transmitting host (Address or Virtual channel no.)

C B A

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Packet Switching

ABCD

AC

Direction of transmission

FFFFFF101010CF00101010111010101111011AF

C is denied the opportunity to transmit

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FFFFFF101010CF00101010111010101111011AF

Packet Switching

ABCD

AC

Direction of transmission

The whole data for A is retransmitted

11111

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FFFFFF101010CF00101010111010101111011AF

Packet Switching

ABCD

AC

Direction of transmission

Solution is break data into small blocks PACKETS

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Packet Switching Techniques

• Connection Oriented– End to end path is setup before any data communication

happens– Every packet need not carry the destination address– Destination address is send to the network only once

during the call setup process • Connectionless

– Path setup is not required. Drop the packet in network and network takes it to destination

– Every packet should must carry the source and destination address

– Every packet is examined independently by the nodes for its routing

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Connection Oriented Data Communications

• A path is established before actual data transfer.• All packets take the same path.• Routing decision is taken before actual data transfer.• Actual data packets contains the routing labels.• All packets follow the same path• Packets reach its destination in sequence• Disruption in communication if link fails during data transfer.• Quality of service can be guaranteed.• Example X.25, Frame Relay, Asynchronous Transfer

Mode(ATM).

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Connection Oriented Data Communications

.I/C O/GP CHL P CHL1 5 3 7

.I/C O/GP CHL P CHL2 7 5 20 .I/C O/G

P CHL P CHL1 20 2 49

.I/C O/GP CHL P CHL1 49 3 35

1

2

34

12

34

5

12

1

2

3

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1001010110F010101001F

Virtual Channel

Connect B Chl No.1

Connect C Chl No.2

Routing TableI/c O/g

P Chl P Chl

AC

BA

B

C

A 1 B xyC2A

FF 0110

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Permanent Virtual Circuit-PVC

4 3 2 1

2 1

2 1

2 1

21

1234

4 3 2 1

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Switched Virtual Circuit-SVC

4 3 2 1

1234

4 3 2 1

2 1

2 1

2 1

21

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Connectionless Data Communications• A path is not established before actual data transfer.• All packets do not take the same path• Routing decision is taken on the arrival of every packet at every node.• Every packet contains the full destination address.• No disruption in communication if link fails during data transfer and

an alternate path exists.• Quality of service is not guaranteed.• Packet can follow different path• Packet can arrive out of sequence at destination• Example Internet

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Connectionless Data Communications

Packet 1

Packet 2

Routing TableDest. Next Hop

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Datagram Approach

1

24

3

22

41

3 41

1 131

4

4 3 2 1

2 1

1432

31

12

24

2

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