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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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