April 2000
AN215 : ADVANCED NETWORKING

QUESTION 2

Total Marks: 15 Marks

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Please do not award any half marks
The network layer may provide services using packet switching,message switching or circuit switching.

(a)What is the purpose of these switching techniques?[2 marks ]
They are used because in most networks it is not possible to arrange point to point communications between all possible nodes (1),so data for various nodes must share the same communications path (1).
Award credit for other relevant points.

(b)Describe the operation of message switching.[6 marks ]
The entire message contains one header and trailer information(overhead miminal).(1) The path is not dedicated (1),so the message is then sent from node to node (store and forward)until it reaches its .nal destination (1).Each node must have large enough buyers to store the message (1).Message switching is also subjected to delays (1) and therefore is not suitable for interactive traffic.(1).
Award credit for other relevant points.

(c)Is message switching suitable for interactive traffic?Explain your answer [3 marks ]
No (1).Subjectes to delays such as queueing delay (1) and propagation delay (1).

(d)A 4096 byte message,which has to be sent.You have a choice between using a virtual circuit (VC)or a datagram network.The addressing overheads involved in the VC is a 2 byte VC identifier per packet that it takes 2 seconds to establish a logical path.On the other hand,the overheads in the datagram packet is 16 bytes.You may ignore any other overheads.Assuming it takes ne second to transfer one packet across the network.Which type of network is more efficient Justify your choice.State any assumptions you wish to make.(Show all workings) [4 marks ]
VC
4096 /(256 -2 bytes (header))
=16.1
=17 packets
17 *1 secperpacket
=17 seconds +2 secondssetuptime
=19 seconds
datagram
4096 /(256 -16 bytes (header))
=17 .
=18 packets
18 *1 sec
=18 seconds
[therefore the datagram network would be more efficient ].
Award up to (2) for each calculation. Do not award a mark for the conclusion unless justified by a calculation. Other methods of calculation may receive credit.