August 1999
AN215 : ADVANCED NETWORKING

QUESTION 5

Total Marks: 20 Marks

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The network layer may provide services using packet, message or circuit switching.

 
(a) What is the purpose of these switching techniques? [2]
Switching techniques are used because in most network it is not possible to arrange point-point communications between all nodes , so data for various nodes must have share common communication paths.

 
(b) Describe the operation of message switching. [6]
There is no dedicated path between the sender and receiver. The entire message is routed from node to node. It works by a store and forward mechanism. The receiving node must receive the whole message before it can attempt to send the message to the next node. If any errors occur during transmission, the sending router will have to re-send the entire message. It has a relatively low overhead as there is only one header and one trailer for the entire message. Subject to various forms of delay. Not suitable for interactive traffic. Low security as routers can be hacked into and the entire message would be available to the hackers.

 
(c) There are 2 forms of packet switching. What are they called? What are the differences between them? [8]
Datagram and Virtual Circuit.
Issue Datagram Virtual Circuit
Circuit setup Not needed Required
Routing Each packet routed independently Route fixed during set-up, all packet follow this route. Each packet contains only a short VC identifier.
Addressing Each packet has to contain full source and destination addresses Entire path has to be re-established.
Call termination Not needed (no overhead) Every router has to delete the entry of the VC number (overhead).

 
(d) The Internet Protocol (IP) uses one of these switching techniques. Which one is it, and why is this technique suited to the Internet? [4]
IP uses the datagram approach (or 'packet switching' ).

Packet switching is the most adaptable -- its does not commit resources for any particular connection. The datagram approach is most well-suited to the possible failure of arbitrary links/routers etc. . Message switching would be inappropriate for interactive use.