Dec 1998 AN215: Advanced Networking (Question 2)

(a)

Briefly describe an advantage and a disadvantage of message switching.

[2]

One mark for an advantage and one mark for a disadvantage:
Advantage: it does not require a dedicated path (1mark). Award credit for any other acceptable advantage provided it is justified (1 mark).
Disadvantage: For large messages, lots of buffer space is required at the switching nodes (i.e. routers) (1 mark). If an error occurs the whole message has to be retransmitted (1 mark)

(b)

The message switching technique is subject to two kinds of delays. What are these delays?

[2]

Propagation delay (1 mark). Queueing delay (1 mark).

(c)

Briefly compare circuit switching and packet switching.

[6]

Item	Circuit switched	Packet switched
Dedicated path	YES	NO
Bandwidth available	FIXED	DYNAMIC
Bandwidth wastage	YES	NO
Store and forward transmission	NO	YES
Call Setup	REQUIRED	NOT NEEDED
Each packet follows the same route	YES	NO

(1 mark) for each difference.

(d)

Packet switching may use the datagram technique or the virtual circuit technique.

The first form does not guarantee that packets will be delivered in order, and requires that they be acknowledged. Explain why this is the case, and why the same concerns does not apply to the virtual circuit approach.

[6]

Award marks up to (6 marks) for points from the following, or other valid, clear points.
In the datagram technique, each packet is treated independently (1 mark). Each of these is routed individually, depending on the current network conditions (1 mark). Due to possible loss of packets, an acknowledgement is needed from the recipient (1 mark). Re-sequencing of packets is needed at the recipient's end (1 mark).
A virtual circuit (VC) is a dedicated route set up to transfer all of the packets of a particular message (1 mark). All packets follow the same route to the destination (1 mark). All packets arrive in sequence at the destination (1 mark). When the connection is released the VC will also be terminated (1 mark).

(e)

Suppose you have to send a 2048-byte message. You may use a virtual circuit which has a set-up time of 2 seconds and a transmission rate of 256 bytes per second. Alternatively, you could use a packet-switched network where each packet may contain up to 256 bytes in total, and on average one packet takes one second to pass through the network.

Decide which will be the more efficient approach. Show your working. You will need to make an additional assumption in order to do the calculation. State your assumption carefully.

[4]

In packet-switching, assume 4 bytes for the source and 4 bytes for the destination addresses. This leaves 248 bytes in each packet for the data (1 mark). 2048/248 = 8.3, so 9 packets will be required for the packet-switching (1 mark); it will take 9 seconds. 2048/256 = 8 seconds, plus 2 for set-up, for the virtual circuit (1 mark). Thus packet-switching will be more efficient (1 mark).
Other assumptios about source and destination fields will give different answers. Allow full credit if calculation is correct. If no allowance for source/destination is made, then allow a maximum of (2 marks), if the calculations are correct.