Aug 1997 AN215: Advanced Networking (Question 3) Suggested Solutions

3.	(a) Two approaches to the use of a shared (broadcast) medium are polling and contention.
		(i) Explain the concept of polling.	[5]
		Central controller gives each of the stations control of the communication facility according to the same strategy.	[1]
		The approach depends on the fact that most stations will have nothing to transmit, whereas those stations which do have data to transmit may have a substantial quantity, and need to transmit it quickly.	[1]
		The primary controller signals to each station in turn to determine if the secondary wishes to transmit or is ready to receive.	[1]
		In the first case, the station wishing to transmit will be given access to the shared medium for a period of time.	[1]
		An alternative is a 'balanced approach', where each station is a combined station; control of the link is distributed among them	[1]
		(ii) A simple contention-based scheme is Pure ALOHA. Explain how this works. [Your answer should include a careful explanation of why random backoff periods are needed.]	[5]
		A station wishing to transmit, puts its message onto the shared medium.	[1]
		It also listens to the medium, to see whether the message was transmitted without colliding with another station's message.	[1]
		*In the event that the message did collide with another, the station backs off* for a random period before trying again to transmit.**	[1]
		The random backoff is essential because if two stations' messages collided, and both delayed for the same time, then they would collide again next time they try to transmit, and so on, forever.	[2]

	(b) A very different approach to sharing the single medium is to use Statistical Time Division Multiplexing (STDM). Explain how STDM works, and why you would expect a higher utilisation rate when using STDM than using ALOHA. What additional overheads are associated with STDM?		[5]
	Statistical TDM entails an extra system---the multiplexer--- collecting messages from stations that wish to transmit.		[1]
	These are then transmitted on the shared medium in a time-sliced manner.		[1]
	Because the multiplexer controls the medium, no contention will arise, and so the utilisation rate can be higher.		[2]
	The multiplexer is an additional overhead; it may also need to buffer messages.		[1]

	(c) An alternative ALOHA scheme is called Slotted ALOHA. In this arrangement, time is divided up into discrete time slots, each slot corresponding to one message frame (all messages are the same size). The stations on the network are synchronized by having one special station emit a pip at the start of each interval, like a clock. A station must wait until the start of a slot before beginning transmission.		[5]
	Do you think that S9lotted ALOHA will have a different probability of collision from Pure ALOHA? If so, will it be higher or lower? Will the capacity of the network be increased or decreased? Explain your answer.
	Slotted ALOHA has a lower probability of collision,		[1]
	because messages can only collide at the start of transmission, rather than at any time.		[2]
	As a result, the network capacity is higher.		[2]
	[Answers which fail to give the correct answer, but give logical reasons, may gain up to [3], as appropriate.]