August
1997 QUESTION 3 Total Marks: 20 Marks |
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| 3. | (a) Two approaches to the use of a shared (broadcast) medium are polling and contention. | ||
| (i) Explain the concept of polling. | [5] | ||
| (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] | ||
| (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] | ||
| (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. | |||