August 1999
AN215 : ADVANCED NETWORKING

QUESTION 2

Total Marks: 20 Marks

 Click here to access other questions

SUGGESTED SOLUTIONS
Solutions and allocated marks are indicated in green.
Return to Question 2
Channel allocation is a procedure of dividing the communication facility (the media) in a manner that maximizes the channel capacity. The allocation procedure is independent of the media access control technique. Access Control deals with how to regulate the use of a shared medium.

 
(a) Explain what is meant by multiplexing in the context of data communications. [2]
Multiplexing is a technique to allow several low speed/bandwidth transmissions to share a common high speed/bandwidth transmission medium.

 
(b) How does the statistical time division multiplexing (Statmux) make it possible that twelve 1200bps terminals could share a composite line with a rate of only 4800bps? What line capacity would a synchronous TDM multiplexer need in order to support the same terminals? [4]
Synchronous TDM would need 14,400bps.

Statmux assigns a time slot to a station only when the station has something to transmit. Terminal traffic will tend to be bursty, so each terminal is an average only using a small fraction of its capacity. Statmux exploits this, to use a much lower bandwidth than might appear necessary.

 
(c) Briefly explain Code Division Multiplexing (CDM). [4]
Code Division multiplexing is based on the spread spectrum theory. There are two categories - direct sequence spreading(DS) and frequency hopping(FH). The signal is encoded with an identification code that only the receiver recognizes. Receiver knows in advance how the sender will spread the frequency spectrum and acquires the signal and continues to track that signal pattern.

 
(d) What is the full form of the acronym CSMA/CD? What media access technique does CSMA/CD employ? Briefly explain this technique. [6]
Carrier Sense Multiple Access with Collision Detection.

It employs the contention technique for media access. All stations contend for time on the medium transmitting whenever the medium is idle. There is no control to determine whose turn is it to transmit. When a station is ready to transmit, it places a frame onto the shared medium. It also monitors the medium to check whether its frame is colliding with another. If so, it signals that a collision has occurred and backs off for a random interval.

 
(e) Explain why random backoff is important in the media access technique described in part (d). [4]
When two stations' frame collide, both must back off before attempting retransmission. If both back off by the same amount, collision will continue to happen indefinitely. To overcome this, a random back-off is used, so that the probability of collision is decreased.