December 1999
AN215 : ADVANCED NETWORKING

QUESTION 4

Total Marks: 15 Marks

• Question 1
• Question 2
• Question 3
• Question 5
• Cover Page

SUGGESTED SOLUTIONS
Solutions and allocated marks are indicated in green.
Return to Question 4

Give two advantages of using coaxial cables rather than twisted pair.
Wider bandwidth (1 mark),higher data rate (1 mark).

Besides noise,there is another transmission impairment,called attenuation .Explain what this is,why it is a problem,and how it may be overcome.
Award marks for these or other valid points:

Attenuation is the weakening of signal power with distance (usually)(1 mark). If the received signal is too weak,it will be hard to distinguish from noise (1 mark) and may be unable to be interpreted. A repeater (1 mark) can be used to overcome this problem, as it regenerates and boosts the receiving signal (1 mark).

In contention-based systems two hosts may broadcast messages simultaneously, which overlap with each other.The result will be loss of data in both messages.Is this a fault?Explain how the hosts recover from this situation.
No (1 mark),this is not a fault.The system is designed to work this way.The hosts detect that a collision has occurred,so they must retransmit the message (1 mark). Each one backs-off for a random interval,then tries again (1 mark).The process is repeated if necessary (1 mark).

Explain the main problem associated with parallel transmission.Why is this is not a problem in serial transmission?
In parallel transmission,separate cables carry separate bits of a character simultaneously (1 mark).Over large distances,the bits loose their timing relationship (1 mark)Thus bits from the same character arrive at different times and this results in wrong characters being received (1 mark).This problem is called skew (1 mark).

In serial transmission all bits are sent over one path contiguously,and so bits must retain their timing relationship (i.e they cannot overtake each other)(1 mark).