August 1997
CS202: COMPUTER ARCHITECTURE AND DATA COMMUNICATION

QUESTION 2

Total Marks: 20 Marks

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2. (a) A number of different transmission media can be used to implement a network.
(i) A twisted pair of wires is the simplest form. What data rate can typically be achieved over such a connection when it is used in a LAN? [2]
20 Mbits per second [2]
(ii) Alternative cable connections are co-axial cable and fibre optic. Write down one advantage and one disadvantage which twisted pair wire has over these other media. [2]
Advantage :
inexpensive or easy to install. [1]
Disadvantage :
relatively low bandwidth, dosen't pass high frequencies well, no shielding
(iii) List two wireless transmission media, giving a data rate for each. Which of these would be ideal for providing data communications between two offices of a company within one city? What condition must apply for this choice of medium to be suitable? Explain your answers. [6]
Terrestrial microwave, [1]
250M bits per second. [1]
Satellite, [1]
50M bits per second. [1]
Choose terrestrial microwave [1]
(provided there is line-of-sight) [1]
higher bandwidth, lower initial cost.
(b) (i) Explain the difference between digital and analog signals, giving examples of how each arises. [3]
analog varies continuously within a range of values; [1]
e.g. telephone, radio, TV [0.5]
digital involves two discrete voltages denoting binary 0s and 1s [1]
e.g. binary data [0.5]
represented as sequence of voltage pulses.
(ii) What is a modem? [2]
Modem : takes binary electrical pulses and converts or modulates signal so that it can be transmitted [1]
(iii) Describe, with the aid of diagrams, two methods of analog modulation. [5]
For any two of the following; [1]
for a suitable diagram (as in the study guide, pages 9-3 - 9-5), and [1.5] for explanation
1. Amplitude modulation (AM) amplitude or height or sine wave is varied to transmit ones (higher) and zeros (lower).
2. Frequency modulation (FM) frequency of carrier is used to transmit the ones and zeros - lower frequency for zeros, higher for ones.
3. Phase modulation (PM) data is transmitted by changing phase of sine wave, e.g by leaving wave unaltered for zero, and inserting a 180 phase shift for a one.