December
1998 QUESTION 1 (Compulsory) Total Marks: 20 Marks |
Click here to access other
questions
SUGGESTED SOLUTIONS
|
| (a) | Explain briefly with the aid of diagram the following LAN topologies | [6] |
| (i) Bus | ||
| Maximum of
(2 marks): Diagram (1 mark). Each node attached (or tapped) to the transmission medium (1 mark). Medium access can either be by contention or by token control (1 mark). Other relevant explanation (1 mark).
|
||
| (ii) Ring | ||
| Maximum of (2 marks): Diagram (1 mark). Consists of a series of nodes connected by a unidirectional transmission link to form a closed loop (1 mark). Each node listens for bits transmitted to it from the preceding node and then repeats them to the next node (1 mark). Other relevant explanation (1 mark).
|
||
| (iii) Star | ||
| Maximum of
(2 marks): Diagram (1 mark). Uses a central controller node as the hub of the network (1 mark). All communications between nodes must pass through the central node (1 mark). Other relevant explanation (1 mark).
|
||
| (b) | Frequency division multiplexing (FDM) and time division multiplexing (TDM) are two channel allocation techniques. Briefly describe one of these, and one other channel allocation technique. | [4] |
| Up to two marks for points from
ONE of (or other relevant points): FDM: Available bandwidth is divided into subchannels (1 mark) separated by guard bands (1 mark). Each is able to support transmission at a lower rate (1 mark). TDM: Time is divided into slots (1 mark); each user/process is allocated one or more slots (1 mark). Up to two marks for points from ONE
of (or other relevant points): CDM: Code division multiplexing. Digital multiplexing based on spread-spectrum theory of communication (1 mark). User is assigned an identification code which can be a distinct sequence (pattern) of amplitudes of frequencies (1 mark). Receiver knows in advance how the transmitter will spread the frequency spectrum; acquires the signal and continues to track the transmitted pattern (1 mark).
|
||
| (c) | What are the two sub-layers that make up the data-link layer? | [2] |
| The Medium
Access Cotrol sublayer (MAC sublayer) (1 mark). The Logical Link Control (LLC) (1 mark).
|
||
| (d) | Briefly describe one encryption technique (include a suitable example). | [3] |
| Either: | ||
| Transportation Cipher (1 mark). The order of the letters is changed but the letter remain the same (1 mark). Appropriate example (1 mark) | ||
| or: | ||
| Substitution Cipher (1
mark). A group of letters are substituted by another group of letters. (1 mark).
Appropriate example (1 mark).
|
||
| For parts (e) - (i), write down the letter of the appropriate answer in each case. | ||
| (e) | At which layer of the OSI model do bridges
operate?
|
[1] |
| (f) | Which device would allow a LAN to access
different devices on a WAN over a public network?
|
[1] |
| (g) | IEEE has defined a standard for Token-bus
networks. Which of the following defines this standard?
|
[1] |
| (h) | Which of the following access control methods is
an example of contention technique?
|
[1] |
| (i) | Which of the following types of transmission
medium has the fastest data-rate?
|
[1] |