April 2000
AN215 : ADVANCED NETWORKING

QUESTION 1 (Compulsory)

Total Marks: 30 Marks

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Solutions and allocated marks are indicated in green.
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Question 1

Please do not award any half marks

(a)Describe two reasons for using layered protocols.[4 marks ]
Any two of the following, or similar Vendor Independence (1) -hardware/software at different layers can be purchased from different suppliers (1)
Separation of concerns (1) -high level software does not need to know the details of underlying layers (1)
Re-Use of components (1) -network protocol does not need to be built in to each Application (1)

(b)Describe two advantages of using a ring topology for a network.[2 marks ]
permits the use of mixed media (1) connection simplicity (1)
Other valid answers should receive credit.

(c)What is noise,in the context of data communications?Explain two ways in which noise may arise.[4 marks ]
Noise is unwanted energy from sources other than the transmitter (1).Noise is a problem because it may mask the original data (1),wipe out suprious bits (1) and create spurious messages (1)

(d)Name and brief3y describe the 3 stages in circuit switching.[3 marks ]
Call setup (1) data transfer (1) call termination (1)
For full marks, an explanation of each step must be given. If the three steps are
merely named, award (1); if an inadequate explanation is given, (2).

(e)What are the components of the RS232-C standard?Briefly describe each one. [6 marks ]
Award marks for any of these points:
Mechanical specification (1) 25-pin connector organized in 2 rows (1).
Functional specifications (1) Grouped into 4 groups-data,control,timing and ground. Details of each group and pin identification (1).
Electrical specifications (1).Voltage greater than +4V is classified as binary ’0 ’while voltage less than -3V is binary ’1 ’.maximum distance 15M and data rate of 20kbps (1).
Procedural Specification (1).Specifies the sequence of events that occur between DCE and DTE in order to transmit or receive data (1).

(f)Of which layer of the OSI reference model is the RS232-C specification a part? [1 mark ]
The physical layer (1)

(g)Briefly describe one encryption technique (include a suitable example).[3 marks ]
Either:
Transposition Cipher (1).Order of letter are changed but the letters remain the same (1).Appropriate example (1)
or:
Substitution Cipher (1).A group of letters are substituted by another group of letters. (1).Appropriate example (1).

(h)Delay distortion is a type of network fault.Explain what this is.[2 marks ]
The propagation velocity tends to be greater near the center frequency of a band and decreases towards the edges of the band (1) causing the different components of a signal to arrive at the destination at different times (1)
Award credit for other relevant points.

(i)Why is flow control necessary in data communications?[2 marks ]
Flow control is needed because the sending node and the receiving node may operate at different speeds (1).It is to prevent a fast sender from sending too may packets to the extent of causing the buffers at the receiver to over flow (1)
Award credit for other relevant points.

(j)Message switching is prone to ’hacking ’.Why is this?Suggest possible solutions to combat this type of risk.[3 marks ]
Since the entire message is switched from node to node,a hacker may be hack a particular and gain access to the entire message (1).One way is to encrypt the message (1),here only the intended receiver is able to decrypt the message (1).
Award credit for other relevant points.