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.
|