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Please do not award any half marks
(a)Explain the term multiplexing.[2
marks ]
Multiplexing is a technique hereby several
low speed lines are combined into a high speed line (1) for long distance
transmission (1).
(b)Explain the concept of Time Division
multiplexing.What is meant by statistical time division multiplexing?What
kind of device would serve this purpose?What is the drawback of using
this technique?[6 marks ]
TDM is here the time of the channel is shared
(1).Every user is allocated a time slot on a rotational basis.During
this time the user may send data after hich control is passed to another
user (1) .Statistical TDM allocates time slots to only active stations
(1), hence eliminating the problem of unused time slots(slots allocated
to stations ho have nothing to transmit),therefore results in better
bandwidth utilization (1) .
Award credit for other relevant points.
The device is known as Statmux (1)
The drawback of this technique is the additional addressing overhead
information needed to identify the owner of a particular time slot
(1)
(c)What is meant by the term guard band
?Explain why guard bands are needed.
[3 marks ]
Guard bands are unused bandwidth between
the sub-channels (1) created by frequency division multiplexing (1).They
are needed to provide adequate separation between frequency channels
(1)
(d)Ten 9600bps lines are to be multiplexed
using TDM.Ignoring verhead bits,what is the total capacity required
by synchronous TDM?
Assuming that we wish to limit average line utilization to 0.8,and
assuming each line is busy 50%of the time,what is the capacity required
by statistical TDM?[4 marks ]
Synchronous TDM:9600 ×10 =96 kbps (1)
Statistical TDM:9600 ×10 ×(0.5/0.8) =60 kbps (3)
[Marks for Statistical TDM broken down as:(1) for re-using 9600 ×10
;(1) for
multiplying by 0.5;(1) for dividing by 0.8]
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