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The network layer may provide services
using packet switching,message switching or circuit switching.
(a)What is the purpose of these switching
techniques?[2 marks ]
(a) Multiplexing is a technique where several
low speed lines are combined into a high-speed line [1] for long distance
transmission[1].
(b)Describe the operation of message
switching.[6 marks ]
(b) Time Division Multiplexing divides the
bandwidth amongst the users
based on time slots, where each user is given a time slot to send
his
frame after which the control is passed to the next user[1].
synchronous multiplexing[1]- all stations attached to the multiplexor
are
given time slots on a rotational basis regardless of whether the station
has data to send or not.
diagram [1]
statistical multiplexing- only active stations attached to the statmux
are
given time slots on a rotational basis [1].
Idle time slots
diagram [1]
(c)Is message switching suitable for
interactive traffic?Explain your answer [3 marks ]
(c) CDM is a digital multiplexing technique
based on the spread spectrum
theory of communications[1]. A signal is assigned a unique id code
that
only the user recognizes. The receiver will then obtain the signal
and
continue to track it[1]
CDM offers better privacy in transmission[1]
Used in wireless LANs [1] Also used in digital audio broadcasting.
(d)A 4096 byte message,which has to be
sent.You have a choice between using a virtual circuit (VC) or a datagram
network.The addressing verheads involved in the VC is a 2 byte VC
identifier per packet that it takes 2 seconds to establish a logical
path.On the other hand,the overheads in the datagram packet is 16
bytes.You may ignore any other overheads.Assuming it takes one second
to transfer one packet across the network.Which type of network is
more efficient Justify your choice.State any assumptions you wish
to make.(Show all workings)[4 marks ]
(d) Synchronous TDM => 9600bps X 10 =
96kbps [2]
Statistical TDM => 9600 X 10 X (0.5/0.8) = 60kbps [2]
In each case, award [1] for setting up the calculation correctly,
and [1] for
getting the right answer.
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