Page 87 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes FDM was the first multiplexing scheme that was widely used in network deployment. They are
still in use today and are used with analog transmission. However, Time Division Multiplexing
is preferred over FDM. The main disadvantage of FDM is the wastage of frequency resources as
it was dedicated to a particular channel for all time. This also puts an upper limit on the number
of channels.

6.2.2 Time Division Multiplexing (TDM)

In digital transmission, Time Division Multiplexing (TDM) and Code Division Multiplexing
(CDM) are widely used. TDM is a process to merge data from several sources into a single
channel for communication over transmission media like telephone lines, microwave system
or satellite system. TDM is implemented in two ways. They are synchronous TDM and
asynchronous TDM. Asynchronous TDM is known as Statistical TDM (STDM). The synchronous
TDM techniques divides a single channel into time slots and each transmitting device is assigned
at least one of the time slots for its transmission as shown in Figure 6.4. Time slots are assigned
in such a way that each transmitting device gets its required share of the available bandwidth.
Because of this time-bandwidth multiplexing technique, TDMs are protocol insensitive and are
capable of combining various protocols onto a single high-speed transmission link. In other
words we can say that multiplexer allocates exactly the same time slot to each device at all times
whether the device is active or idle. Some devices, such as voice and video systems may require
more slots to ensure that data arrives at the distant link-end without becoming distorted from
slower data rates. These different time slots are grouped into frames. A frame consists of one
complete cycle of time slots. Alternatively Figure 6.4 explains more clearly the concept of TDM
in a data communication environment where three PCs are sharing the common circuit. The
packets generated by each PC are multiplexed on the common line as A1, B1, C1 and so on.
It is more flexible than the FDM. Unlike to FDM, the whole bandwidth for a certain amount of
time is provided to the user. All the users are using the same frequency but at a different time.
This time allotment may be varied as per the requirement and priority of the users’ services. In
the Figure 6.4 spaces between different time slots are shown, these are known as guard spaces in
time dimension. These are used to eliminate co-channel interference.

Figure 6.4: Time Division Multiplexing

t

C 1

C 2

C 3

C 4

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