Explain TDM technique with relevant diagram.

Explain TDM technique with relevant diagram. 
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Answer :

TIME DIVISION MULTIPLEXING (TDM): TDM is a digital multiplexing technique in which many signals are transmitted for very short time (time slot) over common transmission channel. Here each signal can utilize the entire bandwidth of the channel. Figure 5.6 illustrates the concept of TDM. Here each signal will be transmitted for a short duration of time. One cycle or frame is said to be complete when each time slot is dedicated to each signal. With n input signals (transmitting devices), each frame has n time slots, with each slot allocated for carrying data from a specified device. The TDM signal in the form of frames is transmitted on the common communication medium. TDM can be used to multiplex analog or digital signals but it is suitable for digital signal multiplexing. 

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BLOCK DIAGRAM OF TDM SYSTEM: TDM is a digital multiplexing process that can be applied when the data capacity of the transmission medium is greater than the data rate required by the transmitting and receiving devices. In such cases, multiple transmissions can occupy a single link by subdividing them and interleaving the portions. Figure 5.7 shows the block diagram of TDM system consisting of four channels.


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TDM TRANSMITTER:

Four input signals, all band-limited to fx by the input filters (LPF) are sequentially sampled at the transmitter by the rotary switch or commutator. The switch makes fs revolutions per second and extracts one sample from each input during each revolution.

The output of the switch is a PAM waveform containing samples of the input signals periodically interlaced in time. The samples from adjacent input message channels are separated by Ts/M where M is the number of input channels.

A set of M pulses containing one sample from each of the M input channels is called a frame.

TDM RECEIVER:

At the receiver, the samples from the channel are separated and distributed by another rotary switch called as a distributor or de-commutator.

The samples from each channel are filtered to produce the original message signal.

The rotary switches at the transmitter and receiver are usually electronic circuits that are carefully synchronized. There are two levels of synchronization in TDM:

1. Frame Synchronization

2. Sample (or word) synchronization

Frame synchronization is necessary to establish the beginning of each frame and sample (or word) synchronization is necessary to properly separate the samples within each frame.

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