Multiplexing is the sending of a number of separate signals together, over the same cable or bearer, simultaneously and without interference.

There are generally two classifications. Time-division multiplexing, or TDM, is a method of separating, in the time domain, pulses belonging to different transmissions.

Use is made of the fact that pulses are generally narrow, and the separation between successive pulses is rather wide.

It is possible, provided that both ends of a link are synchronized, to use the wide spaces for pulses belonging to other transmissions.

On the other hand, frequency-division multiplexing, or FDM, concerns itself with combining continuous signals.

It may be thought of as an outgrowth of independent-sideband transmission, on a much-enlarged scale; i.e., 12 or 16 channels are combined into a group,5 groups into super group ,and so on ,using frequencies and arrangements that are standard on a worldwide scale.

Each group, super group or larger aggregate is then sent as a whole unit on one microwave link, cable or other broadband system.

When a large number of PCM signals are to be transmitted over a common channel, multiplexing of these PCM signals is required.

Figure shows the basic time division multiplexing scheme, called as the $T_1$ digital system.

This system is used to convey multiple signals over telephone lines using wideband coaxial cable.

Operation of the $T_1$ system

The operation of the PCM-TDM system is as follows

This system has been designed to accommodate 24 voice channels marked $S_1$ to $S_{24}$ .

Each signal is bandlimited to 3.3kHz, and the sampling is done at a standard rate of 8 kHz.

This is higher than the Nyquist rate.

The sampling is done by the commutator switch S$W_1$ .

• These voice signals are selected one by one and connected to a PCM transmitter by the commutator switch S$W_1$.

These voice signals are selected one by one and connected to a PCM transmitter by the commutator switch S$W_1$.

Each sampled signal is then applied to the PCM transmitter which converts it into a digital signal by the process of A to D conversion and companding, as explained earlier.

The resulting digital waveform is transmitted over a co-axial cable.

Periodically, after every 6000 ft, the PCM-TDM signal is regenerated by amplifiers called “repeaters”.

They eliminate the distortion introduced by the channel and remove the superimposed noise and regenerate a clean PCM-TDM signal at their output.

This ensures that the received signal is free from the distortions and noise.

At the destination the signal is companded, decoded and demultiplexed, using a PCM receiver.

The PCM receiver output is connected to different low pass filters via commutator switch S$W_2$ .

Synchronization between the transmitter and receiver commutators S$W_1$ and S$W_2$ is essential in order to ensure proper communication.