• Multiplexing is the process of simultaneously transmitting two or more individual signals over a single communication channel. due to multiplexing it is possible to increase the number of communication channels so that more information can be transmitted.

• The typical applications of multiplexing are in telemetry and telephony or in the satellite communication.

• Basic types of multiplexing.

1] Frequency division multiplexing (FDM)

2] Time division multiplexing (TDM)

3] Wave length division multiplexing (WDM)

Block diagram of two channel PCM-TDM system.

• The modulating signals are passed through band pass filters, the outputs of filters are given to sample and hold circuits.

• The sample and hold circuit gets a sample of input signal and hold it for predefined duration.

• The pulse output of sample and hold circuit is given to analog to digital converter.

• The digital output is in parallel form and this parallel digital output is given to parallel to serial converter.

• The serial output is given to one of the inputs of the TDM multiplexer.

• The multiplexer, multiplex the two PCM channel on time AXIS.

• The PCM code for each channel occupies a fixed time slot within the total TDM frame.

• In two channel system, half frame time is given to channel-1 and remaining half time is given to channel-2.

• Total 16 bits are to be transmitted per frame thence, the line speed at the output of the multiplex is,

$\frac{2 \ Channels}{Frame} \times \frac{8000 \ frames}{second} \times \frac{8 \ bits}{channels} = 128 \ kbps$