Tecnical Notes

Multiplexing Techniques in PSTN

Multiplexing
Frequency Division Multiplexing (FDM)
Time Division Multiplexing

Standardization
Pulse Code Modulation (PCM)
PDH
SDH

1. Multiplexing
Multiplexing allows the transmission of multiple communications over a single line. A special device called multiplexer combines the incoming signals into a single signal to be transmitted over, for instance, an optical fibre or a coaxial cable. The single signal at the end of the transmission is demultiplexed: each original signal is separated and sent where appropriate.
1.1 Frequency Division Multiplexing

Frequency-division multiplexing (FDM) is a scheme in which numerous signals are combined for transmission on a single communication line or channel. Each signal is assigned a different frequency (sub channel) within the main channel.

Example

In the case of telephony, a single telephone circuit has a bandwidth of 4kHz. If we consider 6 telephone circuits, they can be multiplexed onto a single carrier with a bandwidth of 24 kHz. The signal in each circuit is combined with a different carrier frequency (f ) by 4 kHz. A different carrier frequency is used for each circuit. (Figure 1)

It is now possible to multiplex composite 24kHz signals, with 4 new carriers, to form a higher order signal with a bandwidth of 96kHz. This is shown in the stage 2 of the figure1.
In this way it is theoretically possible to continue multiplexing by adding more and more multiplexing stages. Later, you will see that this characteristic is shared by PDH and SDH, where the final limitation is the bandwidth of the transmission medium.