What is Time Division Multiplexing (TDM) with regards to Transmissions?

In public switched telephone network (PSTN) and other circuit switched networks there is the need to transmit multiple subscribers' calls down the same transmission path. To accomplish this task; network designers make use of time division multiplexing (TDM). This allows switches to create channels called tributaries within a transmission stream. A typical digital signal 0 (DS0) voice signal with a data rate of 64 kbits per second, this is determined using Nyquist's sampling. Time division multiplexing takes frames of each of the voice signals and multiplexes them into a TDM frame that runs at a much higher bandwidth. For example: if a TDM frame contains ‘n' voice frames, the bandwidth will be: n x 64 kbits per second.

Each sampled voice timeslot in the time division multiplexing frame is known as a channel. In Europe, TDM frames are made up of 30 digital voice channels, whereas in America , they are made up of 24 channels. Both systems also have extra bits for signalling as well as bits for synchronisation.

Multiplexing any more than 30 or 24 digital voice channels is known as higher order multiplexing. Higher order multiplexing is achieved by multiplexing the time division multiplexing frames. For instance, European 120 channel TDM frame is created by multiplexing 4 standard 30 channel TDM frames. For each higher order of multiplexing, 4 time division multiplexing frames from the immediate lower order are then combined.