Time Division Multiplexing (TDM)

Multiplexing is a technique that allows multiple logical signals to share a single physical channel. The two common types of multiplexing are time-division multiplexing (TDM) and statistical time-division multiplexing (STDM). In this article, I am going to discuss the time-division multiplexing.

Time Division Multiplexing (TDM) transmits more than one streaming digital signal over a common channel. Time Division Multiplexing differentiates between synchronous and asynchronous multiplexing.

Incoming signals are divided into equal fixed-length time slots. The multiple signals are transmitted over a shared medium. The signals are de-multiplexed and reassembled into their original format at the receiving end. Time-division multiplexing (TDM) is a digital circuit-switched technique. Before multiplexing, each telephone call required its physical medium. Bell Laboratories invented TDM to decrease the physical medium needs for every telephone call.

TDM is working in the physical layer. It is independent of the Layer 2 protocol used by the input channels. It does not care about the nature of the information multiplexed to the output channel.

Figure 1 illustrates the function of the multiplexer (MUX). The multiplexer at the transmitting end accepts three separate signals from three different sources and breaks each signal into segments. After breaking the signals into segments, the MUX puts each segment into a single channel by inserting each segment into a time slot.

A MUX at the receiving end receives the stream and reassembles it into three separate output signals based on the arrival timing of each bit. A MUX keeps every bit of interleaving number and sequence from each specific transmission so that they can be quickly and efficiently reassembled into their original form when received. Byte interleaving does the same functions, but eight bits are in each byte, so the process requires a bigger and longer time slot. The T1/E1 and ISDN telephone lines are common examples of synchronous TDM. In the synchronous method, single data streams are classified in defined time segments for subsequent transmission in a predetermined order.