Time-division multiplexing (TDM) is a type of digital multiplexing in which two or more apparently simultaneous channels are derived from a given frequency spectrum, i.e., bit stream, by interleaving pulses representing bits from different channels.
In some TDM systems, successive pulses represent bits from successive channels. In other systems different channels take turns using the channels for a group of successive pulse-times (a so-called "time slot") eg voice channels in E1/T1 systems.
What distinguishes coarse time-division multiplexing from packet switching is that the time-slots are pre-allocated to the channels, rather than arbitrated on a per-time slot basis.
Uses of time-division multiplexing:
• The RIFF (WAV) audio standard interleaves channel data on a per-sample basis
• The PDH and SDH network transmission standards
• The GSM telephone system
• The left-right channel splitting in use for Stereoscopic Liquid Crystle shutter glasses
• Multichannel digital audio processing sub-systems, notably the Motorola 56000 based hardware acceleration used in Pro Tools
Time Division Multiplexing (TDM) is the means by which multiple digital signals (or analogue signals carrying digital data) can be carried on a single transmission path by interleaving portions of each signal in time. Interleaving can be done at bits or blocks of bytes. This enables digitally encoded speech signals to be transmitted and switched optimally within a circuit-switched network. This article consists of two sections, namely, Transmission using TDM and Synchronous Digital Hierarchy (SDH). The first section examines the basic principles underlying TDM, while the second section discusses how SDH is used to switch TDM frames.
TDM was a synchronous technique invented during World War II to encrypt transatlantic radio conversation between Churchill and Roosevelt. By the early '60s, engineers from Bell Labs had developed the first T1 Channel Banks, which combined 24 digitised voice calls over a 4-wire copper trunk between Bell central office analogue switches. A channel bank sliced a 1.544 Mbit/s digital signal into 8,000 separate frames, each composed of 24 contiguous bytes. Each byte represented a single telephone call encoded into a constant bit rate signal of 64 Kbit/s. Channel banks used a byte's fixed position (temporal alignment) in the frame to determine which call it belonged to .
Transmission using Time Division Multiplexing (TDM)
In circuit switched networks such as the Public Switched Telephone Network (PSTN) there exists the need to transmit multiple subscribers’ calls along the same transmission medium . To accomplish this, network designers make use of TDM. TDM allows switches to create channels, also known as tributaries, within a transmission stream . A standard voice signal has a bandwidth of 64 kbit/s, determined using Nyquist’s Sampling Criterion . TDM takes frames of the voice signals and multiplexes them into a TDM frame which runs at a higher bandwidth. So if the TDM frame consists of n voice frames, the bandwidth will be n*64 kbit/s .
Each voice frame in the TDM frame is called a channel or tributary. In European systems, TDM frames contain 30 digital voice frames and in American systems, TDM frames contain 24 digital voice frames . Both of the standards also contain extra space for signalling and synchronisation data .