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, e.g., voice channels in a T1 system. In other systems different channels take turns using the channels for a group of successive pulse-times (a so-called "time slot").

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 PDH and SDH network transmission standards
• The GSM telephone system

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 ^[1]. This enables digitally encoded speech signals to be transmitted and switched optimally within a circuit-switched network ^[2]. 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.

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 ^[2]. To accomplish this network designers make use of TDM. TDM allows switches to create channels, also known as tributaries, within a transmission stream ^[2]. A standard voice signal has a bandwidth of 64kb/s, determined using Nyquist’s Sampling Criterion ^[2][3]. 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*64kb/s ^[2].

Each voice frame in the TDM frame is called a channel or tributary ^[2]. In European systems, TDM frames contain 30 digital voice frames and in American systems, TDM frames contain 24 digital voice frames ^[2]. Both of the standards also contain extra space for signalling and synchronisation data ^[2].

Multiplexing more than 24 or 30 digital voice frames is called Higher Order Multiplexing ^[2]. Higher Order Multiplexing is accomplished by multiplexing the standard TDM frames ^[2]. For example, a European 120 channel TDM frame is formed by multiplexing four standard 30 channel TDM frames ^[2]. At each higher order multiplex, four TDM frames from the immediate lower order are combined, creating multiplexes with a bandwidth of n x 64kb/s, where n = 120, 480, 1920, etc. ^[2].

Synchronous Digital Hierarchy (SDH)

Plesiochronous Digital Hierarchy (PDH) was developed as a standard for multiplexing higher order frames ^[2][3]. PDH created larger numbers of channels by multiplexing the standard Europeans 30 channel TDM frames ^[2]. This solution worked for a while; however PDH suffered from several inherent drawbacks which ultimately resulted in the development of the Synchronous Digital Hierarchy (SDH). The requirements which drove the development of SDH were as follows ^[2][3]:

• Be synchronous – All clocks in the system must align with a reference clock.
• Be service-oriented – SDH must route traffic from End Exchange to End Exchange without worrying about exchanges in between, where the bandwidth can be reserved at a fixed level for a fixed period of time.
• Allow frames of any size to be removed or inserted into an SDH frame of any size.
• Easily manageable with the capability of transferring management data across links.
• Provide high levels of recovery from faults.
• Provide high data rates by multiplexing any size frame, limited only by technology.
• Give reduced bit rate errors.

SDH has become the primary transmission protocol in most PSTN networks ^[2][3]. It was developed to allow streams 1.544Mb/s and above to be multiplexed, so as to create larger SDH frames known as Synchronous Transport Modules (STM) ^[2]. The STM-1 frame consists of smaller streams that are multiplexed to create a 155.52Mb/s frame ^[2][3]. SDH can also multiplex packet based frames such as Ethernet, PPP and ATM ^[2].

While SDH is considered to be a transmission protocol (Layer 1 in the OSI Reference Model), it also performs some switching functions, as stated in the third bullet point requirement listed above ^[2]. The most common SDH Networking functions are as follows:

• SDH Crossconnect – The SDH Crossconect is the SDH version of a Time-Space-Time crosspoint switch. It connects any channel on any of its inputs to any channel on any of its outputs. The SDH Crossconnect is used in Transit Exchanges, where all inputs and outputs are connected to other exchanges ^[2].
• SDH Add-Drop Multiplexer – The SDH Add-Drop Multiplexer (ADM) can add or remove any multiplexed frame down to 1.544Mb. Below this level, standard TDM can be performed. SDH ADMs can also perform the task of an SDH Crossconnect and are used in End Exchanges where the channels from subscribers are connected to the core PSTN network ^[2].

SDH Network functions are connected using high-speed Optic Fibre. Optic Fibre uses light pulses to transmit data and is therefore extremely fast ^[2]. Modern optic fibre transmission makes use of Wavelength Division Multiplexing (WDM) where signals transmitted across the fibre are transmitted at different wavelengths, creating additional channels for transmission ^[2][3]. This increases the speed and capacity of the link, which in turn reduces both unit and total costs ^[2].

References

Stallings, W., Data and Computer Communications, Prentice-Hall, New Jersey, Sixth Edition, 2000.

2. ⇧ Hanrahan H.E., Integrated Digital Communications, School of Electrical and Information Engineering, University of the Witwatersrand, Johannesburg, 2005.

3. ⇧ Ericsson Ltd, Understanding Telecommunications, http://www.ericsson.com/support/telecom/index.html, last accessed 22nd February 2005.

This article was originally based on a Federal Standard 1037C entry in support of MIL-STD-188.

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