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Information about Time Division Multiplexing

Multiplex techniques
Circuit mode — for constant bandwidth
TDM | FDM | WDM | Polarization multiplexing | Spatial multiplexing (MIMO)
Statistical multiplexing — for variable bandwidth
Packet mode | Dynamic TDM | FHSS | DSSS | OFDMA
Related topics
Channel access methods | Media Access Control
Time-Division Multiplexing (TDM) is a type of digital or (rarely) analog multiplexing in which two or more signals or bit streams are transferred apparently simultaneously as sub-channels in one communication channel, but physically are taking turns on the channel. The time domain is divided into several recurrent timeslots of fixed length, one for each sub-channel. A sample, byte or data block of sub-channel 1 is transmitted during timeslot 1, sub-channel 2 during timeslot 2, etc. One TDM frame consists of one timeslot per sub-channel. After the last sub-channel the cycle starts all over again with a new frame, starting with the second sample, byte or data block from sub-channel 1, etc.

Application examples

  • The Plesiochronous Digital Hierarchy (PDH) system, also known as the PCM system, for digital transmission of several telephone calls over the same four-wire copper cable (T-carrier or E-carrier) or fiber cable in the circuit switched digital telephone network
  • The SDH and Synchronous Optical Networking (SONET) network transmission standards, that has superseded PDH.
  • The RIFF (WAV) audio standard interleaves left and right stereo signals on a per-sample basis
  • The left-right channel splitting in use for Stereoscopic Liquid Crystal shutter glasses
TDM can be further extended into the time division multiple access (TDMA) scheme, where several stations connected to the same physical medium, for example sharing the same frequency channel, can communicate. Application examples include:
  • The GSM telephone system

TDM versus packet mode communication

In its primary form, TDM is used for circuit mode communication with a fixed number of channels and constant bandwidth per channel.

What distinguishes time-division multiplexing from statistical multiplexing such as packet mode communication (also known as statistical time-domain multiplexing, see below) is that the time-slots are recurrent in a fixed order and pre-allocated to the channels, rather than scheduled on a packet-by-packet basis. Statistical time-domain multiplexing resembles, but should not be considered as, time division multiplexing.

In dynamic TDMA, a scheduling algorithm dynamically reserves a variable number of timeslots in each frame to variable bit-rate data streams, based on the traffic demand of each data stream. Dynamic TDMA is used in

History

For the SIGSALY encryptor of 1943, see PCM.

In 1962, engineers from Bell Labs developed the first D1 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.[1]

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.<ref name="hanrahn" /> A standard DS0 voice signal has a data bit rate of 64 kbit/s, determined using Nyquist’s Sampling Criterion.<ref name="hanrahn" />[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*64 kbit/s.<ref name="hanrahn" />

Each voice frame in the TDM frame is called a channel or tributary.<ref name="hanrahn" /> In European systems, TDM frames contain 30 digital voice frames and in American systems, TDM frames contain 24 digital voice frames.<ref name="hanrahn" /> Both of the standards also contain extra space for signalling (see Signaling System 7) and synchronisation data.<ref name="hanrahn" />

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

Synchronous Digital Hierarchy (SDH)

Plesiochronous Digital Hierarchy (PDH) was developed as a standard for multiplexing higher order frames.<ref name="hanrahn" /><ref name="ericsson" /> PDH created larger numbers of channels by multiplexing the standard Europeans 30 channel TDM frames.<ref name="hanrahn" /> 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:<ref name="hanrahn" /><ref name="ericsson" />
  • 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.<ref name="hanrahn" /><ref name="ericsson" /> It was developed to allow streams 1.544 Mbit/s and above to be multiplexed, so as to create larger SDH frames known as Synchronous Transport Modules (STM).<ref name="hanrahn" /> The STM-1 frame consists of smaller streams that are multiplexed to create a 155.52 Mbit/s frame.<ref name="hanrahn" /><ref name="ericsson" /> SDH can also multiplex packet based frames such as Ethernet, PPP and ATM.<ref name="hanrahn" />

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.<ref name="hanrahn" /> The most common SDH Networking functions are as follows:
  • SDH Crossconnect – The SDH Crossconnect 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.<ref name="hanrahn" />
  • 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.<ref name="hanrahn" />
SDH Network functions are connected using high-speed Optic Fibre. Optic Fibre uses light pulses to transmit data and is therefore extremely fast.<ref name="hanrahn" /> 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.<ref name="hanrahn" /><ref name="ericsson" /> This increases the speed and capacity of the link, which in turn reduces both unit and total costs.<ref name="hanrahn" />

Statistical Time-division Multiplexing (STDM)

STDM is an advanced version of TDM in which both the address of the terminal and the data itself are transmitted together for better routing. Using STDM allows bandwidth to be split over 1 line. Many college and corporate campuses use this type of TDM to logically distribute bandwidth.

If there is one 10MBit line coming into the building, STDM can be used to provide 178 terminals with a dedicated 56k connection (178 * 56k = 9.96Mb). A more common use however is to only grant the bandwidth when that much is needed. STDM does not reserve a time slot for each terminal, rather it assigns a slot when the terminal is requiring data to be sent or received.

References

1. ^ Carriedo, M.I.G, ATM: Origins and State of the Art, [1] last accessed 4rd November 2005.
2. ^ Hanrahan H.E., Integrated Digital Communications, School of Electrical and Information Engineering, University of the Witwatersrand, Johannesburg, 2005.
3. ^ Ericsson Ltd, Understanding Telecommunications, [2] last accessed April 11, 2006.


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

See also

Understanding Telecommunications has been removed from the Ericsson website but can be found at [3] .
In electronics, telecommunications and computer networks, multiplexing (short muxing) is a term used to refer to a process where multiple analog message signals or digital data streams are combined into one signal. The aim is to share an expensive resource.
..... Click the link for more information.
In telecommunications, a circuit switching network is one that establishes a dedicated circuit (or channel) between nodes and terminals before the users may communicate. Each circuit that is dedicated cannot be used by other callers until the circuit is released and a new
..... Click the link for more information.
In fibre-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes multiple optical carrier signals on a single optical fibre by using different wavelengths (colours) of laser light to carry different signals.
..... Click the link for more information.
Statistical multiplexing is a type of communication link sharing. In statistical multiplexing, a communication channel is divided into an arbitrary number of variable bit-rate digital channels or data streams.
..... Click the link for more information.
Packet switching is a communications paradigm in which packets (discrete blocks of data) are routed between nodes over data links shared with other traffic. In each network node, packets are queued or buffered, resulting in variable delay.
..... Click the link for more information.
Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver.
..... Click the link for more information.
In telecommunications, direct-sequence spread spectrum (DSSS) is a modulation technique. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated.
..... Click the link for more information.
Orthogonal Frequency Division Multiple Access (OFDMA) is a multi-user version of the popular OFDM digital modulation scheme. Multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual users as shown in the figure below.
..... Click the link for more information.
In telecommunications and computer networks, a channel access method or multiple access method allows several terminals connected to the same physical medium to transmit over it and to share its capacity.
..... Click the link for more information.
Media Access Control (MAC) data communication protocol sub-layer, also known as the Medium Access Control, is a part of the data link layer specified in the seven-layer OSI model (layer 2).
..... Click the link for more information.
A digital system is one that uses discrete values (often electrical voltages), representing numbers or non-numeric symbols such as letters or icons, for input, processing, transmission, storage, or display, rather than a continuous range of values (ie, as in an analog system).
..... Click the link for more information.
Pulse-amplitude modulation, acronym PAM, is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses.
..... Click the link for more information.
In electronics, telecommunications and computer networks, multiplexing (short muxing) is a term used to refer to a process where multiple analog message signals or digital data streams are combined into one signal. The aim is to share an expensive resource.
..... Click the link for more information.
In computer science, a call stack is a special stack which stores information about the active subroutines of a computer program. (The active subroutines are those which have been called but have not yet completed execution by returning.
..... Click the link for more information.
The Plesiochronous Digital Hierarchy (PDH) is a technology used in telecommunications networks to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems.
..... Click the link for more information.
T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and Korea.
..... Click the link for more information.
E-carrier system, which revised and improved the earlier American T-carrier technology, and this has now been adopted by the ITU-T. This is now widely used in almost all countries outside USA, Canada and Japan.
..... Click the link for more information.
Synchronous optical networking, is a method for communicating digital information using lasers or light-emitting diodes (LEDs) over optical fiber. The method was developed to replace the Plesiochronous Digital Hierarchy (PDH) system for transporting large amounts of telephone and
..... Click the link for more information.
Synchronous optical networking, is a method for communicating digital information using lasers or light-emitting diodes (LEDs) over optical fiber. The method was developed to replace the Plesiochronous Digital Hierarchy (PDH) system for transporting large amounts of telephone and
..... Click the link for more information.
The Resource Interchange File Format (RIFF) is a generic meta-format for storing data in tagged chunks.

It was introduced in 1991 by Microsoft and IBM, and was presented by Microsoft as the default format for Windows 3.1 multimedia files.
..... Click the link for more information.
This article or section needs sources or references that appear in reliable, third-party publications. Alone, primary sources and sources affiliated with the subject of this article are not sufficient for an accurate encyclopedia article.
..... Click the link for more information.
Time division multiple access (TDMA) is a channel access method for shared medium (usually radio) networks. It allows several users to share the same frequency channel by dividing the signal into different timeslots.
..... Click the link for more information.
Frequency-division multiplexing (FDM) is a form of signal multiplexing where multiple baseband signals are modulated on different frequency carrier waves and added together to create a composite signal.
..... Click the link for more information.
    For other uses, see GSM (disambiguation).
Global System for Mobile communications (GSM: originally from Groupe Spécial Mobile) is the most popular standard for mobile phones in the world.
..... Click the link for more information.
In telecommunications, a circuit switching network is one that establishes a dedicated circuit (or channel) between nodes and terminals before the users may communicate. Each circuit that is dedicated cannot be used by other callers until the circuit is released and a new
..... Click the link for more information.
Statistical multiplexing is a type of communication link sharing. In statistical multiplexing, a communication channel is divided into an arbitrary number of variable bit-rate digital channels or data streams.
..... Click the link for more information.
Packet switching is a communications paradigm in which packets (discrete blocks of data) are routed between nodes over data links shared with other traffic. In each network node, packets are queued or buffered, resulting in variable delay.
..... Click the link for more information.
scheduling algorithm is the method by which threads or processes are given access to system resources, usually processor time. This is usually done to load balance a system effectively.
..... Click the link for more information.
HIPERLAN (HIgh PErformance Radio LAN) is a Wireless LAN standard. It is a European alternative for the IEEE 802.11 standards (the IEEE is an American organization). It is defined by the European Telecommunications Standards Institute (ETSI).
..... Click the link for more information.
The IEEE 802.16 Working Group on Broadband Wireless Access Standards, which was established by IEEE Standards Board in 1999, aims to prepare formal specifications for the global deployment of broadband Wireless Metropolitan Area Networks.
..... Click the link for more information.


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