multiplexing

[muhl-tuh-pleks]

multiplexing, in communication, technique whereby two or more independent messages, or information-bearing signals, are carried by a single common medium, or channel. When multiplexing is performed, two or more channels are combined into a single channel, or, in a process often called demultiplexing, a single channel is divided into several subchannels. Many different types of multiplexing are possible. One type is frequency-division multiplexing, in which a single frequency channel is subdivided into two or more subchannels, each of which can then carry a smaller range of frequencies than could the original channel. Frequency-division multiplexing is used in television broadcasting, when audio and video signals share a single channel; in stereophonic FM radio broadcasting, when two audio signals share a single channel; and in microwave transmission of long-distance telephone calls, when 60 or more conversations are carried by a single microwave beam. A second type of multiplexing is time-division multiplexing, in which successive small time intervals are used for the transmission of messages over a single channel. Time-division multiplexing is often used in the construction of digital computers. When information can be stored into or retrieved from the computer's memory at a much greater rate than it can be supplied or used by an external device such as a card reader, printer, or teletype terminal, several such low-speed devices can share a single multiplexed data channel.

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multiplexing

Process of transmitting multiple (but separate) signals simultaneously over a single channel or line. Because the signals are sent in one complex transmission, the receiving end has to separate the individual signals. The two main types of multiplexing methods are time-division multiplexing (TDM) and frequency-division multiplexing (FDM). In TDM (typically used for digital signals) a device is given a specific time slot during which it can use the channel. In FDM (typically used for analog signals) the channel is subdivided into subchannels, each with a different frequency width that is assigned to a specific signal. Optical-fibre networks can use DWDM (dense wavelength-division multiplexing), in which different data signals are sent in different wavelengths of light in the fibre-optic medium.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

Multiplexing

For multiplexing in electronics and signal processing, see Multiplexer.

In telecommunications and computer networks, multiplexing (known as muxing) is a term used to refer to a process where multiple analog message signals or digital data streams are combined into one signal over a shared medium. The aim is to share an expensive resource. For example, in telecommunications, several phone calls may be transferred using one wire.

The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred. A reverse process, known as demultiplexing, can extract the original channels on the receiver side.

A device that performs the multiplexing is called a multiplexer (MUX), and a device that performs the reverse process is called a demultiplexer (DEMUX).

Inverse multiplexing (IMUX) has the opposite aim as multiplexing, namely to break one data stream into several streams, transfer them simultaneously over several communication channels, and recreate the original data stream.

Categories of multiplexing

The two most basic forms of multiplexing are time-division multiplexing (TDM) and frequency-division multiplexing (FDM), both either in analog or digital form. FDM requires modulation of each signal.

In optical communications, FDM is referred to as wavelength-division multiplexing (WDM).

Variable bit rate digital bit streams may be transferred efficiently over a fixed bandwidth channel by means of statistical multiplexing, for example packet mode communication. Packet mode communication is an asynchronous mode time-domain multiplexing, which resembles but should not be considered as time-division multiplexing.

Digital bit streams can be transferred over an analog channel by means of code-division multiplexing (CDM) techniques such as frequency-hopping spread spectrum (FHSS) and direct-sequence spread spectrum (DSSS).

In wireless communications, multiplexing can also be accomplished through alternating polarization (horizontal/vertical or clockwise/counterclockwise) on each adjacent channel and satellite, or through phased multi-antenna array combined with a Multiple-input multiple-output communications (MIMO) scheme.

Relation to multiple access

A multiplexing technique may be further extended into a multiple access method or channel access method, for example TDM into Time-division multiple access (TDMA) and statistical multiplexing into carrier sense multiple access (CSMA). A multiple access method makes it possible for several transmitters connected to the same physical medium to share its capacity.

Multiplexing is provided by the Physical Layer of the OSI model, while multiple access also involves a media access control protocol, which is part of the Data Link Layer.

Application areas

Telegraphy

The earliest communication technology using electrical wires, and therefore sharing an interest in the economies afforded by multiplexing, was the electric telegraph. Early experiments allowed two separate messages to travel in opposite directions simultaneously, first using an electric battery at both ends, then at only one end. In 1874, the quadruplex telegraph developed by Thomas Edison transmitted two messages in each direction simultaneously, for a total of four messages transiting the same wire at the same time. Alexander Graham Bell was investigating a frequency-division multiplexing technique with his "Harmonic Telegraph", which led to the telephone.

Telephony

In telephony, a customer's telephone line now typically ends at the remote concentrator box down the street, where it is multiplexed along with the telephone lines for that neighbourhood or other similar area. The multiplexed signal is then carried to the central switching office on significantly fewer wires and for much further distances than a customer's line can practically go. This is likewise also true for digital subscriber lines (DSL).

Fiber in the loop (FITL) is a common method of multiplexing, which uses optical fibre as the backbone. It not only connects POTS phone lines with the rest of the PSTN, but also replaces DSL by connecting directly to Ethernet wired into the home. Asynchronous Transfer Mode is often the communications protocol used.

Because all of the phone (and data) lines have been clumped together, none of them can be accessed except through a demultiplexer. This provides for more-secure communications, though they are not typically encrypted.

The concept is also now used in cable TV, which is increasingly offering the same services as telephone companies. IPTV also depends on multiplexing.

Video processing

In video editing and processing systems, multiplexing refers to the process of interleaving audio and video into one coherent transport stream (time-division multiplexing).

In digital video, such a transport stream is normally a feature of a container format which may include metadata and other information, such as subtitles. The audio and video streams may have variable bit rate. Software that produces such a transport stream and/or container is commonly called a statistical multiplexor or muxer. A demuxer is software that extracts or otherwise makes available for separate processing the components of such a stream or container.

Digital broadcasting

In digital television and digital radio systems, several variable bit-rate data streams are multiplexed together to a fixed bitrate transport stream by means of statistical multiplexing. This makes it possible to transfer several video and audio channels simultaneously over the same frequency channel, together with various services.

In the digital television systems, this may involve several standard definition television (SDTV) programmes (particularly on DVB-T, DVB-S2, and ATSC-C), or one HDTV, possibly with a single SDTV companion channel over one 6 to 8 MHz-wide TV channel. The device that accomplishes this is called a statistical multiplexer. In several of these systems, the multiplexing results in an MPEG transport stream.

On communications satellites which carry broadcast television networks and radio networks, this is known as multiple channel per carrier or MCPC. Where multiplexing is not practical (such as where there are different sources using a single transponder), single channel per carrier mode is used.

In digital radio, both the Eureka 147 system of digital audio broadcasting and the in-band on-channel HD Radio, FMeXtra, and Digital Radio Mondiale systems can multiplex channels. This is essentially required with DAB-type transmissions (where a multiplex is called an ensemble), but is entirely optional with IBOC systems.

Analog broadcasting

In FM broadcasting and other analog radio media, multiplexing is a term commonly given to the process of adding subcarriers to the audio signal before it enters the transmitter, where modulation occurs. Multiplexing in this sense is sometimes known as MPX, which in turn is also an old term for stereophonic FM, often seen on stereo systems of the 1960s and 1970s.

Other meanings

In spectroscopy the term is used in a related sense to indicate that the experiment is performed with a mixture of frequencies at once and their respective response unravelled afterwards using the Fourier transform principle.

Multiplexing may also refer to a juggling technique where multiple objects are released from one hand at the same time.

In computer programming, it may refer to using a single in-memory resource (such as a file handle) to handle multiple external resources (such as on-disk files).

Some electrical multiplexing techniques do not require a physical "multiplexer" device, they refer to a "matrix" or "Charlieplexing" design style:

Multiplexing may refer to the design of a multiplexed display (non-multiplexed displays are immune to theAliasing#.22Filming_a_spoked_wheel.22 ).
Multiplexing may refer to the design of a "switch matrix" (non-multiplexed buttons are immune to "phantom keys" and also immune to "phantom key blocking").