In
telecommunications
Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that ...
and
computer networking, multiplexing (sometimes contracted to muxing) is a method by which multiple analog or digital signals are combined into one signal over a
shared medium. The aim is to share a scarce resource - a physical
transmission medium
A transmission medium is a system or substance that can mediate the propagation of signals for the purposes of telecommunication. Signals are typically imposed on a wave of some kind suitable for the chosen medium. For example, data can modulat ...
. For example, in telecommunications, several
telephone call
A telephone call is a connection over a telephone network between the called party and the calling party.
First telephone call
The first telephone call was made on March 10, 1876, by Alexander Graham Bell. Bell demonstrated his ability to "ta ...
s may be carried using one wire. Multiplexing originated in
telegraphy in the 1870s, and is now widely applied in communications. In
telephony
Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is i ...
,
George Owen Squier is credited with the development of telephone carrier multiplexing in 1910.
The multiplexed signal is transmitted over a communication channel such as a cable. The multiplexing divides the capacity of the communication channel into several logical channels, one for each message signal or data stream to be transferred. A reverse process, known as demultiplexing, extracts the original channels on the receiver end.
A device that performs the multiplexing is called a
multiplexer (MUX), and a device that performs the reverse process is called a
demultiplexer (DEMUX or DMX).
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.
In
computing
Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, ...
, I/O multiplexing can also be used to refer to the concept of processing multiple
input/output
In computing, input/output (I/O, or informally io or IO) is the communication between an information processing system, such as a computer, and the outside world, possibly a human or another information processing system. Inputs are the signals ...
events from a single
event loop, with system calls like
poll and
select (Unix).
Types
Multiple
variable bit rate
Variable bitrate (VBR) is a term used in telecommunications and computing
Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processe ...
digital
bit stream
A bitstream (or bit stream), also known as binary sequence, is a sequence of bits.
A bytestream is a sequence of bytes. Typically, each byte is an 8-bit quantity, and so the term octet stream is sometimes used interchangeably. An octet ma ...
s may be transferred efficiently over a single fixed
bandwidth channel by means of
statistical multiplexing. This is an
asynchronous
Asynchrony is the state of not being in synchronization.
Asynchrony or asynchronous may refer to:
Electronics and computing
* Asynchrony (computer programming), the occurrence of events independent of the main program flow, and ways to deal wit ...
mode time-domain multiplexing which is a form of time-division multiplexing.
Digital bit streams can be transferred over an analog channel by means of code-division multiplexing 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
Two-dimensional rotation can occur in two possible directions. Clockwise motion (abbreviated CW) proceeds in the same direction as a clock's hands: from the top to the right, then down and then to the left, and back up to the top. The opposite ...
/
counterclockwise
Two-dimensional rotation can occur in two possible directions. Clockwise motion (abbreviated CW) proceeds in the same direction as a clock's hands: from the top to the right, then down and then to the left, and back up to the top. The opposite ...
) on each
adjacent channel
In broadcasting an adjacent channel is an AM, FM, or TV channel that is next to another channel. First-adjacent is immediately next to another channel, second-adjacent is two channels away, and so forth. Information on adjacent channels is ...
and satellite, or through
phased multi-antenna array combined with a
multiple-input multiple-output communications
In radio, multiple-input and multiple-output, or MIMO (), is a method for multiplying the capacity of a radio link using multiple transmission and receiving antennas to exploit multipath propagation. MIMO has become an essential element of wir ...
(MIMO) scheme.
Space-division multiplexing
In wired communication,
space-division multiplexing
Space-division multiple access (SDMA) is a channel access method based on creating parallel spatial pipes (focused signal beams) using advanced antenna technology next to higher capacity pipes through spatial multiplexing and/or diversity, by ...
, also known as space-division multiple access (SDMA) is the use of separate point-to-point electrical conductors for each transmitted channel. Examples include an analogue stereo audio cable, with one pair of wires for the left channel and another for the right channel, and a multi-pair
telephone cable, a switched
star network such as a telephone access network, a switched Ethernet network, and a
mesh network.
In wireless communication, space-division multiplexing is achieved with multiple antenna elements forming a
phased array antenna. Examples are
multiple-input and multiple-output
In radio, multiple-input and multiple-output, or MIMO (), is a method for multiplying the capacity of a radio link using multiple transmission and receiving antennas to exploit multipath propagation. MIMO has become an essential element of wi ...
(MIMO), single-input and multiple-output (SIMO) and multiple-input and single-output (MISO) multiplexing. An IEEE 802.11g wireless router with ''k'' antennas makes it in principle possible to communicate with ''k'' multiplexed channels, each with a peak bit rate of 54 Mbit/s, thus increasing the total peak bit rate by the factor ''k''. Different antennas would give different
multi-path propagation
In radio communication, multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflec ...
(echo) signatures, making it possible for
digital signal processing
Digital signal processing (DSP) is the use of digital processing, such as by computers or more specialized digital signal processors, to perform a wide variety of signal processing operations. The digital signals processed in this manner are ...
techniques to separate different signals from each other. These techniques may also be utilized for
space diversity (improved robustness to fading) or
beamforming (improved selectivity) rather than multiplexing.
Frequency-division multiplexing
Frequency-division multiplexing (FDM) is inherently an analog technology. FDM achieves the combining of several signals into one medium by sending signals in several distinct frequency ranges over a single medium. In FDM the signals are electrical signals.
One of the most common applications for FDM is traditional radio and television broadcasting from terrestrial, mobile or satellite stations, or cable television. Only one cable reaches a customer's residential area, but the service provider can send multiple television channels or signals simultaneously over that cable to all subscribers without interference. Receivers must tune to the appropriate frequency (channel) to access the desired signal.
A variant technology, called
wavelength-division multiplexing (WDM) is used in
optical communications.
Time-division multiplexing
Time-division multiplexing (TDM) is a digital (or in rare cases, analog) technology which uses time, instead of space or frequency, to separate the different data streams. TDM involves sequencing groups of a few bits or bytes from each individual input stream, one after the other, and in such a way that they can be associated with the appropriate receiver. If done sufficiently quickly, the receiving devices will not detect that some of the circuit time was used to serve another logical communication path.
Consider an application requiring four terminals at an airport to reach a central computer. Each terminal communicated at 2400
baud, so rather than acquire four individual circuits to carry such a low-speed transmission, the airline has installed a pair of multiplexers. A pair of 9600 baud modems and one dedicated analog communications circuit from the airport ticket desk back to the airline data center are also installed.
Some
web proxy servers (e.g.
polipo
Polipo is a lightweight caching and forwarding web proxy server. It has a wide variety of uses, from aiding security by filtering traffic; to caching web, DNS and other computer network lookups for a group of people sharing network resources; t ...
) use TDM in
HTTP pipelining of multiple
HTTP
The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide We ...
transactions onto the same
TCP/IP connection.
Carrier sense multiple access and
multidrop communication methods are similar to time-division multiplexing in that multiple data streams are separated by time on the same medium, but because the signals have separate origins instead of being combined into a single signal, are best viewed as
channel access methods, rather than a form of multiplexing.
TD is a legacy multiplexing technology still providing the backbone of most National fixed line Telephony networks in Europe, providing the 2m/bit voice and signalling ports on Narrow band Telephone exchanges such as the DMS100. Each E1 or 2m/bit TDM port provides either 30 or 31 speech timeslots in the case of CCITT7 signalling systems and 30 voice channels for customer connected Q931, DASS2, DPNSS, V5 and CASS signalling systems.
Polarization-division multiplexing
Polarization-division multiplexing uses the
polarization of electromagnetic radiation to separate orthogonal channels. It is in practical use in both radio and optical communications, particularly in 100 Gbit/s per channel
fiber optic transmission systems.
Differential Cross-Polarized Wireless Communications is a novel method for polarized antenna transmission utilizing a differential technique.
Orbital angular momentum multiplexing
Orbital angular momentum multiplexing
Orbital angular momentum (OAM) multiplexing is a physical layer method for multiplexing signals carried on electromagnetic waves using the orbital angular momentum of the electromagnetic waves to distinguish between the different orthogonal signa ...
is a relatively new and experimental technique for multiplexing multiple channels of signals carried using electromagnetic radiation over a single path. It can potentially be used in addition to other physical multiplexing methods to greatly expand the transmission capacity of such systems. it is still in its early research phase, with small-scale laboratory demonstrations of bandwidths of up to 2.5 Tbit/s over a single light path. This is a controversial subject in the academic community, with many claiming it is not a new method of multiplexing, but rather a special case of space-division multiplexing.
Code-division multiplexing
Code-division multiplexing
Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communication ...
(CDM),
code-division multiple access (CDMA) or
spread spectrum is a class of techniques where several channels simultaneously share the same
frequency spectrum, and this spectral bandwidth is much higher than the bit rate or
symbol rate. One form is frequency hopping, another is direct sequence spread spectrum. In the latter case, each channel transmits its bits as a coded channel-specific sequence of pulses called chips. Number of chips per bit, or chips per symbol, is the
spreading factor
In digital communications, a chip is a pulse of a direct-sequence spread spectrum (DSSS) code, such as a pseudo-random noise (PN) code sequence used in direct-sequence code-division multiple access (CDMA) channel access techniques.
In a binary di ...
. This coded transmission typically is accomplished by transmitting a unique time-dependent series of short pulses, which are placed within chip times within the larger bit time. All channels, each with a different code, can be transmitted on the same fiber or radio channel or other medium, and asynchronously demultiplexed. Advantages over conventional techniques are that variable bandwidth is possible (just as in
statistical multiplexing), that the wide bandwidth allows poor signal-to-noise ratio according to
Shannon-Hartley theorem, and that multi-path propagation in wireless communication can be combated by
rake receivers.
A significant application of CDMA is the
Global Positioning System
The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite ...
(GPS).
Multiple access method
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
The Open Systems Interconnection model (OSI model) is a conceptual model that 'provides a common basis for the coordination of SOstandards development for the purpose of systems interconnection'. In the OSI reference model, the communications ...
, while multiple access also involves a
media access control protocol, which is part of the
Data Link Layer
The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer p ...
.
The Transport layer in the OSI model, as well as TCP/IP model, provides statistical multiplexing of several application layer data flows to/from the same computer.
Code-division multiplexing
Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communication ...
(CDM) is a technique in which each channel transmits its bits as a coded channel-specific sequence of pulses. This coded transmission typically is accomplished by transmitting a unique time-dependent series of short pulses, which are placed within chip times within the larger bit time. All channels, each with a different code, can be transmitted on the same fiber and asynchronously demultiplexed. Other widely used multiple access techniques are
time-division multiple access (TDMA) and
frequency-division multiple access (FDMA).
Code-division multiplex techniques are used as an access technology, namely code-division multiple access (CDMA), in Universal Mobile Telecommunications System (UMTS) standard for the third-generation (3G) mobile communication identified by the ITU.
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.
Émile Baudot developed a
time-multiplexing system of multiple
Hughes
Hughes may refer to:
People
* Hughes (surname)
* Hughes (given name)
Places Antarctica
* Hughes Range (Antarctica), Ross Dependency
* Mount Hughes, Oates Land
* Hughes Basin, Oates Land
* Hughes Bay, Graham Land
* Hughes Bluff, Victoria La ...
machines in the 1870s. In 1874, the
quadruplex telegraph developed by
Thomas Edison
Thomas Alva Edison (February 11, 1847October 18, 1931) was an American inventor and businessman. He developed many devices in fields such as electric power generation, mass communication, sound recording, and motion pictures. These inventi ...
transmitted two messages in each direction simultaneously, for a total of four messages transiting the same wire at the same time. Several researchers were investigating
acoustic telegraphy, a
frequency-division multiplexing technique, which led to the
invention of the telephone.
Telephony
In
telephony
Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is i ...
, a
customer
In sales, commerce, and economics, a customer (sometimes known as a client, buyer, or purchaser) is the recipient of a good, service, product or an idea - obtained from a seller, vendor, or supplier via a financial transaction or exchan ...
's
telephone line
A telephone line or telephone circuit (or just line or circuit industrywide) is a single-user circuit on a telephone communication system. It is designed to reproduce speech of a quality that is understandable. It is the physical wire or ot ...
now typically ends at the
remote concentrator box, where it is multiplexed along with other
telephone line
A telephone line or telephone circuit (or just line or circuit industrywide) is a single-user circuit on a telephone communication system. It is designed to reproduce speech of a quality that is understandable. It is the physical wire or ot ...
s for that
neighborhood 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 line
Digital subscriber line (DSL; originally digital subscriber loop) is a family of technologies that are used to transmit digital data over telephone lines. In telecommunications marketing, the term DSL is widely understood to mean asymmetric dig ...
s (DSL).
Fiber in the loop (FITL) is a common method of multiplexing, which uses
optical fiber
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass ( silica) or plastic to a diameter slightly thicker than that of a human hair
Hair is a protein filament that grows ...
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
Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 1 ...
wired into the
home.
Asynchronous Transfer Mode is often the
communications protocol
A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synch ...
used.
Cable TV has long carried multiplexed
television channel
A television channel is a terrestrial frequency or virtual number over which a television station or television network is distributed. For example, in North America, "channel 2" refers to the terrestrial or cable band of 54 to 60 MHz, with ...
s, and late in the 20th century began offering the same services as
telephone companies
A telephone company, also known as a telco, telephone service provider, or telecommunications operator, is a kind of communications service provider (CSP), more precisely a telecommunications service provider (TSP), that provides telecommunicat ...
.
IPTV
Internet Protocol television (IPTV) is the delivery of television content over Internet Protocol (IP) networks. This is in contrast to delivery through traditional terrestrial, satellite, and cable television formats. Unlike downloaded med ...
also depends on multiplexing.
Video processing
In
video
Video is an electronic medium for the recording, copying, playback, broadcasting, and display of moving visual media. Video was first developed for mechanical television systems, which were quickly replaced by cathode-ray tube (CRT) sy ...
editing and processing systems, multiplexing refers to the process of interleaving audio and video into one coherent data stream.
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 multiplexer 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
Digital television (DTV) is the transmission of television signals using digital encoding, in contrast to the earlier analog television technology which used analog signals. At the time of its development it was considered an innovative adva ...
systems, several variable bit-rate data streams are multiplexed together to a fixed bit rate 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. This may involve several
standard-definition television
Standard-definition television (SDTV, SD, often shortened to standard definition) is a television system which uses a resolution that is not considered to be either high or enhanced definition. "Standard" refers to it being the prevailing sp ...
(SDTV) programmes (particularly on
DVB-T
DVB-T, short for Digital Video Broadcasting – Terrestrial, is the DVB European-based consortium standard for the broadcast transmission of digital terrestrial television that was first published in 1997 and first broadcast in Singapore in Feb ...
,
DVB-S2,
ISDB
Integrated Services Digital Broadcasting (ISDB; Japanese: , ''Tōgō dejitaru hōsō sābisu'') is a Japanese broadcasting standard for digital television (DTV) and digital radio.
ISDB supersedes both the NTSC-J analog television system and ...
and ATSC-C), or one
HDTV
High-definition television (HD or HDTV) describes a television system which provides a substantially higher image resolution than the previous generation of technologies. The term has been used since 1936; in more recent times, it refers to the g ...
, 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
MPEG transport stream (MPEG-TS, MTS) or simply transport stream (TS) is a standard digital container format for transmission and storage of audio, video, and Program and System Information Protocol (PSIP) data. It is used in broadcast syste ...
. The newer DVB standards DVB-S2 and
DVB-T2 has the capacity to carry several
HDTV
High-definition television (HD or HDTV) describes a television system which provides a substantially higher image resolution than the previous generation of technologies. The term has been used since 1936; in more recent times, it refers to the g ...
channels in one multiplex.
In
digital radio, a multiplex (also known as an ensemble) is a number of radio stations that are grouped together. A multiplex is a stream of digital information that includes audio and other data.
On
communications satellite
A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth ...
s which carry
broadcast television network
A television network or television broadcaster is a telecommunications network for distribution of television program content, where a central operation provides programming to many television stations or pay television providers. Until the mid ...
s and
radio network
There are two types of radio network currently in use around the world: the one-to-many ( simplex communication) broadcast network commonly used for public information and mass-media entertainment, and the two-way radio ( duplex communication) t ...
s, 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.
Analog broadcasting
In
FM broadcasting
FM broadcasting is a method of radio broadcasting using frequency modulation (FM). Invented in 1933 by American engineer Edwin Armstrong, wide-band FM is used worldwide to provide high fidelity sound over broadcast radio. FM broadcasting is capab ...
and other
analog radio
Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a tr ...
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. (In fact, the stereo multiplex signal can be generated using time-division multiplexing, by switching between the two (left channel and right channel) input signals at an ultrasonic rate (the subcarrier), and then filtering out the higher harmonics.) Multiplexing in this sense is sometimes known as MPX, which in turn is also an old term for
stereophonic FM, seen on
stereo system
Stereophonic sound, or more commonly stereo, is a method of sound reproduction that recreates a multi-directional, 3-dimensional audible perspective. This is usually achieved by using two independent audio channels through a configuration ...
s since the 1960s.
Other meanings
In
spectroscopy
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter ...
the term is used to indicate that the experiment is performed with a mixture of frequencies at once and their respective response unravelled afterwards using the
Fourier transform
A Fourier transform (FT) is a mathematical transform that decomposes functions into frequency components, which are represented by the output of the transform as a function of frequency. Most commonly functions of time or space are transformed ...
principle.
In
computer programming
Computer programming is the process of performing a particular computation (or more generally, accomplishing a specific computing result), usually by designing and building an executable computer program. Programming involves tasks such as anal ...
, 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 "
keyboard matrix" or "
Charlieplexing
Charlieplexing (also known as tristate multiplexing, reduced pin-count LED multiplexing, complementary LED drive and crossplexing) is a technique for driving a multiplexed display in which relatively few I/O pins on a microcontroller are used ...
" design style:
* Multiplexing may refer to the design of a
multiplexed display
Multiplexed displays are electronic display devices where the entire display is not driven at one time.
Instead, sub-units of the display (typically, rows or columns for a dot matrix display or individual characters for a character oriented dis ...
(non-multiplexed displays are immune to
break up).
* 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").
In high-throughput
DNA sequencing
DNA sequencing is the process of determining the nucleic acid sequence – the order of nucleotides in DNA. It includes any method or technology that is used to determine the order of the four bases: adenine, guanine, cytosine, and thymine. T ...
, the term is used to indicate that some artificial sequences (often called ''barcodes'' or ''indexes'') have been added to link given sequence reads to a given sample, and thus allow for the sequencing of multiple samples in the same reaction.
In
sociolinguistics
Sociolinguistics is the descriptive study of the effect of any or all aspects of society, including cultural norms, expectations, and context, on the way language is used, and society's effect on language. It can overlap with the sociology of ...
, multiplexity is used to describe the number of distinct connections between individuals who are part of a
social network. A multiplex network is one in which members share a number of ties stemming from more than one social context, such as workmates, neighbors, or relatives.
See also
*
Add-drop multiplexer
An add-drop multiplexer (ADM) is an important element of an optical fiber network. A multiplexer combines, or multiplexes, several lower-bandwidth streams of data into a single beam of light. An ''add-drop'' multiplexer also has the capabilit ...
*
Central office multiplexing
*
Channel bank
*
Multiplexed display
Multiplexed displays are electronic display devices where the entire display is not driven at one time.
Instead, sub-units of the display (typically, rows or columns for a dot matrix display or individual characters for a character oriented dis ...
*
Optical add-drop multiplexer
*
Orthogonal frequency-division multiplexing (OFDM) (which is a modulation method)
*
Statistical multiplexing
References
*
Efficient beam multiplexing using a spatial light modulator
External links
*
*
{{Telecommunications, state=closed
Digital television
Digital radio
Broadcast engineering
Physical layer protocols
Television terminology