and computer networks
, 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. For example, in telecommunications, several telephone call
s may be carried using one wire. Multiplexing originated in telegraphy
in the 1870s, and is now widely applied in communications. In telephony
, 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).
(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.
, I/O multiplexing can also be used to refer to the concept of processing multiple input/output events
from a single event loop
, with system calls like poll
and select (Unix)
Multiple variable bit rate
digital bit stream
s may be transferred efficiently over a single fixed bandwidth
channel by means of statistical multiplexing
. This is an asynchronous
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
In wireless communication
s, multiplexing can also be accomplished through alternating polarization
) on each adjacent channel
and satellite, or through phased multi-antenna array
combined with a multiple-input multiple-output communications
In wired communication, space-division multiplexing
, 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
(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
(echo) signatures, making it possible for digital signal processing
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.
(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 communication
(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
) use TDM in HTTP pipelining
of multiple HTTP
transactions onto the same TCP/IP connection
Carrier sense multiple access
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 method
s, 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.
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 system
Orbital angular momentum multiplexing
Orbital angular momentum multiplexing
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
(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
. 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 receiver
A significant application of CDMA is the Global Positioning System
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
, while multiple access also involves a media access control
protocol, which is part of the Data Link Layer
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.
(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
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.
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.
developed a time-multiplexing
system of multiple Hughes
machines in the 1870s. 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. Several researchers were investigating acoustic telegraphy
, a frequency-division multiplexing
technique, which led to the invention of the telephone
, a customer
's telephone line
now typically ends at the remote concentrator
box, where it is multiplexed along with other telephone line
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
Fiber in the loop
(FITL) is a common method of multiplexing, which uses optical fiber
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
has long carried multiplexed television channel
s, and late in the 20th century began offering the same services as telephone companies
also depends on multiplexing.
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 statistical multiplexer
or muxer. A demuxer is software that extracts or otherwise makes available for separate processing the components of such a stream or container.
In digital television
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. This may involve several standard definition television
(SDTV) programmes (particularly on DVB-T
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
. The newer DVB standards DVB-S2 and DVB-T2
has the capacity to carry several HDTV
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
s which carry broadcast television network
s and radio network
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.
In FM broadcasting
and other analog radio
media, multiplexing is a term commonly given to the process of adding subcarrier
s 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
s since the 1960s.
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
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 "keyboard matrix
" or "Charlieplexing
" design style:
* Multiplexing may refer to the design of a multiplexed display
(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
, 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.
*Central office multiplexing
*Optical add-drop multiplexer
*Orthogonal frequency-division multiplexing
(OFDM) (which is a modulation method)
Efficient beam multiplexing using a spatial light modulator
Category:Physical layer protocols