Multiplexing
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| Multiplex techniques |
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| Circuit mode (constant bandwidth) |
| TDM · FDM · SDM Polarization multiplexing Spatial multiplexing (MIMO) |
| Statistical multiplexing (variable bandwidth) |
| Packet mode · Dynamic TDM FHSS · DSSS OFDMA · SC-FDM · MC-SS |
| Related topics |
| Channel access methods Media Access Control (MAC) |
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- For other uses of multiplex, see multiplex (disambiguation).
- For multiplexing in electronics and signal processing, see Multiplexer.
In telecommunications and computer networks, multiplexing (also known as muxing) is 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. It originated in telegraphy, and is now widely applied in communications.
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.
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[edit] Types of multiplexing
There are three fundamental types of multiplexing, all of which have significant variations.[1] These main types are:
- Space-Division Multiplexing (SDM)
- Frequency-division multiplexing (FDM)
- Time-Division Multiplexing (TDM)
[edit] Space-Division Multiplexing (SDM)
It is the easiest multiplexing technology to understand. In fact, it is so simple, it would hardly rate its own special term, except that SDM is the primary method by which literally millions of telephone signals reach private homes. With SDM, signals are placed on physically different media. Then those media are combined into larger groups and connected to the desired end points.
For example, telephone wire pairs--each of which can carry either a voice conversation or other data--are aggregated into cables with hundreds or even thousands of pairs. The latter are run as units from central offices out to wiring center locations, from there splitting out to individual customer buildings.[2]
[edit] Frequency-division multiplexing (FDM)
It is inherently an analog technology. FDM achieves the combining of several digital signals into one medium by sending signals in several distinct frequency ranges over that medium.
One of FDM's most common applications is cable television. Only one cable reaches a customer's home but the service provider can send multiple television channels or signals simultaneously over that cable to all subscribers. Receivers must tune to the appropriate frequency (channel) to access the desired signal.[3]
[edit] Time-Division Multiplexing (TDM)
TDM is a digital technology. 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 and 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 bps, 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 bps modems and one dedicated analog communications circuit from the airport ticket desk back to the airline data center are also installed.[4]
[edit] More information
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.
[edit] 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.
[edit] Application areas
[edit] 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 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 workers were investigating acoustic telegraphy, a frequency-division multiplexing technique, which led to the invention of the telephone.
[edit] 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 other telephone lines 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 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.
[edit] Video processing
In video editing and processing systems, multiplexing refers to the process of interleaving audio and video into one coherent MPEG 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.
[edit] 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, ISDB 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. Even the original DVB standards can carry more HDTV channels in a multiplex if the most advanced MPEG-4 compressions hardware is used.
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.
Signal multiplexing of satellite TV and radio channels is typically carried out in a central signal playout and uplink centre, such as ASTRA Platform Services in Germany, which provides playout, digital archiving, encryption, and satellite uplinks, as well as multiplexing, for hundreds of digital TV and radio channels.
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.
[edit] 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, seen on stereo systems since the 1960s.
[edit] 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.
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).[5]
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 the Dorito effect).
- 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").
[edit] See also
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Look up multiplexing in Wiktionary, the free dictionary. |
- Channel access method
- Code-division multiple access (CDMA)
- Codec (coder-decoder)
- Frequency-division multiplexing
- Multiplexer
- Multiplexed display
- Orthogonal frequency-division multiplexing (OFDM) (which is a modulation method)
- Statistical multiplexing
- Time-division multiplexing
[edit] References
- ^ Book: Voice and Data Communications by Regis J. Bates and Marcus Bates ISBN-13: 978-0-0-07-225732-9
- ^ Book: Voice and Data Communications by Regis J. Bates and Marcus Bates ISBN-13: 978-0-0-07-225732-9
- ^ Book: Voice and Data Communications by Regis J. Bates and Marcus Bates ISBN-13: 978-0-0-07-225732-9
- ^ Book: Voice and Data Communications by Regis J. Bates and Marcus Bates ISBN-13: 978-0-0-07-225732-9
- ^ "Multiplexing filehandles with select() in perl". http://www.perlfect.com/articles/select.shtml.
- "Federal Standard 1037C: Glossary of Telecommunications Terms". Institute for Telecommunication Services. http://www.its.bldrdoc.gov/fs-1037/fs-1037c.htm. Retrieved 2009-10-19.
