Multiplexing

Multiplexing (also MUXing) is a term used in electrical engineering to refer to a process where multiple sources of information are combined in order to ease the organization, conversion, and transportation of the material from one place to another. The information is usually held completely intact after it has been multiplexed but is transported in a different manner than normal.

In telecommunications, multiplexing is the combination of two or more higher-level channels into a single lower-level channel such that a reverse process, known as inverse multiplexing, demultiplexing, or demuxing, can extract the original channels. The individual channels are identifiable by a predetermined coding scheme, such as a packet identifier (PID) for digital communications.

The two basic forms of multiplexing here are time-division multiplexing (TDM) and frequency-division multiplexing (FDM). In optical communications, FDM is referred to as wavelength-division multiplexing (WDM). Time-division multiplexing can be either synchronous or asynchronous. Digital communications can also be transmitted with code-division multiplexing. Multiplexing can also be accomplished through alternating polarisation (horizontal/vertical or clockwise/counterclockwise) on each adjacent channel and satellite, however this is only applicable to wireless communications.

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 all of 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).

Fibre 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 due to convergence is increasingly offering the same services as telephone companies. IPTV also depends on multiplexing.

Broadcasting

In digital television, DVB, ATSC, and ISDB can all multiplex several channels together. This may involve several SDTV channels (particularly on DVB-T, DVB-S2, and ATSC-C), or one HDTV, possibly with a single SDTV companion channel.

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.

Note that when encoding video, multiplexing sometimes refers to the process of interleaving audio and video into one coherent transport stream.

In FM broadcasting and other analog radio mediums, 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.

References

Federal Standard 1037C

multiplexing | digital television | digital radio | broadcast engineering

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Telephony

Broadcasting

Other meanings

See also

References