Multiplexing

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Telecommunication multiplexing

In electronics, telecommunications and computer networks, multiplexing (short muxing) is a term used to refer to a process where multiple analog message signals or digital data streams are combined into one signal. The aim is to share an expensive resource. For example, in electronics, multiplexing allows several analog signals to be processed by one analog-to-digital converter (ADC), and in telecommunications, several phone calls may be transferred using one wire. 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).

The two most basic forms of multiplexing are time-division multiplexing (TDM) and frequency-division multiplexing (FDM).

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.

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.

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 seven layer model or TCP/IP five layer model, while multiple access also involves a media access control protocol, which is part of the data link layer.

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

1 Telephony
2 Digital broadcasting
3 Analog broadcasting
4 Other meanings
5 See also
6 References

[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 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.

[edit] Digital broadcasting

In digital television and digital audio broadcasting 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 datacasting 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 8 MHz- or 6 MHz-wide TV channel. The device that accomplish this is called a statistical multiplexer. In several of these systems, the multiplexing results in a so called MPEG transport stream, consisting of 188 byte long packets.

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.

When encoding video, multiplexing refers to the process of interleaving audio and video into one coherent transport stream (time-division multiplexing).

[edit] Analog broadcasting

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.

[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.

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