In telecommunications 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 calls may be carried using one wire.
Multiplexing originated in telegraphy in the s, and is now widely applied in communications. In telephony , George Owen Squier is credited with the development of telephone carrier multiplexing in 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. 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. Multiple variable bit rate digital bit streams 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 DSSS.
In wired communication, space-division multiplexing , also known as Space-division multiple access 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. An IEEE 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.
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 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 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 baud modems and one dedicated analog communications circuit from the airport ticket desk back to the airline data center are also installed.
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.
Polarization-division multiplexing uses the polarization of electromagnetic radiation to separate orthogonal channels.
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.
Practical example of frequency division multiplexing pdf
As of [update] it is still in its early research phase, with small-scale laboratory demonstrations of bandwidths of up to 2. 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 receivers. 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. Code-division multiplexing CDM is a technique in which each channel transmits its bits as a coded channel-specific sequence of pulses. All channels, each with a different code, can be transmitted on the same fiber and asynchronously demultiplexed.
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.
In telephony , a customer 's telephone line now typically ends at the remote concentrator box, 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 fiber as the backbone. Asynchronous Transfer Mode is often the communications protocol used. Cable TV has long carried multiplexed television channels , and late in the 20th century began offering the same services as telephone companies. IPTV also depends on multiplexing.
In video 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. A demuxer is software that extracts or otherwise makes available for separate processing the components of such a stream or container.
File Extensions and File Formats
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.
The device that accomplishes this is called a statistical multiplexer. In several of these systems, the multiplexing results in an MPEG transport stream. 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 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 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.
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 systems since the s. In spectroscopy 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 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.
Frequency Division Multiplexing (FDM)
Some electrical multiplexing techniques do not require a physical " multiplexer " device, they refer to a " keyboard matrix " or " Charlieplexing " design style:. 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.
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