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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.
For example, a customer's telephone line stops at the Remote Concentrator (box) down the street, where it is multiplexed along with all the neighbouring lines for that 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. Because all the phone lines have been clumped together, none of them can be accessed except through what is called a demultiplexer.
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. 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.
When encoding video, multiplexing often refers to the process of interleaving audio and video into one coherent stream. In digital television, DVB, ATSC, and ISDB can all multiplex several channels together. In digital radio, both the EUREKA 147 system of Digital audio broadcasting and the HD Radio and Digital Radio Mondiale systems of IBOC can multiplex channels. This is essentially required with DAB-type transmissions, but is entirely optional with IBOC systems.
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 transmission equipment where modulation occurs. Multiplexing in this sense is sometimes known as MPX.
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.
Time-division multiplexingTime-division multiplexing (TDM) is a type of digital multiplexing in which two or more apparently simultaneous channels are derived from a given frequency spectrum, i.e., bit stream, by interleaving pulses representing bits from different channels.
In some TDM systems, successive pulses represent bits from successive channels, e.g., voice channels in a T1 system. In other systems different channels take turns using the channels for a group of successive pulse-times (a so-called "time slot").
What distinguishes coarse time-division multiplexing from packet switching is that the time-slots are pre-allocated to the channels, rather than arbitrated on a per-time slot basis.
Uses of time-division multiplexing:
Time Division Multiplexing (TDM) is the means by which multiple digital signals (or analogue signals carrying digital data) can be carried on a single transmission path by interleaving portions of each signal in time [1]. Interleaving can be done at bits or blocks of bytes [1]. This enables digitally encoded speech signals to be transmitted and switched optimally within a circuit-switched network [2]. This article consists of two sections, namely, Transmission using TDM and Synchronous Digital Hierarchy (SDH). The first section examines the basic principles underlying TDM, while the second section discusses how SDH is used to switch TDM frames.
Frequency-division multiplexingFrequency-division multiplexing (FDM) is a form of signal multiplexing where multiple baseband signals are modulated on different frequency carrier waves and added together to create a composite signal.
Historically, telephone networks used FDM to carry several voice channels on a single physical circuit. In this, 12 voice channels would be modulated onto carriers spaced 4 kHz apart. The composite signal, occupying the frequency range 60 – 108 kHz, was known as a group. In turn, five groups could themselves be multiplexed by a similar method into a supergroup, containing 60 voice channels. There were even higher levels of multiplexing, and it became possible to send thousands of voice channels down a single circuit.
Modern telephone systems employ digital transmission, in which time-division multiplexing (TDM) is used instead of FDM.
FDM can also be used to combine multiple signals before final modulation onto a carrier wave. In this case the carrier signals are referred to as subcarriers: an example is stereo FM transmission, where a 38 kHz subcarrier is used to separate the left-right difference signal from the central left-right sum channel, prior to the frequency modulation of the composite signal.
Where frequency division multiplexing is used as to allow multiple users to share a physical communications channel, it is called frequency-division multiple access (FDMA).
FDMA is the traditional way of separating radio signals from different transmitters.
The concept corresponding to frequency division multiplexing in the optical domain is known as wavelength division multiplexing.
Statistical multiplexingStatistical multiplexing is a type of digital multiplexing similar to time-division multiplexing (TDM), except that, rather than arbitrarily assigning a time slot to each signal, each signal is assigned a slot according to priority and need.
Statistical multiplexing is an "on-demand" service rather than one that preallocates resources for a transmission link. The transmission capacity of the link will be shared by only those users who have packets.
Statistical multiplexing ensures that slots will not be wasted (whereas TDM can waste slots). However, time within a slot may still be wasted. Also, data may be lost due to buffer overflow.
An example of statistical multiplexing is Packet Switching.
Code Division MultiplexingCode Division Multiplexing (CDM) or Synchronous CDMA exploits at its core mathematical properties of orthogonality.
Code division multiple access (CDMA) is a form of multiplexing (not a modulation scheme) and a method of multiple access that does not divide up the channel by time (as in TDMA), or frequency (as in FDMA), but instead encodes data with a special code associated with each channel and uses the constructive interference properties of the special codes to perform the multiplexing. CDMA also refers to digital cellular telephony systems that make use of this multiple access scheme, such as those pioneered by Qualcomm, or W-CDMA.
CDMA is a military technology first used during World War II by English allies to foil German attempts at jamming transmissions. The allies decided to transmit over several frequencies, instead of one, making it difficult for the Germans to pick up the complete signal.
CDMA has since been used in many communications systems, including the Global Positioning System (GPS) and in the OmniTRACS satellite system for transportation logistics. The latter system was designed and built by Qualcomm, and became the seed which helped Qualcomm engineers to invent Soft Handoff and fast power control, the necessary technologies that made CDMA practical and efficient for terrestrial cellular communications.
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Last modified at : Thursday, December 11st 2008 14:03:00.
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Albert Einstein
