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.
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.
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.
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.
*
multiplexer*
time-division multiplexing*
frequency-division multiplexing*
orthogonal frequency-division multiplexing (OFDM)
*
code-division multiple access (CDMA)
*
codec (coder-decoder)
*
polarization-division multiplexing (PDM)
*
statistical multiplexing*
Federal Standard 1037C
