Method of combining multiple signals into one signal over a shared medium
For multiplexing in electronics and signal processing, see
Multiplexer
.
In
telecommunications
and
computer networking
,
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 ? a physical
transmission medium
.
[
citation needed
]
For example, in telecommunications, several
telephone calls
may be carried using one wire. Multiplexing originated in
telegraphy
in the 1870s, and is now widely applied in communications. In
telephony
,
George Owen Squier
is credited with the development of telephone carrier multiplexing in 1910.
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.
A device that performs the multiplexing is called a
multiplexer
(MUX), and a device that performs the reverse process is called a
demultiplexer
(DEMUX or DMX).
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.
In
computing
,
I/O multiplexing
can also be used to refer to the concept of processing multiple
input/output
events
from a single
event loop
, with system calls like
poll
[1]
and
select (Unix)
.
[2]
Types
[
edit
]
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
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.
Space-division multiplexing
[
edit
]
In wired communication,
space-division multiplexing
, also known as space-division multiple access (SDMA) 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
. Examples are
multiple-input and multiple-output
(MIMO), single-input and multiple-output (SIMO) and multiple-input and single-output (MISO) multiplexing. An IEEE 802.11g wireless router with
k
antennas makes it in principle possible to communicate with
k
multiplexed channels, each with a peak bit rate of 54 Mbit/s, thus increasing the total peak bit rate by the factor
k
. 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
[
edit
]
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.
[3]
A variant technology, called
wavelength-division multiplexing
(WDM) is used in
optical communications
.
Time-division multiplexing
[
edit
]
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 2400
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 9600 baud modems and one dedicated analog communications circuit from the airport ticket desk back to the airline data center are also installed.
[3]
Some
web proxy servers
(e.g.
polipo
) use TDM in
HTTP pipelining
of multiple
HTTP
transactions onto the same
TCP/IP connection
.
[4]
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.
TD is a legacy multiplexing technology still providing the backbone of most National fixed line Telephony networks in Europe, providing the 2m/bit voice and signalling ports on Narrow band Telephone exchanges such as the DMS100. Each E1 or 2m/bit TDM port provides either 30 or 31 speech timeslots in the case of CCITT7 signalling systems and 30 voice channels for customer connected Q931, DASS2, DPNSS, V5 and CASS signalling systems.
[5]
Polarization-division multiplexing
[
edit
]
Polarization-division multiplexing
uses the
polarization
of electromagnetic radiation to separate orthogonal channels. It is in practical use in both radio and optical communications, particularly in 100 Gbit/s per channel
fiber optic transmission systems
.
Differential Cross-Polarized Wireless Communications is a novel method for polarized antenna transmission utilizing a differential technique.
[6]
Orbital angular momentum multiplexing
[
edit
]
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.
[7]
It can potentially be used in addition to other physical multiplexing methods to greatly expand the transmission capacity of such systems. As of 2012
[update]
it is still in its early research phase, with small-scale laboratory demonstrations of bandwidths of up to 2.5 Tbit/s over a single light path.
[8]
This is a controversial subject in the academic community, with many claiming it is not a new method of multiplexing, but rather a special case of space-division multiplexing.
[9]
Code-division multiplexing
[
edit
]
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 significant application of CDMA is the
Global Positioning System
(GPS).
Multiple access method
[
edit
]
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
.
The Transport layer in the OSI model, as well as TCP/IP model, provides statistical multiplexing of several application layer data flows to/from the same computer.
Code-division multiplexing
(CDM) is a technique in which each channel transmits its bits as a coded channel-specific sequence of pulses. 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 and asynchronously demultiplexed. Other widely used multiple access techniques are
time-division multiple access
(TDMA) and
frequency-division multiple access
(FDMA).
Code-division multiplex techniques are used as an access technology, namely code-division multiple access (CDMA), in Universal Mobile Telecommunications System (UMTS) standard for the third-generation (3G) mobile communication identified by the ITU.
[
citation needed
]
Application areas
[
edit
]
Telegraphy
[
edit
]
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.
Emile Baudot
developed a
time-multiplexing
system of multiple
Hughes
machines in the 1870s. In 1874, the
quadruplex telegraph
developed by
Thomas Edison
transmitted two messages in each direction simultaneously, for a total of four messages transiting the same wire at the same time. Several researchers were investigating
acoustic telegraphy
, a
frequency-division multiplexing
technique, which led to the
invention of the telephone
.
Telephony
[
edit
]
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
. 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.
[
citation needed
]
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.
Video processing
[
edit
]
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. Software that produces such a transport stream and/or container is commonly called a multiplexer or
muxer
. A
demuxer
is software that extracts or otherwise makes available for separate processing the components of such a stream or container.
Digital broadcasting
[
edit
]
In
digital television
systems, several variable bit-rate data streams are multiplexed together to a fixed bit rate 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. This may involve several
standard-definition television
(SDTV) programmes (particularly on
DVB-T
,
DVB-S2
,
ISDB
and ATSC-C), or one
HDTV
, possibly with a single SDTV companion channel over one 6 to 8 MHz-wide TV channel. The device that accomplishes this is called a
statistical multiplexer
. In several of these systems, the multiplexing results in an
MPEG transport stream
. The newer DVB standards DVB-S2 and
DVB-T2
has the capacity to carry several
HDTV
channels in one multiplex.
[
citation needed
]
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.
[10]
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.
[
citation needed
]
Analog broadcasting
[
edit
]
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 1960s.
Other meanings
[
edit
]
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).
[11]
Some electrical multiplexing techniques do not require a physical "
multiplexer
" device, they refer to a "
keyboard matrix
" or "
Charlieplexing
" design style:
- Multiplexing may refer to the design of a
multiplexed display
(non-multiplexed displays are immune to
break up
).
- Multiplexing may refer to the design of a "switch matrix" (non-multiplexed buttons are immune to "phantom keys" and also immune to
"phantom key blocking"
).
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.
In
sociolinguistics
, multiplexity is used to describe the number of distinct connections between individuals who are part of a
social network
. A multiplex network is one in which members share a number of ties stemming from more than one social context, such as workmates, neighbors, or relatives.
See also
[
edit
]
References
[
edit
]
- ^
Charles M. Hannum; The
NetBSD
Foundation (1998).
"poll, pollts ? synchronous I/O multiplexing"
.
BSD Cross Reference
.
NetBSD
.
- ^
Computer Systems Research Group
(1994).
"select, pselect ? synchronous I/O multiplexing"
.
BSD Cross Reference
.
NetBSD
.
- ^
a
b
Bates, Regis J; Bates, Marcus (2007),
Voice and Data Communications
, McGraw-Hill Irwin,
ISBN
9780072257328
- ^
Fielding, R.; Gettys, J.; Mogul, J.; Frystyk, H.; Berners-Lee, T. (1997).
"rfc2068 - HTTP/1.1"
.
doi
:
10.17487/RFC2068
. Retrieved
2010-09-23
.
- ^
Curley, Robert, ed. (2012).
Breakthroughs in telephone technology: from Bell to smartphones
. New York: Britannica Educational. p. 69.
ISBN
978-1-61530-724-1
.
OCLC
769190186
.
- ^
Siamack Ghadimi (2019-04-02),
Differential Cross-Polarized Wireless Communications
, Scientific Research
- ^
Tamburini, Fabrizio; Mari, Elettra; Sponselli, Anna; Thide, Bo; Bianchini, Antonio; Romanato, Filippo (2012-01-01).
"Encoding many channels on the same frequency through radio vorticity: first experimental test"
.
New Journal of Physics
.
14
(3): 033001.
arXiv
:
1107.2348
.
Bibcode
:
2012NJPh...14c3001T
.
doi
:
10.1088/1367-2630/14/3/033001
.
ISSN
1367-2630
.
S2CID
3570230
.
- ^
"
'Twisted light' carries 2.5 terabits of data per second"
.
BBC News
. 2012-06-25
. Retrieved
2012-06-25
.
- ^
Tamagnone, Michele; Silva, Joana S.; Capdevila, Santiago; Mosig, Juan R.; Perruisseau-Carrier, Julien (2015). "The orbital angular momentum (OAM) multiplexing controversy: OAM as a subset of MIMO".
2015 9th European Conference on Antennas and Propagation (EuCAP)
: 1?5.
- ^
"All about DAB multiplexes"
.
Radio & Television Investigation Service
.
BBC
. Retrieved
17 June
2018
.
- ^
"Multiplexing filehandles with select() in perl"
.
External links
[
edit
]