Editing Carrier system

{{more footnotes|date=January 2013}}

A '''carrier system''' is a [[transmission system]] that [[transmission (telecommunications)|transmits]] information, such as the voice signals of a [[telephone call]] and the video signals of [[television]], by modulation of one or multiple [[carrier signal]]s above the principal voice frequency or data rate.<ref name=weco>Western Electric (1969) ''Fundamentals of Telephone Communication Systems'', p.16.2</ref>

Carrier systems typically transmit multiple channels of communication simultaneously over the [[shared medium]] using various forms of [[multiplexing]]. Prominent multiplexing methods of the [[carrier signal]] are [[time-division multiplexing]] (TDM) and [[frequency-division multiplexing]] (FDM). A [[cable television system]] is an example of frequency-division multiplexing. Many television programs are carried simultaneously on the same [[coaxial cable]] by sending each at a different frequency. Multiple layers of multiplexing may ultimately be performed upon a given input [[signal]]. For example, in the [[public switched telephone network]], many telephone calls are sent over shared trunk lines by time-division multiplexing. For long-distance calls several of these channels may be sent over a [[communications satellite]] link by frequency-division multiplexing. At a given receiving [[node (networking)|node]], specific channels may be demultiplexed individually.

== History ==
Carrier systems increase economic efficiency by carrying more traffic on comparable cost of [[Infrastructure#Communications| communication infrastructure]]. 19th century telephone systems, operating by direct [[baseband]] transmission, could only carry one telephone call on each wire pair, hence routes with heavy traffic needed many wire pairs.

In the 1920s, frequency-division multiplexing could carry several circuits on the same [[balanced pair|balanced wires]], and by the 1930s [[L-carrier]] and similar systems carried hundreds of calls simultaneously on [[coaxial cable]]s. The capacity of these systems increased in the middle of the century.

In the 1950s, research began into further increasing the throughput of terminal equipment by using digital signals with [[time-division multiplexing]] (TDM). This work led to [[T-carrier]], [[E-carrier]] and other similar digital systems.

Due to the shorter [[repeater]] spacings required by digital systems, long-distance transmission still used [[Frequency-division multiplexing|FDM]] until the late 1970s when [[optical fiber]] was improved to the point that digital connections became the cheapest ones for all distances, short and long. By the end of the century, analog connections between and within [[telephone exchange]]s became rare.

==See also==
* [[Channel access method]]

==References==
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*{{FS1037C MS188}}
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==External links==
* [http://long-lines.net/tech-equip/misc/J-Carrier.html J-carrier]

[[Category:Multiplexing]]