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In telecommunication, a carrier system (loosely, a synonym with carrier) is a multichannel telecommunications system in which a number of individual channels (e.g. data, audio, video or combination thereof) are multiplexed for transmission. The transmission occurs between nodes of a network.
In carrier systems, many different forms of multiplexing may be used, such as time-division multiplexing and frequency-division multiplexing. A cable television system is an example of a carrier system that uses frequency-division multiplexing. Many different 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; i.e., the output resulting from one stage of modulation may in turn be modulated. For example, in the public telephone network, many telephone calls are sent over shared trunklines by time-division multiplexing; then for long distance calls several of these channels may be sent over a communications satellite link by frequency-division multiplexing. At a given node, specified channels, groups, supergroups, etc. may be demultiplexed by add-drop multiplexers without demultiplexing the others.
The purpose of carrier systems is to save money by carrying more traffic on less infrastructure. 19th century telephone systems, operating at baseband, could only carry one telephone call on each wire, hence routes with heavy traffic needed many wires.
In the 1920s, frequency-division multiplexing could carry several circuits on the same balanced wires, and by the 1930s L-carrier and similar systems carried hundreds of calls at a time on coaxial cables.
Capacity of these systems increased in the middle of the century, while in the 1950s researchers began to take seriously the possibility of saving money on the terminal equipment by using time-division multiplexing. This work led to T-carrier and similar digital systems for local use.
Due to the shorter repeater spacings required by digital systems, long-distance still used 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.
- This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).