In telecommunication, a carrier system (loosely, a synonym with carrier) is a multichannel telecommunications system in which a number of individual channels (eg 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.
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.
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. 19th century telephone systems, operating at baseband, could only carry one conversation, hence routes with heavy traffic needed many wires. In the 1920s, frequency-division multiplexing could carry several circuits on the same balanced wires, and in the 1930s L-carrier and similar systems carried hundreds on co-axial 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.
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