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A carrier system is a transmission system that transmits information, such as the voice signals of a telephone call and the video signals of television, by modulation of one or multiple above the principal voice frequency or data rate.Western Electric (1969) Fundamentals of Telephone Communication Systems, p.16.2
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, specific channels may be demultiplexed individually.
In the 1920s, frequency-division multiplexing could carry several circuits on the same balanced pair, and by the 1930s L-carrier and similar systems carried hundreds of calls simultaneously on . 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 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 exchanges became rare.
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