Carrier telephony

Carrier telephony is a method for carrying several simultaneous long-distance telephone conversations over the same transmission line. Each conversation is shifted to a different frequency band, allowing the channels to share the line by frequency-division multiplexing (FDM). At the receiving end, filters and demodulators separate the channels and return them to ordinary voice frequencies.

The method was developed to increase the capacity of expensive long-distance lines without adding more wire. American inventor George Owen Squier demonstrated what he called "wired wireless" in 1910, and the Bell System introduced commercial carrier systems in the United States after World War I to meet growing demand for long-distance service without adding a new pair of wires for every conversation. Early systems added a few channels to open-wire routes (copper wires on poles). By the late 1930s, Bell Telephone Laboratories had adopted a standard group of twelve voice channels as a building block for open wire, cable and coaxial cable systems; it was later used in microwave transmission and submarine communications cable systems.

Carrier systems grew steadily in capacity. Early open-wire routes carried on utility pole systems carried only a few additional conversations, while the Bell System's L5 coaxial system, introduced in 1974, could carry 10,800 calls on one coaxial pair and 108,000 calls in a ten-pair cable.

Carrier telephony also changed the practice of electrical engineering. The systems needed filters to separate signals only a few kilohertz apart. Amplifiers for cable and coaxial lines had to add gain that matched the loss of the cable, without distortion. On the longest routes where hundreds or thousands of repeaters were in the path, a repeated small error grew to a failure. Consumers expected telephone service simply to work, but behind that apparent simplicity were thousands of miles of cable, thousands of circuits, and new approaches to accuracy, maintenance, testing, and reliability. Much of the later theory of filters, feedback amplifiers, equalizers, and communication channels grew out of this work. Analog carrier systems were gradually replaced by digital transmission systems using pulse-code modulation (PCM), beginning with systems such as T1 in the early 1960s.