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A telecommunication circuit may be defined as follows:
- The complete path between two terminals over which one-way or two-way communications may be provided.
- An electronic path between two or more points, capable of providing a single or multiple communication channels.
- An electronic closed-loop path among two or more points used for signal transfer.
In operational terms, a telecommunication circuit may be capable of transmitting information in only one direction (simplex circuit), or it may be bi-directional (duplex circuit). Bi-directional circuits may support half-duplex operation, when only one end of the channel transmits at any one time, or they may support full-duplex operation, when independent simultaneous transmission occurs in both directions.
Originally, telecommunication circuits transmitted analog information. Radio stations used them as studio transmitter links (STLs) or as remote pickup unit (RPU) for sound reproduction, sometimes as a backup to other means. Later lines were digital, used in pair-gain applications, such as carrier systems, or in enterprise data networks.
A leased line, private circuit, or dedicated circuit, is a circuit that is dedicated to only one use and is typically not switched at a central office. The opposite is a switched circuit, which can be connected to different paths in a switching center or telephone exchange. Plain old telephone service (POTS) and ISDN telephone lines are switched circuits.
- Patrick D. van der Puije (2002). Telecommunication Circuit Design. New York: John Wiley & Sons.