Four-wire circuit
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In telecommunication, a four-wire circuit is a two-way circuit using two paths so arranged that the respective signals are transmitted in one direction only by one path and in the other direction by the other path.
The four-wire circuit gets its name from the fact that, historically, two conductors were used in each of two directions for full-duplex operation. The name may still be applied, e.g. to a communications link supported by optical fibers, even though only one fiber is required for transmission in each direction.
Contrast with two-wire circuit.
