Four-wire circuit

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. Late in the 20th century, almost all connections between telephone exchanges were four-wire circuits, while conventional phone lines into residences and businesses are two-wire circuits.

The four-wire circuit gets its name from the fact that, historically, a balanced pair of 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. When transmission directions are separated by frequency duplex, it still gets the benefits of a four-wire circuit even if the same wire pair is used in both directions.

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