# Four-wire circuit

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In [telecommunications](/source/telecommunications), a '''four-wire circuit''' is a two-way [circuit](/source/Telecommunication_circuit) using two paths so arranged that the respective [signals](/source/Signal_(information_theory)) 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 is uses four conductors to create two complete electrical circuits, one for each direction. The two separate circuits (channels) allow full-[duplex](/source/Duplex_(telecommunications)) operation with low [crosstalk](/source/crosstalk).

In telephony a four-wire circuit was historically used to transport and switch [baseband](/source/baseband) audio signals in the phone company [telephone exchange](/source/telephone_exchange) before the advent of [digital modulation](/source/digital_modulation) and the [electronic switching system](/source/electronic_switching_system) eliminated baseband audio from the telco plant except for the [local loop](/source/local_loop). The local loop is a [two-wire circuit](/source/two-wire_circuit) for one reason only: to save copper. Using half the number of copper wire conductors per circuit means that the infrastructure cost for wiring each circuit is halved. Although a lower quality circuit, the local loop allows full duplex operation by using a [telephone hybrid](/source/telephone_hybrid) to keep near and far voice levels equivalent.

As the [public switched telephone network](/source/public_switched_telephone_network) expanded in size and scope, using many individual wires inside the telco plant became so impractical and labor-intensive that in-office and inter-office signal wiring progressed to high bandwidth coaxial cable (still a popular interconnection method in the 21st century, used with the Lucent 5ESS [Class-5 telephone switch](/source/Class-5_telephone_switch) to present day), [microwave radio relay](/source/microwave_radio_relay) and ultimately [fiber-optic communication](/source/fiber-optic_communication) for high-speed trunk circuits. At the end of the 20th century, four-wire circuits saw renewed growth for corporate local loop service for use in [dedicated line](/source/dedicated_line) service for computer modems to interconnect company [computer networks](/source/computer_networks) and to connect networks to an [Internet service provider](/source/Internet_service_provider) for Internet connectivity before commodity [DSL](/source/DSL) and [cable modem](/source/cable_modem) connectivity was widely available.

==References==
{{Reflist}}
* A History of engineering and science in the Bell System: transmission technology (1925-1975)

{{telecommunications}}

Category:Communication circuits
Category:Telephony

{{telephony-stub}}

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Adapted from the Wikipedia article [Four-wire circuit](https://en.wikipedia.org/wiki/Four-wire_circuit) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Four-wire_circuit?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.