Editing Four-wire circuit

In [[telecommunications]], a '''four-wire circuit''' is a two-way [[Telecommunication circuit|circuit]] using two paths so arranged that the respective [[Signal (information theory)|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 is uses four conductors to create two complete electrical circuits, one for each direction. The two separate circuits (channels) allow full-[[Duplex (telecommunications)|duplex]] operation with low [[crosstalk]].

In telephony a four-wire circuit was historically used to transport and switch [[baseband]] audio signals in the phone company [[telephone exchange]] before the advent of [[digital modulation]] and the [[electronic switching system]] eliminated baseband audio from the telco plant except for the [[local loop]]. The local loop is a [[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]] to keep near and far voice levels equivalent.

As the [[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]] to present day), [[microwave radio relay]] and ultimately [[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]] service for computer modems to interconnect company [[computer networks]] and to connect networks to an [[Internet service provider]] for Internet connectivity before commodity [[DSL]] and [[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]]


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