telecommunications Telecommunication, often used in its plural form or abbreviated as telecom, is the transmission of information over a distance using electronic means, typically through cables, radio waves, or other communication technologies. These means of ...

, 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 is uses four conductors to create two complete electrical circuits, one for each direction. The two separate circuits (channels) allow full- 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 The public switched telephone network (PSTN) is the aggregate of the world's telephone networks that are operated by national, regional, or local telephony operators. It provides infrastructure and services for public telephony. The PSTN consists o ...

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 Fiber-optic communication is a form of optical communication for transmitting information from one place to another by sending pulses of infrared or visible light through an optical fiber. The light is a form of carrier wave that is modul ...

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 An Internet service provider (ISP) is an organization that provides a myriad of services related to accessing, using, managing, or participating in the Internet. ISPs can be organized in various forms, such as commercial, community-owned, no ...

for Internet connectivity before commodity DSL and cable modem connectivity was widely available.

References

* A History of engineering and science in the Bell System: transmission technology (1925-1975) Communication circuits Telephony {{telephony-stub