telecommunication Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...

, a four-wire circuit is a two-way circuit using two paths so arranged that the respective

signals In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...

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 In telecommunications and signal processing, baseband is the range of frequencies occupied by a signal that has not been modulated to higher frequencies. Baseband signals typically originate from transducers, converting some other variable int ...

audio signals in the phone company telephone exchange before the advent of

digital modulation In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...

and the electronic switching system eliminated baseband audio from the telco plant except for the

local loop In telephony, the local loop (also referred to as the local tail, subscriber line, or in the aggregate as the last mile) is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the commo ...

. The local loop is a

two-wire circuit In telecommunication, a two-wire circuit is characterized by supporting transmission in two directions simultaneously, as opposed to four-wire circuits, which have separate pairs for transmit and receive. The subscriber local loop from the telco c ...

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 A telephone hybrid is the component at the ends of a subscriber line of the public switched telephone network (PSTN) that converts between two-wire and four-wire forms of bidirectional audio paths. When used in broadcast facilities to enable the a ...

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 {{No footnotes, date=August 2008 A class-5 telephone switch is a telephone exchange in the public switched telephone network (PSTN) that directly serves subscribers and manages subscriber calling features. Class-5 services include basic dial-tone, ...

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 In computer networks and telecommunications, a dedicated line is a communications cable or other facility dedicated to a specific application, in contrast with a shared resource such as the telephone network or the Internet. It is a communication ...

service for computer modems to interconnect company

computer networks A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are m ...

and to connect networks to an

Internet service provider An Internet service provider (ISP) is an organization that provides services for accessing, using, or participating in the Internet. ISPs can be organized in various forms, such as commercial, community-owned, non-profit, or otherwise privat ...

for Internet connectivity before commodity

DSL Digital subscriber line (DSL; originally digital subscriber loop) is a family of technologies that are used to transmit digital data over telephone lines. In telecommunications marketing, the term DSL is widely understood to mean asymmetric dig ...

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