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 ...

, 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 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 ...

from the telco central office are almost all two wire for analog baseband voice calls (and some digital services like ISDN), and converted to four-wire at the line card back when telephone switching was performed on baseband audio. Today the audio is

digitized DigitizationTech Target. (2011, April). Definition: digitization. ''WhatIs.com''. Retrieved December 15, 2021, from https://whatis.techtarget.com/definition/digitization is the process of converting information into a Digital data, digital (i ...

and processed completely in the digital domain upstream from the local loop. The reason for using two wires rather than four is simple economics—half the materials cost half as much to purchase and install. Note the use of the past-tense "cost," as installation of two-wire copper local loops for telephony was done primarily during the mid 20th century. In the first world there is no new infrastructure planning for new copper-based technology, and as customers are migrating to cellular telephony and high-speed Internet, wireline carriers are abandoning their copper local loops, tearing out the copper and replacing it with fiber-optic cable and/or selling the

rights-of-way Right of way is the legal right, established by grant from a landowner or long usage (i.e. by prescription), to pass along a specific route through property belonging to another. A similar ''right of access'' also exists on land held by a gov ...

to third parties for private use. In developing nations, wireless communications are considered to be the most cost-effective from an infrastructure perspective. Two-wire circuits in new installations are limited to

intercom An intercom, also called an intercommunication device, intercommunicator, or interphone, is a stand-alone voice communications system for use within a building or small collection of buildings which functions independently of the public telephon ...

and military field telephone applications, though these too are being supplanted by modern digital communication modes. To communicate in both directions in the same wire pair, conversion between four-wire and two-wire was necessary, both at the

telephone A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into e ...

and at the central office. A

hybrid coil A hybrid coil (or bridge transformer, or sometimes hybrid) is a transformer that has three windings, and which is designed to be configured as a circuit having four ports that are conjugate in pairs. A signal arriving at one port is divided e ...

accomplishes the conversion for both. At the central office, it is part of a four-wire terminating set, more often as part of a line card. A modern line card has no two-to-four wire conversion whatsoever; it is strictly an analog/digital interface to a system that has a completely digital and integrated signal path internally. Using actual wires to circuit switch a telephone call became obsolete when the

crossbar switch In electronics and telecommunications, a crossbar switch (cross-point switch, matrix switch) is a collection of switches arranged in a matrix configuration. A crossbar switch has multiple input and output lines that form a crossed pattern of int ...

(a mechanical system) was replaced by

4ESS The No. 4 Electronic Switching System (4ESS) is a class 4 telephone electronic switching system that was the first digital electronic toll switch introduced by Western Electric for long-distance switching. It was introduced in Chicago in Janua ...

electronic switches in the 1970s by the Bell System in the US. The old telephone hybrids of yore have been replaced by inexpensive IC chip-based components that perform the same functions at greatly reduced cost. When personal computing and the Internet became popular at the end of the 20th century, the inductive load of traditional hybrids became a liability for computer modem users, and remaining loading coils in subscriber lines were scrapped.

Impedance standards

Different countries have different standards for telephone impedance. * The European regulatory requirement CTR 21 has been officially withdrawn. Some manufacturers prefer to continue meeting CTR 21, but there is little reason to do so.CTR-21 withdrawn
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References

* A History of engineering and science in the Bell System: Transmission Technology (1925-1975) * A History of engineering and science in the Bell System: Switching Technology (1925-1975) Communication circuits {{telecomm-stub