telecommunication 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 two-wire circuit is characterized by supporting transmission in two directions simultaneously, as opposed to

four-wire circuit In telecommunications, 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 ...

s, 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 co ...

from the telco central office are almost all two wire for analog baseband voice calls (and some digital services like

ISDN Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. ...

), and converted to four-wire at the

line card A line card or digital line card is a modular electronic circuit designed to fit on a separate printed circuit board (PCB) and interface with a telecommunications access network. A line card typically interfaces with a twisted pair cable of a ...

back when telephone switching was performed on baseband audio. Today the audio is

digitized Digitization is the process of converting information into a digital (i.e. computer-readable) format.Collins Dictionary. (n.d.). Definition of 'digitize'. Retrieved December 15, 2021, from https://www.collinsdictionary.com/dictionary/english ...

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 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, small collection of buildings or portably within a small coverage area, which funct ...

and military

field telephone Field telephones are telephones used for military communications. They can draw power from their own battery (electricity), battery, from a telephone exchange (via a Common battery, central battery known as CB), or from an external power source. S ...

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, colloquially referred to as a phone, is a telecommunications device that enables 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 ...

and at the central office. A

hybrid coil A hybrid transformer (also known as a bridge transformer, hybrid coil, or just hybrid) is a type of directional coupler which is designed to be configured as a circuit having four ports that are conjugate in pairs, implemented using one or mo ...

accomplishes the conversion for both. At the central office, it is part of a

four-wire terminating set A four-wire terminating set (4WTS) is a balanced transformer used to perform a conversion between four-wire and two-wire operation in telecommunication systems. For example, a 4-wire circuit may, by means of a 4-wire terminating set, be connec ...

, more often as part of a

. 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 (mathematics), matrix configuration. A crossbar switch has multiple input and output lines that form a ...

(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 January ...

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.

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