A two-wire circuit is a telecommunication wiring model that uses a single pair of wires for each user point capable of bi-directional traffic, or, in other words, both sending and receiving signals. This wiring method is utilized in most consumer telephone cables that serve end user terminals. The wiring in most trunk and switching facilities is, in contrast, typically four wire, with each line featuring separate wiring pairs for sending and receiving signals. These four-wire systems require the installation of a device such as a hybrid coil or electronic converter to allow the conversion from four to two wire systems on the outgoing lines. The impedance of the components on both ends of a two-wire circuit have to be carefully matched to each other to prevent echo on the line as a result of the simultaneous reception and transmission of signals.
Telecommunication installations are two-way systems that allow persons or devices on either end of a consumer line to communicate with one another. Generally, in large telephone trunk and switching facilities, a four-wire system consisting of two separate pairs is used for each line. In these installations, one pair is used for sending outgoing signals, and the other for receiving incoming signals. The cables that run from these facilities to the end user points are generally of a two-wire circuit design and feature a single pair of wires for each line. This means that the single pair is used to facilitate bi-directional traffic, allowing for both reception and transmission of signals.
The conversion from the exchange four-wire setup and the consumer line two-wire circuit is made possible by the inclusion of a converter on each line. This device allows the separate outgoing and incoming signals in the facility to be converted to the two-way traffic used in the two-wire circuit. These converters may, in older installations, be hybrid coil transformers or integrated circuit (IC) and resistor packs in newer facilities. These devices also facilitate the balancing of the impedance of both sides of the line.
Impedance balancing in a two-wire circuit is critically important for maintaining a good, clear signal. Impedance is a term used to describe the total alternating current (AC) resistance in terms of amplitude, voltage, current, and phase relationship. If the impedance of the exchange equipment and the user's phone or modem do not match, the signal quality will be poor, with unacceptable levels of volume disparity and echo. This is why most countries have strict standards, which ensure consistency in the specifications of the instruments available to end users.