A two-wire circuit is a telecommunications wiring model that uses a single pair of wires for each user point, allowing for bi-directional traffic. Most consumer telephone cables use this method, while larger systems use four-wire systems with converters to facilitate conversion. Impedance balancing is crucial for maintaining signal quality.
A two-wire circuit is a telecommunications wiring model that uses a single pair of wires for each user point that can handle bi-directional traffic, or in other words, send and receive signals. This wiring method is used in most consumer telephone cables serving end-user terminals. Cabling in most splicing and switching systems is, in contrast, typically four-wire, with each line having separate pairs of wires 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 conversion from four-wire systems to two-wire on the outgoing lines. The impedance of the components at both ends of a two-wire circuit must be carefully matched to each other to avoid echo on the line due to the simultaneous reception and transmission of signals.
Telecommunications systems are two-way systems that allow people or devices on both ends of a consumer line to communicate with each other. Generally, large telephone lines and switching systems use a four-wire system consisting of two separate pairs for each line. In these installations, one pair is used to send out signals and the other to receive incoming signals. The cables running from these facilities to end-user points are usually of a two-wire circuit and have a single pair of wires for each line. This means that the single pair is used to facilitate two-way traffic, allowing for both reception and transmission of signals.
Conversion from the four-wire central configuration and two-wire consumer line circuit is made possible by the inclusion of a converter on each line. This device converts the separate signals entering and leaving the system into the two-way traffic used in the two-wire circuit. These converters may, in older installations, be hybrid coil transformers or integrated circuits (ICs) and resistor packs in newer designs. These devices also facilitate impedance balancing of both sides of the line.
Impedance balancing in a two-wire circuit is of paramount importance in maintaining a good, clear signal. Impedance is a term used to describe the total resistance of alternating current (AC) in terms of amplitude, voltage, current, and phase relationship. If the impedance of the exchange equipment and the user’s telephone or modem do not match, the signal quality will be poor, with unacceptable levels of loudness and echo disparity. This is why most countries have strict standards, which ensure consistency in the specification of tools available to end users.
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