In telecommunication

Telecommunication

Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...

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

Four-wire circuit

In telecommunication, 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...

s, which have separate pairs for transmit and receive. In either case they are twisted pair

Twisted pair

Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...

s. Telephone lines are almost all two wire, while trunks and switching are almost entirely four wire. To communicate in both directions in the same wire pair, conversion between four-wire and two-wire is necessary, both at the telephone

Customer-premises equipment

Customer-premises equipment or customer-provided equipment is any terminal and associated equipment located at a subscriber's premises and connected with a carrier's telecommunication channel at the demarcation point...

 and at the central office

Telephone exchange

In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...

. A hybrid coil

Hybrid coil

A hybrid coil is a transformer that has three windings, and which is designed to be configured as a circuit having four branches, that are conjugate in pairs....

 accomplishes the conversion for both. At the central office, it is part of a four-wire terminating set

Four-wire terminating set

A four-wire terminating set is a balanced transformer used to perform a conversion between 4-wire and 2-wire operation in telecommunication systems.For example, a 4-wire circuit may, by means of a 4-wire terminating set, be connected to a 2-wire telephone set...

, more often as part of a line card

Line card

A line card or Digital Line Card is a modular electronic circuit on a printed circuit board, the electronic circuits on the card interfacing the telecommunication lines coming from the subscribers to the rest of the telecommunications access network.A line card commonly interfaces the twisted pair...

.

Two Wire Impedance Standards

Because the same twisted pair carries telephone signals in both directions,
echo is often a problem on these circuits. Echo

Signal reflection

Signal reflection occurs when a signal is transmitted along a transmission medium, such as a copper cable or an optical fiber, some of the signal power may be reflected back to its origin rather than being carried all the way along the cable to the far end. This happens because imperfections in the...

 is avoided by ensuring matching
impedance

Impedance matching

In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....

at both end of the circuit. Different countries have different
standards for telephone impedance.

Country	Termination Nickname	Summary	Reference
Australia	TN12	220Ω + ( 820Ω	120 nF )	AS/ACIF S002
Canada	600Ω	600Ω	CS-03 Part I
European Union	CTR21*	270Ω + ( 750Ω	150 nF )	?
New Zealand	BT3	370Ω + ( 620Ω	310 nF )	PTC200
North America	600Ω	600Ω	?

• 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.