In telecommunication, 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. In either case they are twisted pairs. 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 and at the central office. A hybrid coil 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.
Two wire impedance standards
Because the same twisted pair carries telephone signals in both directions, echo is often a problem on these circuits. Echo is avoided by ensuring matching impedance at both ends of the circuit. Different countries have different standards for telephone impedance.
|Australia||TN12||220Ω + ( 820Ω || 120 nF )||AS/ACIF S002|
|Canada||600Ω||600Ω||CS-03 Part I |
|European Union||CTR21*||270Ω + ( 750Ω || 150 nF )||ETSI ES 202 971 V1.2.1|
|New Zealand||BT3||370Ω + ( 620Ω || 310 nF )||PTC200 |
- The European regulatory requirement CTR 21 has been officially withdrawn.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. 
- Spectrum Management and Telecommunications - CS-03, Part I - Requirements for Terminal Equipment and Related Access Arrangements Intended for Direct Connection to Analogue Wireline Facilities
- http://www.telepermit.co.nz/PTC200X6.html NZ Telecom access standard
- CTR-21 withdrawn
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