Two-wire circuit

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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.

[edit] 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 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[1]
Canada	600Ω	600Ω	CS-03 Part I [2]
European Union	CTR21	270Ω + ( 750Ω || 150 nF )	?
New Zealand	BT3	370Ω + ( 620Ω || 310 nF )	PTC200 [3]
North America	600Ω	600Ω	?

[edit] References