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 end of the circuit. Different countries have different standards for telephone impedance.
| Country |
Termination Nickname |
Summary |
Reference |
| Australia |
TN12 (same as CTR21) |
220Ω + ( 820Ω || 120 nF ) |
AS/ACIF S0021 |
| Canada |
600Ω |
600Ω |
CS-03 Part I 2 |
| European Union |
CTR21 |
220Ω + ( 820Ω || 120 nF ) |
? |
| New Zealand |
BT3 |
370Ω + ( 620Ω || 310 nF ) |
PTC200 3 |
| North America |
600Ω |
600Ω |
? |
References
