What Is a Two-Wire Circuit?

Two-wire circuits are usually formed by a pair of metal conductors, which are telecommunication circuits for two-wire transmission.

When a telephone user talks, the signal transmitted from the telephone to the program-controlled exchange is

Simplex -Signal transmission can only be performed in one direction. For example, signals can be transmitted from A to B, and vice versa. Broadcasting is a good example.
Half-duplex Signals can be transmitted from A to B or from B to A, but not simultaneously. In other words, only two-way communication at the same time is not possible. A more common example is a walkie talkie.
Full duplex signals can be transmitted simultaneously in both directions. That is to say, at any one time, while performing signal transmission from A to B, signal transmission from B to A can also be performed. Phone is such an example ^[2]

Because the same twisted pair carries telephone signals in both directions, echo is usually a problem on these circuits. Different countries have different phones

The earliest communication system was actually two-wire, because the transmission distance is short, no relay is needed, and the channel is a two-wire cable (more generally, it is a pair of twisted pairs. Don't misunderstand it as UTP, there are four pairs Twisted pair), there is no problem with full-duplex voice communication at this time, as was the case with early telephone switching systems.
Later, people were not satisfied with short-distance communication, so they added repeaters to the channel, which greatly increased the transmission distance. The simplest repeater is an amplifier (in fact, even a complex repeater is indispensable), but the amplifier only has a unidirectional amplification function (seems to have not heard of an amplifier with a bidirectional amplification function, at least, the problem of self-excited oscillation It ca nt be solved), so this second-line communication system has changed from a full-duplex system to a simplex system. In order to maintain the original function, a reverse simplex system was added, so that there was a full-duplex system composed of two simplex systems, and the original two-wire system became a four-wire system. .
Therefore, most of the current communication systems are four-wire systems (optical fiber is dual-core), and only a few systems are two-wire systems, such as the user loop part of the local call.
There was a time, in order to save the cost of long-distance communication trunk lines, the frequency division method was used on the second line, that is, one frequency band is used for AB signal transmission, and the other frequency band is used for BA signal transmission. Such a system can be said to be a nominal two-wire system and an essentially four-wire system. A typical example is a 12-channel carrier. At present, this technology is still basically used for the "cat" on the local user ring, whether it is ISDN or ADSL. But generally not seen on long distance trunk lines.
In the communication system, it mainly appears in the form of four-wire system, and the situation of two-wire system is rare. However, many people think that the communication system is a two-wire system because the user terminals are basically displayed in a two-wire system. In fact, no matter what kind of communication, the trunk line is a four-wire system. As for computer communication (including commonly known as Internet access), it is generally a four-wire system, and also includes a small part of the two-wire system (that is, the part that uses ISDN or XDSL to access the Internet). However, there must be a network card connected to the computer. It's a four-wire system. The two-wire system and the four-wire system can be switched under certain conditions (generally voice communication), but not in most cases (regardless of the nominal two-wire system, which is essentially a four-wire system at any time. Can be converted, provided there is sufficient bandwidth) ^[3] .