information, and the other wire acts as the return path to com- plete the electrical circuit. Two-wire circuits are generally deployed in the analog local loop, which is the last mile between the subscriber and the subscriber's first point of access into the network. Figure 1.1 shows an example of a two-wire circuit. Transmission Lines Figure 1.1 A two-wire circuit Two-Wire and Four-Wire Versus Two-Pair and Four-Pair Don't confuse the terms two-wire circuit and four-wire circuit with the terms two-pair and four-pair. Two-pair and four-pair refer to the number of wires in the internal cabling plan. Two-wire and four-wire have to do with the number of electrical conductors associated with a transmission line. Four-Wire Circuits A four-wire circuit has two pairs of conductors. That is, it has two sets of one-way transmission paths: one path for each direction and a complementary path to com- plete the electrical circuit (see Figure 1.2). Four-wire circuits are used where the distance between the termination points requires that the signal be strengthened (amplified) periodically. So, for example, four-wire circuits connect the various switches that make up the public switched telephone network (PSTN). Four-wire circuits are also used with leased lines, where a customer may be connecting loca- tions of its own that are separated by distance. Also, all digital circuits are provi- sioned on a four-wire basis. There are two types of four-wire circuits: physical four-wire and logical four- wire. In physical four-wire, you can actually count four wires. In logical four-wire, physically there are only two wires, but you derive the four individual paths by splitting the frequency. Half of the frequency band carries the transmit signal, and the other half carries the receive signal. So you can't always tell just by looking what kind of circuit you're dealing with; the application dictates the type of circuit it is. Customer Carrier Figure 1.2 A four-wire circuit Chapter 1 Telecommunications Technology Fundamentals Using Two-