 upon their electrical and physical characteristics. There are three basic types of channels: simplex, half-duplex and full-duplex. As an example of each, consider transmission between points A and B in Fig. 1-3. Transmission from A to B only (and not from B to A) requires a simplex channel. Simplex channels are used in loop mode configurations such as supermarket checkout termi- nals. Transmission from A to B and then from B to A, but not si- multaneously, requires a half-duplex channel. If a two-wire circuit is used, the line must be turned around to reverse the direction of transmission. A four-wire circuit eliminates line turnaround. Trans- mission from A to B and from B to A simultaneously describes a full-duplex channel. Although four wires are most often used, a two- wire circuit can support full-duplex communications if the frequency spectrum is subdivided into receive and transmit channels. Bits 6 5 4 3 2 1 ASC II "C" Fig. 1-2. Serial transmission is most often used. 6 TELECOMMUNICATIONS Simplex Half-duplex Full-duplex Fig. 1-3. Types of communications channels. In addition to the direction of transmission, a channel is charac- terized by its bandwidth. In general, the greater the bandwidth of the assigned channel, the higher the possible transmission speed. This speed it usually measured in terms of the number of line signal ele- ments per second, the baud rate. If a signal element represents one of two binary states, the baud rate is equal to the bit rate. When more than two states are represented, as in multilevel modulation, the bit rate exceeds the baud rate. 1.2.3 Digital Versus Analog Transmission Digital transmission can be applied to digital data or analog voice signals. In either case, information is sent over the communications channel as a stream of pulses. If analog to digital conversion is re- quired, the signal voltage values are sampled and represented in bi- nary format (see Fig. 1-4). Pulses transmitted over a communication line are distorted by line capacitance, inductance, and leakage