 is an alarm circuit. If someone opens a door he or she is not authorized to open, a signal is sent over the wires to the security desk, but nothing comes back over the wires. Half-duplex provides the capability to transmit information in two directions but in only one direction at a time (e.g., with a pair of walkie-talkies). Half-duplex is associated with two-wire circuits, which have one path to carry information and a second wire or path to complete the electrical loop. Because half-duplex circuits can't handle simultaneous bidirectional flow, there has to be a procedure for Chapter 5 Data Communications Basics Simplex Half-Duplex (2-Wire Circuit) Full-Duplex (4-Wire Circuit) Figure 5.5 Simplex, half-duplex, and full-duplex data transmission manipulating who's seen as the transmitter and who's seen as the receiver, and there has to be a way to reverse who acts as the receiver and who acts as the trans- mitter. Line turnarounds handle these reversals, but they add overhead to a session because the devices undertake a dialog to determine who is the transmitter and who is the receiver. For communication that involves much back-and-forth exchange of data, half-duplex is an inefficient way of communicating. Full-duplex, also referred to simply as duplex, involves a four-wire circuit, and it provides the capability to communicate in two directions simultaneously. There's an individual transmit and receive path for each end of the conversation. There- fore, no line turnarounds are required, which means full-duplex offers the most efficient form of data communication. All digital services are provisioned on a four- wire circuit and hence provide full-duplex capabilities. You may be wondering how you can conduct a two-way conversation on your telephone, which is connected to a two-wire local loop. The answer is that the tele- phone set itself is a full-duplex device, containing a circuit called the network inter- face or telephony hybrid, which connects the microphone and speaker to the telephone line and performs the