8.1 Introduction to PC Networking
8.1.7 Simplex, half-duplex, and full-duplex transmission
The data channels over which a signal is sent can operate in one of three ways: simplex, half-duplex, or full-duplex (often just called duplex). The distinction is in the way that the signal can travel.

Simplex Transmission
Simplex is a single one-way baseband transmission. Simplex transmission, as the name implies, is simple. It is also called unidirectional because the signal travels in only one direction, just like traffic flows on a one-way street. Figure illustrates this transmission type.

Contemporary applications for simplex circuits, although rare, include remote station printers, card readers, and a few alarm or security systems (fire and smoke alarms). This type of transmission is not frequently used because it is not a practical mode for transmitting. The only advantage of simplex transmission is that it is inexpensive.

Half-Duplex Transmission
Half-duplex transmission is an improvement over simplex because the traffic can travel in both directions. Unfortunately, the road is not wide enough to accommodate bi-directional signals simultaneously. This means that only one side can transmit at a time, as shown in Figure . Two-way radios, such as Citizens Band (CB) and police/emergency communications mobile radios, work with half-duplex transmissions. When pressing the button on the microphone to transmit, nothing being said on the other end can be heard. If people at both ends try to talk at the same time, neither transmission gets through.

Note: Modems are half-duplex devices. They can send and receive, but not at the same time. However, it is possible to create a full-duplex modem connection with two telephone lines and two modems.

Full-Duplex Transmission
Full-duplex transmission operates like a two-way, two-lane street. Traffic can travel in both directions at the same time, as shown in Figure .

A regular telephone conversation is an example of full-duplex communication. Both parties can talk at the same time, and the person talking on the other end can still be heard by the other party while they are talking (although it might be difficult to understand what is being said).

Full-duplex networking technology increases performance because data can be sent and received at the same time. DSL, two-way cable modem, and other broadband technologies operate in full-duplex mode. With DSL, for example, users can download data to their computer at the same time they are sending a voice message over the line.