A duplex communication system is a method of transmitting signals, allowing for two people, or pieces of equipment, to communicate with each other in opposite directions—meaning at the same time. This system has become an important standard in the area of telecommunications, especially in telephony and computer networking. Although the definition of duplex means to transmit in opposite directions at the same time, this is not the case in every duplex communication system. Two primary areas exist: full-duplex and half-duplex. In the half-duplex system, both parties can transmit data, so it technically works in opposite directions, but not at the same time.
To understand a full-duplex communication system, think of two groups of people standing at opposite ends of a field in separate lanes. A trigger is released and both sides walk or run toward each other at the same time, with no signals informing the groups to stop. The traffic of people continues in an orderly manner, with no collisions, until another trigger is released informing them to stop. This is the method in which full-duplex operates in a duplex communication system. The telephone, both land-line and mobile varieties, are the most well-known examples of full-duplex systems. Phones allow people to speak and hear concurrently.
Computer networks also are examples of full-duplex communications. When connecting to the Internet, a computer on one end sends data while a computer on the other ends sends data at the same time—a simultaneous exchange of information. Both receive and send data without stopping until triggered to do so. Dial-up telephone connections and high-speed broadband Internet are capable of communicating in full-duplex, depending on the networks used. Use of full-duplex allows for faster connections because more information can be sent and received.
Half-duplex communications allows communicating in two directions, but only in one direction at a time. In the example of the two groups standing at opposite ends of a field, only one group can take off when the trigger is released. The opposite group can't take off until the first group is stopped and a second trigger is released. Walkie-Talkie radios are a good example of a half-duplex system—each person has to wait until the other has stopped speaking in order to transmit. One person speaks, and the person on the opposite end receives the message. When the message is completed, the person often states, "over," which informs the other person they may relay their message.