Any point that can send and receive signals can function as a data communications site. Data communications uses a wide range of signal carriers, including the public telephone system or other circuits that can deliver electrical tones and digital signals. Simplex A simplex circuits consist of a single channel that handles transmissions in only one direction at a time. To communicate in the other direction, it is necessary for people at each end of a circuit to reset switches that adapt the line for transmission in the other direction. The original telegraph lines were simplex circuits.
Duplex circuits allow information to be transmitted on the line in both directions.
One way this is done is to reverse frequencies above a certain level for traffic in one direction, and reserve frequencies below that level for traffic in the other direction. Another way is to have a pair of circuits that connect two or more communicating points, one pair having communication in one direction, and the other pair communication in the other. The telephone system is an example of a Full-Duplex system. Half-duplex circuits consist of a single circuit and allows for transmission in either direction, one at a time. Once a transmitting station has completed a message, either party can claim the line and transmit data.