When a person is giving a lecture or speech, information is primarily conveyed in one direction. However, during a conversation between two people, spoken messages (information) are usually exchanged in both directions. These messages are normally exchanged alternatively but can , of course, be exchanged simultaneously. This analogy can be used to explain the three main communication modes that a channel can operate under.

Simplex Mode

This is when data is to be transmitted in one direction only. For example, in a data logging system in which a monitoring device returns a reading at regular intervals to the gathering facility.

Half-Duplex

This is used when the two interconnected devices wish to exchange information (data) alternatively. For example, if one of the devices returns some data only in response to a request from the other. Clearly, the two devices must be able to switch between send and receive modes after each transmission.

Full Duplex

This is used when data is to be exchanged between two interconnected devices in both directions simultaneously. For example, if for throughput reasons data can flow in each direction independently.

It is imperative to realise that the alternative communication modes are important. In many distributed systems the circuits (lines) used to provide the communication facilities are often leased from the post, telephone and telecommunications (PTT) authority, and it is less expensive to lease a single circuit, rather than two complete circuits if only a simple operation is required.

Moreover, in addition to the messages carried by data transmission systems, it should be noted that usually it is also necessary to carry control information e.g. details of whether equipment is ready to receive a transmission and so on. Even in the case of a simplex system there can be transmission of control information in the opposite direction.