A wire pair connecting two telephones is the simplest circuit, allowing communication in both directions. Other channels include coaxial cables, optical fibres, satellites, a light beam, microwaves, and others. Whatever the physical medium over or through which message travels, transmission may take place in a number of different modes. They are :
Simplex Half-duplex and Full-duplex. Simplex In this type of channel, the message travels in only one direction, that is from point A to point B but not back again.
Half-Duplex: Transmission can take place in either direction, but not at the same time. In other words, message can be transmitted either from A to B or by B to A one at a time.
Full-Duplex: In this type of arrangement messages can be transmitted and received over the same channel at the same time. Many librarians and information scientists are familiar with the latter two types because database search terminals must be set to transmit full or half duplex in order to communicate with database vendors' computers.