modes of channel operation - Simplex, Half-Duplex and Full Duplex

Simplex
Data in a simplex channel is always one way. Simplex channels are not often used because it is not possible to send back error or control signals to the transmit end.

It's like a one way street. An example of simplex is Television, or Radio. The simplex channel also corresponds directly to Shannon's model of communication discussed earlier.

Half Duplex
A half-duplex channel can send and receive, but not at the same time. It's like a one-lane bridge where two way traffic must give way in order to cross. Only one end transmits at a time, the other end receives. In addition, it is possible to perform error detection and request the sender to retransmit information that arrived corrupted. In some aspects, you can think of Internet surfing as being half-duplex, as a user issues a request for a web document, then that document is downloaded and displayed before the user issues another request.

Another example of half-duplex is talk-back radio, and CB Radio (Citizens Band). You might have seen movies where truckies (drivers of very big trucks) communicate to each other, and when they want the other person to speak they say "over". This is because only one person can talk at a time.

Full Duplex
Data can travel in both directions simultaneously. There is no need to switch from transmit to receive mode like in half duplex. Its like a two lane bridge on a two-lane highway. Have you ever watched these television talk shows where the host has a number of people on the show, and they all try to talk at once. Well, that's full duplex!