Duplex

In communications, duplex means "two-way" when referring to communications channels.

• A simplex channel is a one-way channel.
• A half-duplex channel is one that can carry information in both directions, but cannot carry it in both directions at the same time.
• A full-duplex channel is a channel which can carry information in both directions at once.

These terms apply also to early PC sound cards. Most are now full-duplex.

Where channel access methods are used for dividing forward and reverse communication channels on the same physical communications medium, they are known as duplexing methods, such as:

• Time division duplex (TDD) is the application of Time division multiple access to separate outward and return signals
• Frequency division duplex (FDD) is the application of Frequency division multiple access to separate outward and return signals