<communications> Used to describe a communications channel that can carry
signals in both directions, in contrast to a simplex channel which only ever
carries a signal in one direction.
If signals can only flow in one direction at a time the communications is
"half-duplex", like a single-lane road with traffic lights at each end.
Walkie-talkies with a "press-to-talk" button provide half-duplex communications.
If signals can flow in both directions simultaneously the communications is
"full-duplex", like a normal two-lane road. Telephones provide full-duplex
The term "duplex" was first used in wireless, telegraph, and telephone
communications. Nearly all communications circuits used by computers are
two-way, so the term is seldom used.
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