<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 communications.
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.
