Duplex
<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.
(http://www.cit.ac.nz/smac/dc100www/dc_014.htm).
