A duplex communication system is one where signal can flow in both directions between connected parties.
These systems are employed in nearly all communications networks, either to allow for a "two-way street" between connected parties or to provide a "reverse path" for the monitoring and remote adjustment of equipment in the field.
Sub-Categories of Duplex
Half-Duplex
A half duplex system allows communications in both directions, but only one direction at a time (not simultaneously). Any radio system where you must use "Over" to indicate the end of transmission, or any other procedure to ensure that only one party broadcasts at a time would be a half duplex system.
A good analogy for a half duplex system would be a one lane road with traffic controllers at each end. Traffic can flow in both directions, but only one direction at a time with this being regulated by the controllers.
Full-Duplex
A full duplex system allows communication in both directions, and unlike half duplex allows this to happen simultaneously. Most telephone networks are full duplex as they allow both callers to speak at the same time.
A good analogy for a full duplex system would be a two lane road with one lane for each direction.
Examples
See Also