A duplex communication system is a point-to-point system composed of two connected parties or devices that can communicate with one another in both directions simultaneously. An example of a duplex device is a telephone. The people at both ends of a telephone call can speak at the same time, the earphone can reproduce the speech of the other person as the microphone transmits the speech of the local person, because there is a two-way communication channel between them.
The term multiplexing is used when mediating pairwise communication between more than one pair of parties or devices - on the same point-to-point system. There is no communication between the separate pairs of parties or devices.
Duplex systems are employed in many communications networks, either to allow for a communication "two-way street" between two connected parties or to provide a "reverse path" for the monitoring and remote adjustment of equipment in the field.