Duplex is the having of two principal elements or parts.

A duplex home is the US name for a particular type of housing; see Semi-detached.

A Duplex Lamp is a type of oil lamp.

Telecommunications

In telecommunications, duplex means "two-way" when referring to communications channels.

• A simplex channel is a one-way channel.
• A half-duplex channel is one that can carry information in both directions, but not at the same time.
• A full-duplex channel is a channel which can carry information in both directions at once.

These terms also apply to early PC sound cards, however almost all are now full-duplex.

Where channel access methods are used for dividing forward and reverse communication channels on the same physical communications medium, they are known as duplexing methods, such as:

• Time division duplex (TDD) is the application of time-division multiple access to separate outward and return signals
• Frequency division duplex (FDD) is the application of frequency-division multiple access to separate outward and return signals

Usage in point-to-multipoint networks

In point to multipoint networks such as cellular networks, both TDD and FDD are used in communication between the base station and the mobile transmitter.

Time division duplex has a strong advantage in the case where the asymmetry of the uplink and downlink data speed is variable. As the amount of uplink data increases, more bandwidth can be allocated to that and as it shrinks it can be taken away. Another advantage is that the uplink and downlink radio paths are likely to be very similar in the case of a slow moving system. This means that techniques such as beam forming work well with TDD systems.

Frequency division duplex is much stronger in the case of symmetric traffic. In this case TDD tends to waste bandwidth during switchover from transmit to receive, has greater inherent latency, and may require more complex, more power-hungry circuitry.