Halbduplex

Half-duplex, sometimes written half duplex, describes a communications channel that allows data transmission in both directions but not at the same time. Devices on a half-duplex link must alternate between sending and receiving, unlike simplex links (one-way) or full-duplex links (simultaneous two-way transmission).

On a shared medium, a single channel carries traffic in both directions, and access control determines who may transmit. In wired networks, half-duplex operation often relies on a contention-based mechanism such as CSMA/CD to detect collisions and retry transmissions. In practice, devices switch between transmit and receive modes, or use timing gaps, to avoid overlapping signals.

Common examples include older Ethernet networks that used a shared coaxial cable or a hub, where collision domains existed, and serial bus topologies such as RS-485 that operate in half-duplex mode on a two-wire pair. Wireless and radio systems, such as walkie-talkies, also use half-duplex, with users transmitting only when the channel is clear.

Advantages of half-duplex include simpler hardware and lower cost, particularly on long or noisy physical media. It can be sufficient for traffic that is naturally asynchronous or asymmetric. Limitations include lower sustained throughput due to collisions and turn-taking delays, increased latency under load, and inefficiency for modern applications that require high bandwidth and simultaneous two-way communication.

Related concepts include full duplex, simplex, CSMA/CD, and RS-485 or other bus architectures.