halfduplex

Half-duplex refers to a communication system in which data can be transmitted in both directions, but not at the same time. In a half-duplex channel, one party must finish transmitting before the other can begin, and a control mechanism is often required to manage the direction of flow. This contrasts with simplex systems, which allow one-way transmission only, and full-duplex systems, which support simultaneous two-way communication.

Common examples of half-duplex devices include walkie-talkies, push-to-talk (PTT) radios, and many two-way radio services. In these systems, users press a button to transmit and release it to listen, so commanding the channel as a single shared resource. Some older or simpler computer networks also operate in half-duplex mode, especially when using a single shared medium.

Technically, half-duplex communications rely on a single channel or medium that must be shared between both directions. Media access control mechanisms, such as CSMA/CD used in early Ethernet networks, help prevent and manage collisions that occur when two nodes attempt to transmit simultaneously. When a collision is detected, devices wait for a random period before attempting to retransmit. Because only one direction at a time is active, network efficiency can decline as traffic increases.

In modern networking, half-duplex operation is often found in legacy or coaxial-cable networks and in some older Ethernet setups that connect through hubs. Most contemporary Ethernet and wireless networks aim for full-duplex operation using switches or multiple simultaneous channels to avoid collision-related overhead. Nevertheless, half-duplex remains a useful model for understanding shared-medium communications and certain mobile and radio technologies.