Half duplex is a communication mode in which an interface can send and receive data, but not simultaneously. The interface must wait for the shared medium to become available before transmitting. section 8.1.3
Shared media explain the limitation. Wi-Fi devices use the same radio-frequency medium and take turns communicating. Older Ethernet hubs also required half duplex because they repeated incoming signals to every port. section 8.1.3 section 8.4
Half-duplex operation has several important consequences:
- Only one host in a collision domain can transmit at a time. section 8.4
- Half-duplex devices use CSMA/CD to detect and handle collisions. section 8.4
- A wired Ethernet switch normally enables full duplex, while a hub places connected hosts in one shared half-duplex collision domain. section 8.1.3 section 8.4
- A duplex mismatch can leave both interfaces up while greatly reducing performance; the half-duplex side may record collisions and late collisions, while the full-duplex side may record runts or CRC errors. section 8.3.2
Cisco interface configuration
Use `duplex half` to force half-duplex operation, `duplex full` for simultaneous transmission, or `duplex auto` to use autonegotiation. Verify the result with `show interfaces status`; an `a-` prefix indicates autonegotiation. For example, a manually configured GigabitEthernet interface can show `full`, while an automatically negotiated FastEthernet interface can show `a-full`. section 8.4
