Half duplex means only one device transmits at a time. Think of a walkie-talkie. If two devices transmit simultaneously, a collision occurs. Devices must wait and retry. Half duplex uses CSMA/CD (Carrier Sense Multiple Access with Collision Detection). You'll only see half duplex with hubs or legacy equipment. Modern switched networks don't need it.
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$ curl repovive.com/roadmaps/network-design/physical-and-data-link-fundamentals/half-duplex-explained
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