双工（duplex）， 指二台通信设备之间，允许有双向的数据传输。

半双工（half-duplex）的系统允许二台设备之间的双向数据传输，但不能同时进行。因此同一时间只允许一设备发送数据，若另一设备要发送数据，需等原来发送数据的设备发送完成后再处理。

半双工的系统可以比喻作单线铁路。若铁道上无列车行驶时，任一方向的车都可以通过。但若路轨上有车，相反方向的列车需等该列车通过道路后才能通过。

无线电对讲机就是使用半双工系统。由于对讲机发送及接收使用相同的频率，不允许同时进行。因此一方讲完后，需设法告知另一方讲话结束（例如讲完后加上"OVER"），另一方才知道可以开始讲话。

A duplex communication system is a point-to-point system composed of two connected parties or devices that can communicate with one another in both directions. "Duplex" comes from "duo" that means "double", and "plex" that means "structure" or "parts of"; thus, a duplex system has two clearly defined data transmissions, with each path carrying information in only one direction: A to B over one path, and B to A over the other. There are two types of duplex communication systems: full-Duplex and half-Duplex.