Which system provides communication in both directions, but only one direction at a time?

A half-duplex (HDX) system provides communication in both directions, but only one direction at a time. An example of a half-duplex system is a two-party system such as a walkie-talkie, wherein one must use over or another previously designated keyword to indicate the end of transmission, and ensure that only one party transmits at a time, because both parties transmit and receive on the same frequency.