TWO DEVICES on a link can transmit; the other can only receive. EXAMPLE of simplex systems: Keyboards and traditional monitorsIn half-duplex mode, each station can both transmit and receive, but not at the same TIME, that is, when one DEVICE is sending, the other can only receive, and vice versa.Example of Half-duplex systems: Walkie-talkies and CB (citizens band) radiosIn full-duplex mode, both stations can transmit and receive simultaneously; Telephone is an example of full-duplex system