Communications Terminology
Half-Duplex: Circuit established on a single frequency for two-way communications
Semi-Duplex: One end of circuit is equipment limited (transmit on one freq or receive on another freq but not at the same time)
Full-Duplex: Circuit both parties can transmit and receive on two different frequencies at the same time
Notes:
Half duplex: standard tactical radio circuit with one person talking at a time
Full-duplex: a telephone, and the goal that many systems are going to