In simplex operation one terminal of the system transmits while the other terminal receives.  Simultaneous transmission and reception at a terminal is not possible with simplex operation.  The simplex dispatching system consists of a base station and mobile units, all operating on a single frequency.

Simplex operation is sometimes referred to as single-frequency simplex.

In half-duplex dispatching systems, the base station and the mobile transmit on two different frequencies.  The base station transmits on the mobile's receive frequency and vice versa.  However, the half-duplex terminal equipment does not allow simultaneous transmission and reception.  Half-duplex operation is sometimes referred to as two-frequency simplex.

Half-duplex configuration was designed to allow a repeater-type of operation, when a base station "repeats" and amplifies a mobile's signal on a different frequency.  Half-duplex operation is used most commonly in public safety LMR systems.

In full-duplex operations, radios can transmit and receive simultaneously.  As in half duplex, this operation uses two frequencies, the difference is that transmitter and receiver can be both powered and active full time.  Repeater base stations typically operated in full-duplex mode, receiving on one frequency and re-transmitting the signal on another frequency, using separate transmit and receive antennas.  For full-duplex single-antenna configuration, additional equipment is required such as duplexer, which enables the receiver and transmitter to use the same antenna simultaneously.  LMR systems rarely use this type of operation, since full-duplex subscriber units are typically much more expensive and, if battery-powered, consume more battery power.