Basic radio communications systems use a single channel and what is known as a press to talk system, where the user presses a button or "pressel" on the microphone to talk, and then releases the pressel to listen on the same frequency. This system is known as simplex as it uses a single channel. For a phone system a full duplex system is required where it is possible to speak in both directions at the same time. There are two main ways in which this can be achieved. The first is to transmit in one direction on one frequency, and simultaneously transmit in the other direction on another. To achieve this there must be sufficient frequency separation and filtering to ensure that the transmitter does not interfere with the receiver. A scheme that uses one frequency for transmitting traffic in one direction and another frequency for traffic in the other is known as Frequency Division Duplex (FDD).

The other system uses only a single frequency and can be employed where digital or data systems are used. This requires the analogue audio signal to be digitised. A single frequency is used for the radio frequency signal and short packets of data are sent first in one direction, and then the other. As these data bursts are relatively short the user does not notice the short delay introduced by the fact that the digitised speech signal is not sent immediately. This type of system is known as Time Division Duplex (TDD).

It is often necessary to distinguish between the link from the mobile to the base station, and the link from the base station to the mobile. The first, i.e. the link from the mobile to the base station is often called the uplink or the reverse link as the signal is being transmitted up to the base station. The second, i.e. the link from the base station to the mobile is known as the downlink or the forward link.