TETRA Group communication is most often called talkgroup communication. User that have the same talkgroup selected in the mobile radios are able to communicate with each other on a half duplex basis. Half duplex commununication means that one user is speaking (transmitting) while the others in the same group listen to the person that is transmitting. The listeners can only start to talk (transmit) when the other it finished (with different capabilities/permissions one can interupt which is called pre-empting, this is manufacturer depending). When a control room is available the control room dispatcher is also listening to the talkgroup. Most of the time the dispatcher is able to interupt the other talkgroup users (pre-empt).
In analogue networks users selected a frequency on the mobile radios. The users that selected the same frequency are able to communicate with each other. The number of available frequencies determined to number of groups to be formed. In TETRA a talkgroup has a number (GSSI) that is configured in the network and the mobile radios. Users select the alias on their radios that is associated to it (the talkgroup alias or talkgroup name). The network deals with assigning a temporary frequency for the time of the communication. When the transmission stops the frequency can be reused for another talkgroup. This means that the number of available frequencies doesn’t affect the number of talkgroups. The number of possible talkgroups that can be used can be several thousands. Ofcourse care need to be taken that the number of simultaneous active talkgroups doesn’t exceed the number of available frequencies/channels (since one frequency in TETRA consist of four channels).