The common mode of operation is in a group calling mode in which a single button push will connect the user to the users in a selected call group and/or a dispatcher. For protection against eavesdropping, air interface encryption and end-to-end encryption is available. TETRA provides for authentication of terminals towards infrastructure and vice versa. Status messages and short data services (SDS) are provided over the system's main control channel, while packet-switched data or circuit-switched data communication uses specifically assigned channels. In addition to voice and dispatch services, the TETRA system supports several types of data communication. In emergency situations this feature allows direct communications underground or in areas of bad coverage. This functionality is called DMO gateway (from DMO to TMO) or DMO repeater (from DMO to DMO). As well as allowing direct communications in situations where network coverage is not available, DMO also includes the possibility of using a sequence of one or more TETRA terminals as relays. TETRA Mobile Stations (MS) can communicate direct-mode operation (DMO) or using trunked-mode operation (TMO) using switching and management infrastructure (SwMI) made of TETRA base stations (TBS). Digital data transmission is also included in the standard though at a low data rate. Both point-to-point and point-to-multipoint transfer can be used. TETRA uses time-division multiple access (TDMA) with four user channels on one radio carrier and 25В kHz spacing between carriers. 5.4.5 Cell relinquishable (abandonable).5.4.1 Cell re-selection (or hand-over) in images.

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