TETRA also provides a point-to-point function that traditional analogue emergency services radio systems did not provide.A dedicated transponder system isn't required in order to achieve this functionality, unlike with analogue radio systems.
Gateway mode - where a single mobile with connection to the network can act as a relay for other nearby mobiles that are out of range of the infrastructure.In the absence of a network, mobiles/portables can use 'direct mode' whereby they share channels directly (walkie-talkie mode).So called 'mission critical' networks can be built with TETRA where all aspects are fail-safe/multiple-redundant. This is a unique feature, which dPMR networks typically provide, that allows a number of fall-back modes such as the ability for a base station to process local calls. During a voice call, the communications are not interrupted when moving to another network site.The much lower frequency used gives longer range, which in turn permits very high levels of geographic coverage with a smaller number of transmitters, thus cutting infrastructure costs.The main advantages of TETRA over other technologies (such as GSM) are: Emergency buttons, provided on the terminals, enable the users to transmit emergency signals, to the dispatcher, overriding any other activity taking place at the same time.
TETRA terminals can act as mobile phones ( cell phones), with a full-duplex direct connection to other TETRA Users or the PSTN. It is also possible for the terminal to act as a one-to-one walkie talkie but without the normal range limitation since the call still uses the network. 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 25 kHz spacing between carriers. 5.4.5 Cell relinquishable (abandonable).5.4.1 Cell re-selection (or hand-over) in images.