The Terrestrial Trunked Radio(TETRA) trunked radio standard is suitable as a universal system platform for a wide range of mobile radio services: Paging, trunked radio, telephony, telemetry and data transmission.
The network-related Tetra service is Trunked Mode Operation( TMO) in which a connection is established via the base station. In addition, there is the network-independent direct mode, Direct Mode Operation( DMO), in which two or more radios can communicate directly with each other. This mode is activated by the mobile station and does not operate over the Tetra network; it is similar to voice radio.
Direct mode can be used when handheld radios are outside the coverage area of a base station. Also, a single radio can be used as a mobile relay station for other devices. Thus, on the one hand, a device in the vehicle can act as a repeater between two radios and ensure radio coverage of the handheld radios in an area, and on the other hand, it can act as a gateway to the physical Tetra network. This is the case for radios operating in DMO mode.
There are several different transmission modes for voice service: half- duplex, full-duplex, multicasting and broadcasting. For duplex voice transmission, the voice is compressed in time to allow continuous two-way communication over two staggered time slots.
Data services include the short data service of up to 255 characters per message and data-only transmission, although this does not reach high data rates until the Tetra Enhanced Data Service( TEDS) standardized in Release 2 (Tetra 2). In Tetra 1, one to four time slots can be combined for data transmission. Each time slot offers 7.2 kbit/s. This means that data rates of up to 28.8 kbit/s are possible.