TETRA (Terrestrial Trunked Radio) is a standard of digital trunking communication developed in 1992 by ETSI (European Telecommunication Standard Institute).

In TETRA we have two kinds of terminals:

• mobile terminals
• base stations

Similarly to cellular telephone systems, mobile terminals may have simply functions for voice transmissions but it is also possible to use terminals that allow data transmissions or other services. TETRA network may be connected to other TETRA sites by other infrastructures and have possibilities of roaming connections between operators.

In TETRA we have following type of services:

• Individual Call: This service corresponds to a call in a public mobile radio system (GSM, UMTS). One user calls another individual user and is connected with the latter
• Group Call: One user calls a predefined group. Each member of the group can hear everything and can speak. The group call can be set up so that the individual members have to acknowledge or not. A group can be modified dynamically, i.e. members can be added or removed
• Direct Mode: In direct mode, two or more mobile stations communicate with eachother, without involving a base station (walkie-talkie)
• Broadcast Call: This is a unidirectional point-to-multipoint call within a specified area. The area and the users are defined in advance. The individual users do not acknowledge the call and therefore the caller is unable to verify who has received and who has not received the call
• Emergency Call: An emergency call button sets up a high-priority call to a dispatcher or a predefined group of users
• Include Call: During a call, this type of call makes it possible to ring one or more additional users and include them in the call
• Open Channel: A group of users can converse with ach other on a specific channel for a specific period. Within the group, all participants can hear each other and can speak at any time. In TETRA this service is not explicitly standardized.
• Carrier services (data services):
  -Status Transmission: very short, predefined messages can be transferred from the dispatcher to the mobile stations and vice versa or between mobile stations
  - Short Data Service: predefined messages can be sent to individual subscribers or to a group
• Circuit-switched data services:
  - non-protected data transmission: n x 7.2 kbit/s (n = 1, 2, 3 or 4)
  - protected data transmission: n x 4.8 kbit/s (n = 1, 2, 3 or 4)
  - highly protected data transmission: n x 2.4 kbit/s (n = 1, 2, 3 or 4)

TETRA uses following bands:

• 380 - 400 MHz
• 410 - 420 MHz

There are two base system modes:

• TETRA VD - for voice and data communication
• TETRA POD - for data transmissions only.