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TETRA stands for TErrestrial Trunked RAdio. It follows GSM and DECT as the third digital mobile telecommunications system to be standardized by ETSI, the European Telecommunications Standards Institute. The TETRA standard, which was approved by ETSI in 1997, is a direct result of the 1994 TETRA MoU (Memorandum of Understanding). TETRA MoU set out to define a standard that would meet the needs of private mobile radio (PMR) users, particularly those working in areas of public safety, and to promote the adoption of that standard by international standards bodies. Originally TETRA stood for Trans European Trunked RAdio; the present name reflects a growing world-wide interest in the standard.
TETRA defines an open standard for digital trunked radio. A trunked radio shares available radio resources among users, allocating channels on demand to make most efficient use of radio spectrum. Digital modulation offers advantages over earlier analog systems in terms of clarity of reception and in the very important area of user security (public safety organizations are expected to be main user groups for TETRA). "Open Standard" allows diverse users (both geographically diverse and operationally diverse) to inter-operate, and hopefully creates a competitive market (with associated cost benefits) for both infrastructure and hand terminals.
Primary features of TETRA include the capability for simultaneous voice and data transmission, flexible data rates and on-demand bandwidth, digital encryption, and multi-vender support. Operation can be point-to-point (Direct Mode Operation or DMO) or point-to-multipoint (through infrastructure). The standard has two options: TETRA Voice + Data (V+D), and TETRA Packet Data Optimised (PDO). TETRA V+D provides voice services, both circuit and packet mode data services, and a range of supplementary services; it uses 4 TDMA timeslots on a single 25 kHz RF channel. TETRA PDO provides voice and packet data services, and uses packet switching at 36 kbps gross data rate via Statistical Multiplexing/Multiple Access (SMA/STMA). Voice services include individual call, group call, acknowledged group call, and broadcast call. Both clear and encrypted modes are available. Data services include unprotected data at rates to 28.8 kbps, and protected data to 19.2 kbps. Additional services include call authorized by dispatcher, area selection, access priority, late entry, pre-emptive priority call, discrete listening, ambience listening, dynamic group number assignment, and transfer of control.
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