Terrestial Trunked Radio

TErrestial Trunked RAdio (TETRA) is a specialist cell phone and walkie
talkie standard used by police, ambulance and military. Its main advantage
over technologies such as GSM are:

   * the much lower frequency used, which permits very high levels of
     geographic coverage with a smaller number of transmitters, cutting
     infrastructure cost,
   * the fact that its infrastructure can be separated from that of the
     public cellphone network, and made substantially more diverse and
     resilient by the fact that base stations can be some distance from the
     area served, and that
   * unlike most cellular technologies, TETRA can also be used in a
     point-to-point mode if the infrastructure fails.

Its main disadvantages are:

   * it can only support a much lower teledensity in a given area, compared
     to GSM and similar technologies (which is not a problem in the
     applications for which it is used, but mostly limits it to these
     applications)
   * handsets are more expensive, due to the reduced economies of scale
     compared to mass-market cellphones

TETRA uses Time Division Multiple Access (TDMA) with four user channels on
one radio carrier and 25 kHz spacing between carriers. Both point-to-point
and point-to-multipoint transfer can be used. Mobile Stations (MS) can
communicate Direct Mode or using infrastructure made of TETRA Base Stations
(TBS).

All traffic is normally encrypted. TETRA provides both transfer encryption
and end-to-end encryption.

In Europe, TETRA uses frequencies:

   * Emergency systems 380-383 MHz, 390-393 MHz (also 383-395 MHz and
     393-395 MHz)
   * Civil systems 410-430 MHz, 870-876 MHz / 915-921 MHz, 450-470 MHz,
     385-390 MHz / 395-399,9 MHz,

TETRA is an ETSI standard, first version published 1995.

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