INTRODUCTION TO TETRA
TErrestrial Trunked RAdio (TETRA) (formerly known as Trans
European Trunked RAdio) is a specialist Professional Mobile Radio and two-way transceiver (colloquially known as a walkie talkie) specification. TETRA was specifically designed for use by government agencies, emergency services, (police forces, fire departments, ambulance), rail transportation staff, transport services and the military.
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TETRA is an ETSI standard, first version published 1995. TETRA is
endorsed by the European Radio Communications Committee (ERC) and mandated for use in Europe.
TETRA terminals can act as mobile phones (cell phones), with a direct connection to the PSTN. It is common also for them to operate in a group calling mode in which a single button push will connect the user to a dispatcher and all the other users in a group. 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. 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 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. Digital data transmission is also included in the tandard though at a low data rate.
TETRA Mobile Stations (MS) can communicate Direct Mode or using Trunked infrastructure (Switching and Management Infrastructure or SwMI) made of TETRA Base Stations (TBS). As well as allowing direct communications in situations where network coverage has been lost, Direct Mode or DMO also includes the possibility of using one (or a chain) of TETRA erminals as relays for a signal. This functionality is called DMO gateway (from DMO to TMO) or DMO Repeater (DMO to DMO). In rescue situations this feature could allow direct communications underground or in areas of bad coverage.
In addition to voice and dispatch services, the TETRA system supports
several types of data communication. Status messages and short data
services (SDS) are rovided over the system's main control channel,
while Packet Data or Circuit switched data communication uses
specifically assigned traffic channels.
All traffic is normally encrypted. TETRA provides both over the air
encryption and end-to-end encryption. |
