Terrestrial Trunked Radio
|
|
TErrestrial Trunked RAdio (TETRA) is a specialist Professional Mobile Radio 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 (about 750 EUR in 2003), due to the reduced economies of scale compared to mass-market cellphones
- Data transfer is extremely slow at 7.2 kbit/s per timeslot.
- Due to the very low frequency modulation, handsets interfere with sensitive electronic devices such as heart pacemakers and defibrillators.
TETRA terminals can act as 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. Also possible is 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 emrgency 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 standard though at a low data rate.
TETRA Mobile Stations (MS) can communicate Direct Mode or using infrastructure made of TETRA Base Stations (TBS). As well as allowing direct communications in situations where network coverage has been lost, this also includes the possibility of using one (or a chain) of TETRA terminals as relays for a signal. 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 provided over the system's main control channel, while Packet Data communication uses specifically assigned traffic channels.
All traffic is normally encrypted. TETRA provides both over the air 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. TETRA is endorsed by the European Radio Communications Committee (ERC).
Health Worries
TETRA operates its TDMA (see above) at 17Hz i.e. an amplitude modulation of 17 times a second held in a carrier wave at the microwave frequencies listed above. It is alleged that TETRA causes health problems with those living near masts because this frequency interferes with brain activity. There do not appear to be any studies on people that provide direct information on health effects of exposures to radio frequency fields at about 16Hz modulation.
External links
- European Standards Organization - Telecom Standards - TETRA (http://www.etsi.org/frameset/home.htm?/technicalactiv/tetra/tetra.htm)
- TETRA Memorandum of Understanding - TETRA Facts (http://www.tetramou.com)
- report on health effects of TETRA (http://www.nrpb.org/publications/documents_of_nrpb/abstracts/absd12-2.htm)
- Powerwatch Report on health effects of TETRA (http://www.powerwatch.org.uk/micrwave/masts/tetra.asp)de:TETRA