TETRA, a cellular network for public service professionals
TETRA or Terrestrial Trunked Radio is a specification for professional mobile radio. It was designed specifically with emergency services, public safety networks, the military and other government services in mind. The ETSI standard for Tetra was first published in 1995.
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 operation (DMO) or using trunked-mode operation (TMO) using switching and management infrastructure (SwMI) made of TETRA base stations (TBS). As well as allowing direct communications in situations where network coverage is not available, DMO also includes the possibility of using a sequence of one or more TETRA terminals as relays. This functionality is called DMO gateway (from DMO to TMO) or DMO repeater (from DMO to DMO). In emergency situations this feature allows direct communications underground or in areas of bad coverage.
TETRA provides for authentication of terminals towards infrastructure and vice versa. For protection against eavesdropping; air interface encryption and end-to-end encryption is available. There are several advantages of using TETRA over other regular GSM or UMTS standards:
- A much lower frequency makes a longer range possible, which in turn permits very high levels of geographic coverage with a smaller number of transmitters, cutting infrastructure costs.
- During a voice call, communication is not interrupted when moving to another network site.
- High spectral efficiency - 4 channels in 25 kHz and no guard bands, compared to GSM with 8 channels in 200 kHz and guard bands.
- Very fast call set-up - a one to many group call is generally set-up within 0.5 seconds (typical less than 250 msec for a single node call). On a GSM network, this would take 7 to 10 seconds.
- Communication success is ensured even during overload situations, such as disasters.
- TETRA infrastructure is usually separate from (although connected to) the public phone network.
- A number of fall back modes are available.
- Option to use a 'direct mode' (or walkie-talkie mode) when no network is available. A gateway mode allows a single mobile device with connection to the network to act as a relay for others nearby that are out of range of the infrastructure.
- Multiple high level encryption options.
- Suitable for one-to-many and many-to-many communications.
- Network solutions are available in both older circuit-switched architectures and IP-based architectures with software switches.
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