Tetra

Terrestrial Trunked Radio (TETRA) (formerly known as Trans-European Trunked Radio) is a professional mobile radio and two-way transceiver (popularly known as a walkie talkie) specification. TETRA was specifically designed for use by government agencies, emergency services, (police forces, fire departments, ambulance) for public safety networks, rail transportation staff for train radios, transport services and the military.

TETRA is a European Telecommunications Standards Institute (ETSI) standard that was first published in 1995.

Overview

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.

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-switched data or circuit-switched data communication uses specifically assigned traffic channels.

TETRA provides for authentication of terminals towards infrastructure and vice versa. For protection against eavesdropping air interface encryption and end-to-end encryption are available.

The common mode of operation is in a group calling mode in which a single button push will connect the user to the users in a selected call group and/or a dispatcher. 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. TETRA terminals can act as mobile phones (cell phones), with a full-duplex direct connection to other TETRA Users or the PSTN. 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.

Advantages

The main advantages of TETRA over other technologies (such as GSM) are:

•  The much lower frequency used gives longer range, which in turn permits very high levels of geographic coverage with a smaller number of transmitters, thus cutting infrastructure costs.

• During a voice call, the communications are not interrupted when moving to another network site. This is a unique feature which dPMR and DMR do not offer.

• 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) compared with the many seconds (typically 7 to 10s) that are required for a GSM network.

• Works at high relative speeds >400 km/h. TETRA was used during the French TGV train speed record on 3 April 2007 at 574.8 km/h.

• The system contains several mechanisms, designed into the protocols and radio parameters, to ensure communication success even during overload situations (e.g., during major public events or disaster situations), thus calls will always get through unlike in cellular systems. The system also supports a range of emergency calling modes.

•  TETRA infrastructure is usually separate from (but connected to) that of the public (mobile) phone networks, resulting in (normally) no call charges for the system owners, substantially more diverse and resilient communications and it is easy to customize and integrate with data applications (vehicle location, GIS databases, dispatch systems, etc.).

• Unlike most cellular technologies, TETRA networks typically provide a number of fall-back modes such as the ability for a base station to process local calls. So called 'mission critical' networks can be built with TETRA where all aspects are fail-safe/multiple-redundant.

•  In the absence of a network mobiles/portables can use 'direct mode' whereby they share channels directly (walkie-talkie mode).

• Gateway mode - where a single mobile with connection to the network can act as a relay for other nearby mobiles that are out of range of the infrastructure.

• TETRA also provides a point-to-point function that traditional analogue emergency services radio systems did not provide. This enables users to have a one-to-one trunked 'radio' link between sets without the need for the direct involvement of a control room operator/dispatcher.

•  Unlike cellular technologies, which connect one subscriber to one other subscriber (one-to-one), TETRA is built to do one-to-one, one-to-many and many-to-many. These operational modes are directly relevant to the public safety and professional users.

• TETRA supports both air-interface encryption and end-to-end encryption
•  Rapid deployment (transportable) network solutions are available for disaster relief and temporary capacity provision.

• Equipment is available from many suppliers around the world, thus providing the benefits of interoperable competition.

• Network solutions are available in both the older circuit-switched (telephone like) architectures and flat, IP architectures with soft (software) switches.