Tetra – Basics

Tetra – Basics

Terrestrial Trunked Radio (TETRA; formerly known as Trans-European Trunked Radio), a European standard for a trunked radio system, is a professional mobile radio and two-way transceiver specification. TETRA was specifically designed for use by government agencies, emergency services, (police forces, fire departments, ambulance) for public safety networks, rail transport staff for train radios, transport services and the military. TETRA is the European version of trunked radio similar to Project 25.

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 channels.

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.

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.

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 networks typically provide, that allows 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.
  • Security TETRA supports terminal registration, authentication, air-interface encryption and end-to-end encryption.
  • Rapid deployment (transportable) network solutions are available for disaster relief and temporary capacity provision.
  • Network solutions are available in both reliable circuit-switched (telephone like) architectures and flat, IP architectures with soft (software) switches.

Its main disadvantages are:

  • Requires a linear amplifier to meet the stringent RF specifications that allow it to exist alongside other radio services.
  • Data transfer is slow by modern standards.

Up to 7.2 kbit/s per timeslot, in the case of point-to-point connections, and 3.5 kbit/s per timeslot in case of IP encapsulation. Both options permit the use of between one and four timeslots. Different implementations include one of the previous connectivity capabilities, both, or none, and one timeslot or more. These rates are ostensibly faster than the competing technologies DMR, dPMR, and P25 are capable of). Latest version of standard supports 115.2 kbit/s in 25 kHz or up to 691.2 kbit/s in an expanded 150 kHz channel. To overcome the limitations many software vendors have begun to consider hybrid solutions where TETRA is used for critical signalling while large data synchronization and transfer of images and video is done over 3G / LTE.

Reference: Wikipedia

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