TETRA – A World of new possibilities
TETRA is ideal as a digital alternative for organisations with large numbers of two-way radio users and high traffic capacity. Thanks to the Time Division Multiple Access technology (TDMA), TETRA offers no less than four channels per frequency, which allows more data to be sent simultaneously. Not only is your critical communication of higher quality, your data communication is also much more reliable.
The TETRA standard for the use of two-way radios and radio communication was created to make more efficient use of the available frequency space. In the (analogue) telephone world, it was already common practice to make a bundle with telephone cable available to multiple users. A central unit controlled the allocation of which connection ran over which shared connection. In the world of radio communications, this movement emerged later and often use was made of separate frequencies where the users themselves had to select the correct channel.
The introduction of dynamic frequency allocation gained momentum in the Netherlands in the 1970s and 1980s. Systems work with a control channel to which the mobile users are aligned. A central unit continuously listened on the control channel whether a mobile user wanted to make a call and with whom, and then sent a command to all users involved to switch to the first available radio frequency for the duration of the call. In most cases, this involved a group conversation with multiple users. The systems were less suitable for one-on-one conversations.
In this way, it was possible, for example, to divide 100 users into several groups and have them communicate via four physical frequencies. This would never have been possible without a central unit. Analogue trunking systems had two drawbacks. They used proprietary protocols and could only use peripheral equipment and a system of the same brand. In addition, analogue channels were still used and tapping was relatively easy.
“TETRA is ideal as a digital alternative for organisations with large numbers of two-way radio users and high traffic capacity.”
A Serious Standard
The TETRA standard was developed to solve these two problems. The European Telecommunications and Standardisation Institute (ETSI) published a description of this standard in the mid-1990s, after which suppliers launched the first systems on the market.
The digital TETRA standard was designed by ETSI (European Telecommunications Standards Institute), and is used by government institutions, major transport companies and large industrial complexes.
A TETRA network uses trunking technology or the ‘pooling of use’. This technology makes use of a computer that selects free channels for sending and receiving voice and data. If available, the computer can also use several channels to send or receive a message.
Tetra means “four” in Greek and the principle of a TETRA system is that one frequency is divided into four time slots. Each time slot is suitable for making a call or sending data. The first time slot is the “control channel”. If the system is expanded by a frequency, four additional time slots will be added. Communication is now also digital and the allocation of time slots is done dynamically. Because a central unit (the controller) is used, it’s possible to give users or groups certain priorities and, for example, to ensure that an emergency call always goes through. The above data communication is quite limited, especially if you compare it with current data rates over broadband networks; 7.2 kbit per second per time slot. TETRA systems focus on voice communication with two-way radios as is the standard in many sectors.
Because the communication protocols have been laid down in a standard, it’s now also possible to use a TETRA system of brand A in combination with two-way radios of brand B. The selection of the user is often based on the specific characteristics of the TETRA two-way radios in combination with the total price of the solution.
In many countries, Public Order and Safety agencies use regional and national networks based on TETRA. In the Netherlands that network is known as C2000, in Belgium this network is called ASTRID. The Netherlands is a densely populated country and the efficient handling of available frequencies is important. That’s probably why TETRA is a popular technology. In many factories, but also in convention facilities, amusement parks, and limited geographical areas, the TETRA system is used to support mission- and business-critical communications. The Dutch Authority for Digital Infrastructure issues the frequencies for TETRA systems on the basis of an application in combination with a business plan. For the application in POS in the Netherlands, the frequency band between 380 and 400 MHz is used; 410-430 MHz is available for other applications. Defence applications may use different frequency bands. In addition to the efficient use of the frequency spectrum, TETRA systems offer additional benefits:
- Controlled access to the system;
- Optimised for group calls;
- Multiple priority levels;
- Quick setup of the connection (call setup <300 msec);Good
- audio quality;
- Speech encryption.
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