s are by Motorola, by Ericsson (EDACS) and Johnson (LTR). These systems have not gained particularly widespread acceptance. TETRA Although trunked PMR provides many advantages over the simple PMR, advancing technology has enabled far more sophisticated systems to be envisaged. As a result the most recent PMR system called TETRA has been launched. Its specification is controlled by the European Tele- communications Standards Institute (ETSI), although it has gained use in many countries outside Europe. TETRA provides a number of advantages over trunked private mobile radio. It uses digital speech transmission. This in itself provides a large degree of privacy against listeners, but in addition to this encryption is also supported, enabling it to be completely secure for services such as the police. Data transmissions can take place at a greater rate. The data rate for MPT 1327 trunked radio is 1.2 kbits per second whereas for TETRA it is significantly faster at 7.2 kbits per second for a single channel. This can be increased fourfold to 28.8 kbits per second when multislot operation is employed. It also enables efficient use of the available spectrum to be made with four slots per 25 kHz to be used under its time 244 Newnes Guide to Radio and Communications Technology division multiple access (TDMA) system. This effectively means that each user occupies just 6.25kHz bandwidth. The system also supports a number of other features including call hold, call barring, call diversion and ambience listening. There are three different modes in which TETRA can be run. They are voice plus data (V + D), direct mode operation (DMO), and packet data optimized (PDO). The most commonly used mode is V + D. This mode allows switching between speech and data transmissions, and can even carry both by using different slots in the same channel. Full duplex is supported with base station and mobile radio unit frequencies normally being offset by about 10 MHz to enable interference levels between the transmitter and receiver in the stati