 compatible service in all European countries. 5.4.2.3 Terrestrial Trunked Radio A modern digital standard for a pan-European PMR system has been devel- oped by ETSI and it is known as Terrestrial Trunked Radio (TETRA). This system was originally called Trans-European Trunked Radio and it is differ- ent from GSM but based on GSM's experiences. It uses different frequency bands and provides some services that are not available in GSM, for example, mobile-to-mobile communication. The TETRA networks are built for pub- lic safety organizations such as police, fire brigades, and border guards. These systems use the 380- to 400-MHz frequency band. Later the 410-, 450-, and 870-MHz frequency bands will be put into use by the commercial TETRA service for taxi, transport, railway, and other organizations. Like all trunked systems, TETRA uses a cellular network structure and channel allocation on demand to improve spectral efficiency. It is a digital system, uses an efficient speech-coding method, and tolerates high interfer- ence, which further improves spectral efficiency. Why do we need a separate network when the public cellular networks provide a service that can define a closed user group for an organization? One reason is because the operation of emergency services is SO essential for a community that a separate network is required. The main reasons behind this are as follows: Availability of capacity is independent of the activity of ordinary sub- scribers to the public cellular networks. In an emergency situation public cellular networks may become overloaded. The structure and services of the network can be modified inde- pendently from the public service according to the users' needs. Some required features are not supported by the public cellular net- works, for example, direct mobile-to-mobile communications and end-to-end encryption. General features of TETRA systems are listed here: Efficient use of spectrum, cellular structure, trunked (shared) radio resources; Efficient use of investments; BSs and exchange