 channels only in the cells where the users of the specific group have a significant advantage from traffic resource Thanks to digital technology, the network can be shared between multiple organisations without interfering with them. Each organisation can independently manage the right of the users or dispatchers or the group configuration. Each user or group has a specified priority to define the queing order for the TETRA Network Planning channel resource. This feature allows optimisation of the use of the network, saving the investment put in for implementation of the network. The radio base station fallback feature helps to improve network resilience. This feature allows com- munication within the cell when the radio base station is disconnected by digital switching, for example, due to unavailability of the communication links or unavailability of digital switching itself. The data communication features allow the user to query databases, telemetry, remote control, email and fax, etc. Additional features are: Special calls type such as group calls and emergency calls Priorities for serving the calls Dispatcher workstations for the correct user management and group control Direct mode operation (direct communication between terminals without the support of the radio base station) Short data service (similar to the SMS of GSM) Implementation of end-to-end encryption Multivendor environment because the air interface is standardised (with the possibility of using radio terminals of different vendors under the same infrastructure) Efficient use of the frequency spectrum Fast call setup Radio base station fallback Data communication D.4.1 Physical Layer The TETRA air interface is partitioned in frequency and each frequency is partitioned in time (FDMA and TDMA). The base station and mobile station communicate in duplex; therefore one set of frequencies is allocated to the uplink path and another set to the downlink path. One pair of frequencies for each cell is reserved for the main control channel (MCCH). Each f