on to be reduced to an acceptable level. DMO is used for direct communication between two mobile units and supports both voice and data; however, full duplex is not supported in this mode. Only simplex is used. This is particularly useful as it allows the mobile stations to communicate with each other even when they are outside the range of the base station. Finally the PDO mode is optimized for data only transmissions. It has been devised with the idea that much higher volumes of data will be (a) V+D mode communication in one direction at a time (b) Direct mode operation Figure 9.2 TETRA modes of operation Private mobile radio 245 needed in the future and it is anticipated that further developments will be built upon this standard. TETRA uses TDMA enabling efficient use of the spectrum by allowing several users to share a single frequency. As the speech is digitized, both voice and data are transmitted digitally and multiplexed into the four slots on each channel. Digitization of the speech is accomplished using a system that enables the data to be transmitted at a rate of only 4.567 kbits per second. This low data rate can be achieved because the process that is used takes into account the fact that the waveform is human speech rather than any varying waveform. The digitization process also has the advantage that it renders the transmission secure from casual listeners. For greater levels of security that might be required by the police or other similar organizations it is possible to encrypt the data. This would be achieved by using an additional security or encryption module. The data transmitted by the base station has to allow room for the control data. This is achieved by splitting what is termed a multiframe lasting 1.02 seconds into 18 frames and allowing the control data to be transmitted every 18th frame. Each frame is then split into four timeslots. A frame lasts 56.667 ms. Each timeslot then takes up 14.167 ms. Of the 14.167 ms only 14 ms are used. The remaining time is required for the transmitter t