smitted audio will be heard. The advantage of this system is that the sub-audible tones are transmitted for the whole period of the transmis- sion SO if the signal fades at the beginning of the transmission and then increases in strength, this part of the transmission will be heard. Systems typically are able to provide up to 37 different tones, the lowest frequency of which is 67 Hz and the highest 250.3Hz. In general narrow-band frequency modulation is the chosen form of modulation, although airport services use amplitude modulation. Typi- cally a deviation of 2.5kHz is used for FM and this enables a channel spacing of 12.5kHz to be implemented. As the demands for PMR are high, it is necessary to make effective use of the channels available. This is achieved by reusing the frequencies in different areas. Base stations must be located sufficiently far apart SO that interference is not experienced, and also selective calling techniques such as CTCSS and DTMF are used to ensure that as many mobiles as possible can use a given channel. Trunked PMR The elementary form of private mobile radio, although simple and relatively low cost to implement, has many limitations. It cannot provide for routing of incoming calls over a wide area. To implement a system of this nature adds a considerable degree of complexity and requires a network of base station sites. This in turn requires further complexity because the system that is set up needs to know the location of the mobile station SO that any calls can be routed through to the relevant base station for onward transmission to the mobile. As a result of this, charging structures are different to those used for simple PMR, and are generally on the basis of utilization. Therefore each call is recorded for this purpose. In view of the very high cost of setting up trunked networks, they normally run by large leasing companies or consortia that provide a service to a large number of users. In view of the wider areas covered by these networks and the greater complexity, equipment