the mobile unit in command- ing the base either to route calls to a required PABX or PSTN 110.9 173.8 number, or in a more limited number of cases to selectively address 114.8 179.9 other mobile units for alert indications to be given. In a Rayleigh fading propagation environment, DTMF signalling becomes prone 118.8 186.2 to errors caused by signal loss, e.g. by a single DTMF digit being 123.0 192.8 decoded as a repeated digit during a signal fade with resultant 127.3 203.5 incorrect signalling. 131.8 210.7 46.7.2 CTCSS 136.5 218.1 141.3 225.7 The Continuous Tone Controlled Squelch System (DTI, 1978) is a basic form of selective signalling which is extensively used throug- 146.2 233.6 hout the private mobile radio field. (Figure 46.7) In typical usage a 151.4 241.8 single 'sub-audible' tone frequency is transmitted at 10% to 15% of 100.0 156.7 250.3 103.5 162.2 46.7.3 Sequential tone To overcome the effects of Rayleigh fading in a mobile environ- ment, sequential tone signalling may be used (DTI, 1981b). Here a unique single tone corresponding to each digit is sequentially trans- mitted, together with a unique 'repeat' tone in place of the digit tone 250Hz 300Hz 2550Hz Frequency in the case of repeated digits. Strings of repeated digits are thus sent in the manner digit, repeat, digit, repeat etc. as shown in Figure 46.8. The tones are sent with a defined signalling interval, this varying Speechband between 40mS and 100ms dependent upon the system being used. Thus if a tone is missed due to signal fade, the sequence is recog- nised as invalid and is thus re-transmitted by the mobile unit rather than being received incorrectly. A 'transpond' system is commonly used where inter-unit dialling Figure 46.7 Continuous tone controlled signalling system and status indication is performed. Here the base interrogates the employs tone frequencies below 300Hz mobile user, this unit acknowledges by automatically transmitting a pre-set status number to indicate availability etc. This operation the maximum system FM deviation, 