 services on v.h.f./u.h.f. employ narrowband reception. The effect of the scatter angle between the receiving and FM telephony (F3E) as the principal modulation method. AM Analogue signalling 46/9 telephony (A3E) is used in some cases, often to retain compatibility In normal personal communication use this tone is used in addi- with existing systems but also for emergency services where it is tion to the carrier or noise squelch system inherent in personal advantageous for the base station control operator to be aware of a receivers to control the loudspeaker muting circuitry. Thus if a weaker mobile station attempting to establish communication signal is received with either no tone or an incorrect tone, the user's whilst the channel is occupied by a stronger signal. receiver remains muted to allow privacy between users of a shared Single sideband telephony (J3E) is used in some cases in an effort radio channel in a given geographical area. to attain greater usage of available frequency spectrum, some sys- For telephone interconnection or communal base station oper- tems employing pilot carrier SSB to aid frequency synchronisation ation, through a given dedicated or common base station, this tone between the transmitter and remote receivers. These modes of is used to control line/talkthrough seizure together with user auth- communication are extensively documented elsewhere. orisation, billing and time limitation switching. Mobile units may A variety of signalling methods are used in addition to the speech optionally have circuitry present to also inhibit transmission whilst modulation to provide control of the communication. receiving an incorrect tone, thus preventing interference to other users of a shared channel. An internationally used set of sub-tones are given in Table 46.2. Analogue signalling 46.7.1 DTMF Table 46.2 CTCSS frequencies Dual Tone Medium Frequency signalling, which is comprehens- lively documented elsewhere in this handbook, is commonly used 107.2 167.9 for simple numerical signalling from 