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64 SUBSCRIBER SIGNALINGthe low-group DTMF frequencies detects a signal, and the natural sound is notaccepted as a digit. The same happens with natural sounds that contain one of theother DTMF frequencies.3.3.4 Ringing SignalsThe ringing signal is a high-level 20–25 Hz signal analog, of typically 100 V rms,and designed to drive the electromechanical ringers in the early telephones. Thisringing signal cannot be provided by a service circuit, because there are no PCMcodes to represent these high-level voltages. The ringing signal is therefore injectedinto the line circuit (Fig. 3.3-2).When a subscriber line has to be rung, the processor sets the line circuit to theringing state. In this state, switch (S) connects a common ringing-voltage source(RS) to the A-wire of the subscriber line. The alternating ringing current passesthrough the external circuit (subscriber line, capacitor (C), and ringer (RR)—seeFig. 3.2-1) and detector (CD). However, this current is not large enough to causeCD to indicate “off-hook.”When the called telephone goes off-hook, the ringer current and the direct currentfrom battery (BAT) flow through CD. An off-hook is reported to the processor,which then changes the LC state to normal.3.4 TONES, ANNOUNCEMENTS, AND RINGINGA calling subscriber receives tones or announcements that inform him about the progressof the setup of his call. These tones originate at the local exchange of the caller,at intermediate exchanges along the connection, or at the local exchange of thecalled party (Fig. 3.4-1).3.4.1 Tone FormatsIn the early years of telecommunications, all call-progress signals were tones, atfrequencies of 400–600 Hz. These tones, some of which are still in use, are eithercontinuous or repeating “on–off” cycles with a certain cadence. The frequenciesand cadences were established by the telecoms of individual countries, wellbefore the beginnings of international standards. The meanings of tones, and theirfrequencies and cadences, are therefore somewhat country-specific.3.4.2 Basic TonesThe following functional tone signals are used in all countries.Dial-Tone. This is sent by the local exchange serving the calling subscriber, toindicate that the exchange is ready to receive the called number. In most countries,dial-tone is continuous.