CALL PROGRESS TONE DETECTORS (VoIP)

17.4
This section is presented for the call progress tone detectors (CPTDs) with VoIP services. The tone detection requirements vary with country. In the PSTN telephone service, the CO or DLC generates the call progressive tones, and usually no requirement is made for detecting the tones by the PSTN. The VoIP adapter has an FXO/PSTN interface as marked in Fig. 17.1(b) that allows for dialing into or receiving from the PSTN network. The FXO interface of the VoIP adapter emulates the electronic telephone and continuously monitors
the progress of the call. Call progress tone detection allows a VoIP adapter that dials into or receives from the PSTN network to monitor the progress of the resulting call, and hence, different states with respective call progress tones like dial, ringing, and busy can be determined. In CPTD, amplitude, frequency, and cadence (timing) are required to be analyzed. An audio cadence is a repeating sequence of sound (tone-on) and silence (tone-off) similar to the on-off pattern produced by a ring-back or a busy tone.
CPTD analyzes the signal on the line to detect a repeating pattern of signal and silence, such as the pattern produced by a dial tone, ring-back tone, or busy signal. Once a cadence has been established, it can be classified as a single ring, double ring, or busy signal that allows a PSTN application or any other signaling protocol to keep track of the progress of an incoming or outgoing PSTN call. Various types of call progress tones exist. These tones and country-specific deviations were given in the previous sections.
To detect call progress (CP) tones, many implementations of CPTD comprise filtering for required tones, power estimation, timing analysis, and protection against false detections. Generic tone detection modules are given under DTMF in topic 7 and fax over PSTN in topic 14 for higher center frequency. Some of these techniques can be used for tone detections at different center frequency and frequency combinations. Basic call progress tones are of low frequency in the range of 330 to 620 Hz.
Call progress tones and cadences may differ from country to country. Basic tones are given in E.180 [ITU-T-E.180/Q.35 (1998)] for multiple countries. Thus, one of the most difficult problems facing the user who requires analog tone recognition is the variability of these tones when calling to or from different parts of the world, thereby necessitating the implementation of required tone detection. Tolerances of detections of the transitions could occur from the on period to the off period. The algorithm has to be implemented by considering the tolerances.