VoIP Voice and Fax Signal Processing

8.8 SUMMARY AND DISCUSSIONS

Caller ID, call wait ID, and call wait indication are the basic call features supported in VoIP deployments. On direct terminals of IP and WiFi phones, it is easy to implement call features compared with the VoIP gateway with analog phones. Caller ID features mainly deal with the hook status of telephone lines, signal encoding, and decoding for various modulations and tones. Several options exist, and deviations in implementations widely vary across multiple countries. Caller ID timing and hook status are more sensitive in dealing with implementations. The implementations tolerate for deviations in types of FSK modulations, signal level, impedance of subscribed line interface circuit (SLIC) and COder/DECoder (CODEC) interfaces, and so on.

In this chapter, some of the most popular approaches are given. Refer to country-specific PSTN standards for complete details on the specifications. Some country-specific deviations are also given in this chapter. These caller ID and call wait ID generation and detection features consume very little processing. When the product or software development has to cater to multiple countries, it is suggested to build a basic framework that caters to different methods in FSK, DTMF, signal levels, timing programming, and different line conditions. The telephone does not distinguish whether a call is from VoIP or PSTN; hence, these features have to emulate functions similar to a telephone interfacing with a PSTN network.