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VoIP Voice and Fax Signal Processing by Sivannarayana Nagireddi

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8.7 CALLER ID ON FXO INTERFACES

In VoIP, gateways or adapters use FXS and FXO interfaces. Some of these aspects are given in Chapter 2. Products with FXO interfaces are usually called residential gateways (RGs) or integrated access devices (IADs). As shown in Fig. 8.8(a), gateways use FXS for connecting to the telephone. In the previous sections, caller ID and call wait ID generation are given with an FXS interface. The VoIP gateway sends the required call progress tones, caller ID, and other signals to the FXS phone. On an FXS interface, the processor inside the VoIP system generates caller ID, call wait ID, and other call progress tone signals. A VoIP box uses an FXO interface for connecting to PSTN, digital subscriber line (DSL), or private branch exchange (PBX) system. As shown in Fig. 8.8(a), the FXO interface is similar to an electronic telephone and consists of a codec and data access arrangement (DAA). The codec has the same functionality as the FXS in converting analog speech to digital and vise versa. In a normal telephone, call progress tones are observed by the listener and take required actions based on call progress tones. The PSTN central office sends call progress tones and caller ID information to the FXO interface. The FXO interface has to understand the way the phone understands. On an FXO interface, call progress tones are observed by the listener/phone inside the interface. On an FXO electrical interface, these call progress signals and indications are converted ...

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