VoIP Voice and Fax Signal Processing

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PSTN BASIC INFRASTRUCTURE, INTERFACES, AND SIGNALS

Telephones, fax, and dial-up modems are popularly used with a two-wire TIP-RING foreign exchange subscriber (FXS) interface that supplies a battery. The interface inside the phone or fax machine is a foreign exchange office (FXO). In some countries, a four-wire integrated services digital network (ISDN) is used for telephone services. A T1/E1 family of interfaces is used mainly for higher channel communication. In an office environment, a user may get telephone service through a public switched telephone network (PSTN) central office (CO) or a private branch exchange (PBX) system resident close to the office phones. PBX systems may use multiple FXS or digital phone interfaces for connecting to the user and FXO, ISDN, or T1/E1 family of interfaces to communicate with the nearest PSTN CO or digital loop carrier (DLC). A DLC resides close to the subscribers and extends the reach of central offices. For inter-regional services, the local CO will route the calls to the destination CO. The destination CO then terminates the call directly or through the local DLC. Several handbooks and documents are available on this subject [Freeman (1996), Bellamy (1991), ITU-Handbook (1992)]. The combinations and possibilities in service vary with each service provider and country. VoIP service and user interfaces are closely related to historical PSTN services. In this chapter, an overview of the PSTN telephone infrastructure and some of interfaces ...