VoIP Voice and Fax Signal Processing

17

COUNTRY DEVIATIONS OF THE PSTN MAPPED TO VoIP

In the public switched telephone network (PSTN) system, the closest central office (CO) or digital loop carrier (DLC) drives the telephone lines. DLC is part of the PSTN system located in between the subscribers and the central office to take care of distribution of CO physical interfaces. A user connects multiple phones, fax machines, and dial-up modems on the telephone TIP-RING interface. The CO/DLC is capable of ringing and driving the required current to use three to five telephones referred to as ring equivalent numbers (RENs). As shown in Fig. 17.1(a), the DLC is capable of driving the lines to several miles, usually given in kilo feet (kft) or 1000 feet units. Multiple DLCs and COs communicate using A-law or μ-law compression schemes for digital transmission. The impedance shown in Fig. 17.1(a) at the telephone interface and central office is for North America.

In a complete PSTN system, several deviations are taken care of while dealing with solutions for multiple countries. It is required to take care of deviations and to match these to the local PSTN for better voice quality. The right combination of phones, DLCs, central offices, transmission lines, and other supporting infrastructure can help provide better quality. To map the influences to VoIP, major deviations of the PSTN for multiple countries are categorized as central office deviations, transmission lines, and telephones. VoIP supporting infrastructure focused on ...