VoIP Voice and Fax Signal Processing

1.2 PSTN USER INTERFACES

PSTN end users will get services through FXS, ISDN, and the T1/E1 family of interfaces [URL (TIA-496B), URL (T1/E1), URL (ISDN)]. These interfaces are also used in the migration of VoIP voice and fax solutions. As shown in Fig. 1.1, the telephone, fax, and dial-up modem are connected on an FXS interface. The FXO interface is part of a telephone, fax, and modem. Some COs provide an ISDN interface for residential applications. The T1/E1 families of interfaces are mainly used with PBX and enterprise services.

1.2.1 FXS and FXO Analog Interfaces

A PSTN wall socket is the FXS interface given to the subscriber for connecting a telephone. FXS is the two-wire TIP-RING interface provided by the PSTN CO or DLC. This interface is used for connecting telephones, fax machines, and dial-up modems. FXS supplies battery voltage, high-voltage ring, and sufficient current to drive three to five parallel phones. A subscriber line interface circuit (SLIC) and a subscriber line access circuit (SLAC) are the main components of the FXS interface. SLIC consists of a two-to-four-wire hybrid and of high-voltage electronics. SLAC is the interface between a SLIC analog signal and processor digital interface.

The FXO receives battery voltages from the PSTN FXS interface. Sometimes the FXO interface is known as a passive interface, which means the FXO will not generate a high-voltage battery on analog TIP-RING interfaces. The FXO interface is available on the TIP-RING connections from ...