VoIP Voice and Fax Signal Processing

17.1 COUNTRY-SPECIFIC DEVIATIONS

This section is presented with the country-specific deviations mapped to the PSTN and VoIP. Catering to required deviations in VoIP can closely match the feel of PSTN-based calls.

17.1.1 Central-Office-Specific Deviations Mapped to VoIP

The VoIP adapter is like a PSTN CO for the telephone user. Hence, most functions of the CO have to be emulated through the VoIP adapter. Figure 17.1(b) is a functional representation of the VoIP adapter with foreign exchange subscriber (FXS) and foreign exchange office (FXO) interfaces. The following major parameters can influence the design and configuration of the VoIP adapter for multiple countries:

• Impedance of VoIP adapter telephone interfaces of FXS and FXO
• REN—number of simultaneous phones that can be used on the same lines
• Subscriber line interface circuit (SLIC) line voltages, battery voltages, line reversals, grounding, and various protections
• Ringing voltage, frequency, and cadence
• Diagnostic methods performed on the telephone interfaces
• Call progress tone generation and specifications
• Dual-tone multifrequency (DTMF) digit generation, detection specifications
• Caller ID, call feature tones, and timings
• Overall loudness rating (OLR), gain/loss planning in send and receive path
• Country-dependent transmission characteristics similar to TR-57 [TR-NWT-000057 (1993)] transmission characteristics
• Call switching timing, which is similar to TR-57 switching characteristics that varies with country
• Based on local ...