VoIP Voice and Fax Signal Processing

15.6 FAX OVER IP INTEROPERABILITY CHALLENGES

The primary challenge in FoIP implementation is end-to-end interoperability. By improving interoperability, FoIP calls can be comparable with PSTN-based fax calls. In most situations, PSTN fax calls are completed successfully. Nevertheless, PSTN fax calls can occasionally fail because of line conditions from the user fax machine to the PSTN digital loop carrier or central office. Fax machine anomalies, mismatched messages, and capability exchanges contribute to these failures. Even in the PSTN, intermediate long-distance routing of PSTN fax calls may use intermediate VoIP or FoIP calls.

The type of fax machines used, VoIP gateway features, fax call switching, and deviations in fax call tones can influence FoIP interoperability. IP network impediments—such as delays, bandwidth variations, jitter, packet loss, timing issues, packet formats, redundancy, error correction mode, end-to-end transmission characteristics, clock drifts, as well as various configurations and interactions between voice and fax modules—affect interoperability. However, interoperability concerns can be alleviated by ensuring that FoIP implementations are tolerant of the many anomalies that may occur [URL (CED)]. FoIP interoperability issues are given here.

15.6.1 Interoperability with Fax Machines

Several deviations exist among the available fax machine timings in delivering messages and responses. T.38 FoIP adds delays that may exceed fax-timing limits, resulting ...