VoIP Voice and Fax Signal Processing

13.3 ANALOG FRONT-END VOICE TRANSMISSION TESTS

Connecting fax machines and sending several fax pages in fax pass-through (as explained in Chapter 15) at a high speed confirms the possibility of good transmission quality in the VoIP system. FXS interfaces on VoIP gateways are similar to PSTN central office interfaces provided to the residential users. To make VoIP voice quality match PSTN, it is essential to ensure end-to-end signal transmission and switching delays should be in compliance with TR-57 requirements. TR-57 [TR-NWT-000057 (1993)] is the technical reference for North American PSTN digital loop carrier (DLC). TR-57 measurements are for narrowband voice and are classified into two categories—namely signaling or switching characteristics that analyzes various delays in call progress and transmission characteristics that play a major role in steady-state voice quality.

For TR-57 tests, a gateway has to use a G.711 call that gives the maximum possible quality. These calls are made within the same platform or are made on the local area network (LAN) interface to eliminate IP impediments. Jitter buffer is used with a slightly higher threshold or is configured to operate as fixed jitter buffer to avoid packet adjustments during the transmission tests.

TR-57 is the Telcordia/Bellcore reference for qualifying the PSTN digital loop carriers. Even though the TR-57 standard does not talk about VoIP, meeting its specifications for the FXS/FXO interface is expected to result in better ...