VoIP Voice and Fax Signal Processing

12.5 CLOCK DRIFT INFLUENCE ON VOICE AND FAX CALLS

In voice applications, packet under-run and over-run can happen with clock drifts. This packet mismatch over time cannot be avoided in practical deployments. Based on the long-term jitter buffer growth direction, packet adjustments can be attempted without allowing for complete under-run or over-run. One of the best options of packet adjustment is to correct silence zones in the middle of utterances or talk-spurts. The jitter buffer adjustments are presented in Chapter 10.

In fax pass-through, a silence zone will not be clearly available like voice. Any attempt of removing or creating extra silence zones will create amplitude or phase hits that cause the fax to work at a lower rate. To minimize these issues, jitter buffer has to adjust smaller duration frames at a rate of slower than once every 1 minute to match the acceptance of the hits test. Fixed jitter buffer with higher initial thresholds is also used to avoid getting into requirements of packet adjustments for several pages. In some VoIP boxes, low-bit-rate V.21 modulations are continuously extracting low-speed data. In the transition of low-speed to high-speed data, a small gap of about 75 ms is available. These gaps can be used in adjusting silence zones.

In T.38 [ITU-T-T.38 (2005)] fax, PPM issues may not be dominant because of redundancy, forward error correction, and silence zones available in the middle of data packets. As an overall summary, it is preferred to maintain ...