QUESTION: Is synchronization required for non-traditional services such as voice-over-IP?
ANSWER: The answer to this topical question depends on the performance required (or promised) for the service. Usually, Voice-over-IP is accepted to have a low quality reflecting its low cost (both relative to traditional PSTN voice service). Synchronization, to some degree is definitely required between the end-points. That is, the analog-to-digital-converter (ADC) clock at one end must be “equal” to the digital-to-analog-converter (DAC) clock at the other end. How “equal” do these need to be? For voice only (human mouth to human ear) applications the clocks can be substantially different, allowing the source and destination clocks to free run at even a stratum-4 level (i.e. 32 ppm). If the same terminal equipment is required to support voice-band services, such as facsimile or other modems, then the level of equality is much more stringent. Contributions that have been made to ITU-T Study Group 15 that is examining this requirement, indicate that for support of voice-band modems the level of equality needs to be of the order of 0.1 ppm (100 ppb) and so if the clocks are free-running then an accuracy of 0.05 ppm (50 ppb) is appropriate.
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