VoIP Voice and Fax Signal Processing

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CLOCK SOURCES FOR VoIP APPLICATIONS

Clock precision is one of the important aspects of VoIP for maintaining voice and fax quality. This chapter is presented for some popular clock sources used in VoIP adapters, public switched telephone network (PSTN)-to-VoIP gateways, and the key influencing clock selection parameters that help to maintain voice and fax quality. In PSTN, central offices (COs) and digital loop carriers (DLCs) are made to work with precision stratum clock references. In VoIP, every user's VoIP adapter hosts part of the services of PSTN CO and DLC. The VoIP adapter can be treated as a central office interfacing with the user. Hence, clock precision requirements will propagate to the VoIP adapter or closest interfacing VoIP system. From a clock perspective, maintaining VoIP voice interface clock accuracies matching the PSTN system clock is one of the helpful parameters to maintain better voice and fax quality in VoIP deployments that gives benefits of exceeding PSTN quality through wideband. The VoIP system designer may attempt certain trade-offs in clock selection to optimize for cost, space, and power consumption. It is helpful to know the performance trade-offs with reference to the optimization in the selection process. Providing the right clock is usually a hardware member's responsibility. In the absence of right clocks to voice interfaces, many problems will translate to degradation of voice quality and it takes a lot of effort to debug those directly influenced ...