VoIP Voice and Fax Signal Processing

12.2 VoIP SYSTEM CLOCK OPTIONS

VoIP systems may use several clock options. In this section, some popular clock options are given as listed below:

1. Using precision crystal for clock generation
2. External clock oscillator at multiples of 2.048/1.536-MHz frequency
3. Deriving clock from PSTN T1/E1 family interfaces
4. Deriving clock from network timing reference (NTR)
5. Deriving clock from network time protocol (NTP)

These five options are marked in Fig. 12.1 circled with the serial number of the above list. The recommendations made in this section are broad options. In the figure, all clocks are shown as joined to the same phase locked loop (PLL) with dotted lines. It is implied that only one of these options is used in the system. The user may find a different scheme in the process of reusing the existing hardware resources and clocks available as part of other subsystems and the total system. Hence, the scope of clocks is not limited to the given combinations of Fig. 12.1.

Table 12.1. Stratum Clocks used in PSTN and Relevance with VoIP Systems

Figure 12.1. Functional Representations of Clock Sources in VoIP Applications.

12.2.1 Using Precision Crystal to Work with Processors

Crystal in association with an active circuit is the most popular technique to building the clock oscillator. The clock ...