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Problem: Timing Drift

Voice over IP systems commonly experience timing drift. This occurs when the internal clock in the sending system runs at a slightly different speed to that in the receiving system – causing drift of typically up to 60 microseconds per second. Timing drift can also occur in the system being used to observe the packet stream. Timing drift is can be seen on a delay trace as a constant slope or a sawtooth.

Impact

Low rates of timing drift may cause an periodic audible “tick”. VoIP systems can sometimes hide this by doing necessary timing adjustments during silence periods. If an NTP timing server is used then VoIP systems may resychronize or adjust their clock speed automatically.

High rates of drift can be much more problematic, and may be symptomatic of hardware problems. These can be caused by high temperatures in end systems such as PCs or due to the use of cheap ceramic resonators instead of crystals in low cost IP phones.

Resolution

It can be difficult to resolve timing drift problems without consulting the equipment supplier however initially review the configuration of IP phones and gateways to see what guidance is provided. Use an NTP time server to provide a common reference point. Some operating systems, for example Linux, do provide a method for tuning the internal clock rate of computers which can minimize this effect.

Tools:

Network Analyzer

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