Problem: Gaps in Speech
This may be due to a high rate of packet loss or packet discard due to jitter, or to a problem, with Voice Activity Detection associated with an echo canceller.
The Voice over IP hardware may be configured to use silence insertion instead of packet loss concealment, hence during a period of high loss or congestion many short segments of speech can be missing.
Voice Activity Detection is used with some CODECs to determine if speech energy is present. If there is no speech energy present then packets are not transmitted. Echo cancellers use a similar voice energy detection algorithm to mask echo in low level speech signals (echo suppression) and may insert silence.
Another possible cause is a high rate of link failures.
Impact
Users report gaps in speech or difficulty in understanding speech.
Example audio file - 10% packet loss with silence insertion:
Example audio file - Voice Activity Detection problem:
Example audio file - Echo suppression problem:
Resolution
Investigate the source of packet loss or packet discard problems and verify that the jitter buffer is set to a sufficient depth. Verify that the VoIP hardware is configured to use packet loss concealment. Check for excessive route changes or link failures in core IP network.
If there is no apparent packet loss or discard then check the operation of Gateway echo cancellers.
Tools:
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