Problem: Packet Loss Concealment
Packet Loss Concealment is a technique used to mask the effects of lost or discarded packets. PLC is generally effective only for small numbers of consecutive lost packets, for example a total of 20-30 milliseconds of speech, and for low packet loss rates.
Packet loss can be bursty in nature - with periods of several seconds during which packet loss may be 20-30 percent. The average packet loss rate for a call may be low however these periods of high loss rate can cause noticeable degradation in call quality.
PLC algorithms typically involve either replaying the last packet received ("replay") or some more sophisticated algorithm that uses previous speech samples to generate speech. Simple replay algorithms tend to lead to "robotic" sounding speech when multiple consecutive packets are lost. More sophisticated algorithms can provide reasonable quality at 20% packet loss rates however can consume DSP bandwidth and hence reduce the number of channels that can be supported in, for example, a high density gateway.
Impact
If PLC is not enabled then users may report difficulty in understanding speech due to short gaps. If PLC is used but is not effective then the problem is likely to be bursts of packet loss or periods of high discard rate.
| Silence Insertion | Replay last packet | G.711 Appendix 1 | |
| 5% loss rate |  |
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| 10% loss rate | |
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| 20% loss rate | |
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| 40% loss rate | |
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These example files use G.711
Resolution
Ensure that PLC is enabled.
Try and identify the source of packet loss by examining packet metrics available from switches and routers along the voice path. If packet loss rates are low then check for congestion in limited speed links.
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