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Collaboration, Voice & AV · Intermediate

Packet Loss Concealment (PLC)

Techniques a receiving codec uses to hide missing voice packets by replaying or interpolating audio.

Definition

PLC estimates the content of a lost or late packet, for example by repeating the previous frame with fading or by waveform interpolation, so short gaps are less audible. Modern codecs such as Opus, iLBC and G.729 have built-in concealment, and G.711 Appendix I defines an optional PLC algorithm. PLC is effective for isolated losses of 20 to 30 ms but cannot hide bursts of consecutive losses.

Example

Opus with in-band forward error correction plus PLC keeps a Webex call intelligible at 5 percent random loss.

Exam tip

PLC masks small random loss; bursty loss still degrades MOS and must be fixed in the network with QoS.

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