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What is PLC?

PLC stands for Packet Loss Concealment. PLC describes any method of generating new audio data when packet loss is detected. In Asterisk, there are two main flavors of PLC, generic and native. Generic PLC is a method of generating audio data on signed linear audio streams. Signed linear audio, often abbreviated "slin," is required since it is a raw format that has no companding, compression, or other transformations applied. Native PLC is used by specific codec implementations, such as iLBC and Speex, which generates the new audio in the codec's native format. Native PLC happens automatically when using a codec that supports native PLC. Generic PLC requires specific configuration options to be used and will be the focus of this document.

How does Asterisk detect packet loss?

Oddly, Asterisk does not detect packet loss when reading audio in. In order to detect packet loss, one must have a jitter buffer in use on the channel on which Asterisk is going to write missing audio using PLC. When a jitter buffer is in use, audio that is to be written to the channel is fed into the jitterbuffer. When the time comes to write audio to the channel, a bridge will request that the jitter buffer gives a frame of audio to the bridge so that the audio may be written. If audio is requested from the jitter buffer but the jitter buffer is unable to give enough audio to the bridge, then the jitter buffer will return an interpolation frame. This frame contains no actual audio data and indicates the number of samples of audio that should be inserted into the frame.