RTP Comfort Noise Payload
G.711 with VAD, G.729B, and Opus DTX all rely on comfort noise signaling. Missing CN payloads cause 'dead air' perception and hold-music glitches in contact centers.
What It Defines
Defines RTP payload format for comfort noise (CN) during silence periods when VAD/DTX is active. Payload type 13 (static) or dynamic `CN` with spectral parameters. Prevents callers from thinking a call has dropped during silence.
Canonical (Normative)
Convenient (Practical)
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Related Specs
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Codec mismatch debugging starts here — SDP says `PCMU/8000` which is PT 0 per RFC 3551. Static vs dynamic payload types explain most SIP/WebRTC negotiation confusion.
G.711 is the common denominator of telephony. Every PSTN gateway and legacy SIP trunk speaks G.711. When codec negotiation fails and you're left with 64 kbps PCM, this is why.
G.729 remains common on legacy SIP/PBX trunks where bandwidth is constrained. License/patent history affected adoption — know it when debugging codec negotiation on older enterprise systems.
Opus is the codec for real-time audio on the internet. WebRTC mandates it; Discord, Zoom, and most VoIP apps use it. Understanding bitrate, FEC, and frame sizing directly impacts audio quality and bandwidth.