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Collaboration, Voice & AV · Intermediate

Comfort Noise Generation (CNG)

Synthesizing low-level background noise during silence periods so listeners do not think the call has dropped.

Definition

When VAD suppresses packets during silence, the sending side may send occasional silence insertion descriptor (SID) frames describing background noise, and the receiver generates matching comfort noise. RFC 3389 defines a generic comfort noise RTP payload. Without CNG, sudden total silence makes callers ask whether the other party is still there.

Example

A G.729b call sends SID frames every few hundred milliseconds during silence instead of 50 packets per second.

Exam tip

CNG pairs with VAD; one saves bandwidth and the other masks the resulting silence.

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