Enabling Comfort Noise Generation

The device can generate artificial background noise, known as comfort noise, in the voice channel during periods of silence when no one on the call is speaking. This feature helps reassure participants that the call remains connected, preventing the perception of sudden disconnections.

The device detects silence using its Voice Activity Detection (VAD) mechanism. When Comfort Noise Generation is enabled and silence is detected, the device transmits Silence Identifier Descriptors (SIDs), which allow the receiving side to reproduce local background noise.

The Comfort Noise Generation functionality also depends on the Silence Suppression (SCE) setting of the voice codec (G.711 or G.726) used in the voice channel. Voice coders are configured in the Coders Groups table (see Configuring Coders Groups).

The use of comfort noise is indicated by including a payload type for CN on the media ('m=') description line in the SDP. The device can use CN with a codec whose RTP time stamp clock rate is 8,000 Hz (G.711 / G.726). The static payload type 13 is used.

CN is negotiated between the call parties. If the remote side doesn't support CN, it's not used. Regardless of the device's settings, the device always attempts to adapt to the remote side's request for Comfort Noise Generation.

If you enable Comfort Noise Generation (see below procedure), the following occurs:

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If the device is the initiator, it sends a 'CN' in the SDP only if the SCE of the codec is enabled. If the remote UA responds with a 'CN' in the SDP, then CNG occurs; otherwise, CNG doesn't occur.

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If the device is the receiver and the remote SIP UA doesn't send a 'CN' in the SDP, then no CNG occurs. If the remote side sends a 'CN', the device attempts to be compatible with the remote side and even if the codec’s SCE is disabled, CNG occurs.

If you disable Comfort Noise Generation, the device doesn't send 'CN' in the SDP. However, if the codec’s SCE is enabled, the device performs Comfort Noise Generation.

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To enable Comfort Noise Generation:

1.

Open the RTP/RTCP Settings page (Setup menu > Signaling & Media menu > Media folder > RTP/RTCP Settings).

2.

Select the 'Comfort Noise Generation Negotiation' check box (ComfortNoiseNegotiation):

3.

Click Apply.