Comfort Noise Transcoding - Oracle Help Center

The following scenarios show how the Oracle Communications Session Border Controller (SBC) handles Comfort Noise (CN) packets with transcoding enabled.

General Behavior in the Scenarios

When the SDP produced by the ingress codec-policy O¹ contains an audio codec that interoperates with CN (PCMA, PCMU, and G.726), and the egress codec-policy allows the interoperable codec and is configured to add CN, the SBC adds the default payload type of 13 to the SDP m= line.

When the SDP produced by the ingress codec-policy O¹ does not contain a codec that interoperates with CN, and the egress codec-policy is configured to add an interoperable audio codec plus the CN codec, the SBC adds the default payload type of 13 to the SDP m= line.

When the SBC adds the CN codec, the system adds the following default a= line to the SDP:

a=rtpmap:13 CN/8000

For more information about the concepts and terms used in the scenarios, see "Transcoding Processing Overview."

Comfort Noise Transcoded Scenario

In a typical scenario with CN transcoding enabled, the SBC transcodes the ingress audio codec for the media stream to PCMA. The DSP detects silence in the media stream from the Offerer, translates the silence into a CN packet, and sends the packet to the Answerer. The Answerer sends a CN packet to the DSP, which translates the packet into silence in the media stream and sends the packet to the Offerer. The result is silence on egress.

Ingress codec policy	Setting	Egress codec policy	Setting
allow codecs	*	allow codecs	*
add codecs on egress	N/A	add codecs on egress	PCMA CN
order codecs	N/A	order codecs	N/A

The side of the call that does not support CN, must use an audio codec that the SBC supports as transcodable, represented by "x" in the call flow.

The side of the call that supports CN, must use an audio codec that the SBC supports as interoperable with CN, represented by PCMA in the call flow.

Audio Codec Not Transcoded - Comfort Noise Codec Transcoded Scenario

In the following scenario, the ingress codec policy and the egress codec policy both allow all offered codecs. The setting for add-codecs-on-egress is CN.

1. The Offerer sends an offer O° to the SBC with an SDP m-line for an audio codec PCMU, but with no CN signaling codec
2. The codec-policy for the ingress realm allows the audio codec, PCMU, and outputs O¹ which contains PCMU.
3. The SBC egress codec-policy takes O¹ as input and adds CN after checking that at least one of the audio codecs in O¹ interoperates with CN, resulting in PCMU and CN in output O².
4. The Answerer responds with the answer in Aº containing the PCMU audio codec and the signaling codec CN, indicating the Answerer supports comfort noise. The egress codec-policy produces answer A¹ with no changes.
5. The SBC removes CN from A¹ because it was not offered by the Offerer. The result contains only the audio codec PCMU.

Even with CN transcoding enabled, the SBC does not transcode the audio codec (PCMU). The SBC transcodes silence detection and generation, as well as CN packet detection and generation.

Ingress codec policy	Setting	Egress codec policy	Setting
allow codecs	*	allow codecs	*
add codecs on egress	N/A	add codecs on egress	CN
order codecs	N/A	order codecs	N/A

Comfort Noise and Telephone Event Codecs Both Transcoded Scenario

In the following scenario, the SBC transcodes both Comfort Noise (CN) and telephone event codecs.

1. The Offerer sends an offer (O°) with an SDP m-line for an audio codec PCMU to the SBC, and includes both CN and telephone-event signaling codecs.
2. The codec-policy for the ingress realm allows all codecs and outputs O¹. The SBC egress codec-policy takes O¹ as input, allows all codecs, and produces output O².
3. The Answerer responds with the answer in A° containing the PCMU audio codec. The egress codec-policy produces answer A¹ with no changes.
4. The SBC applies the add-codecs-on-egress parameter from the ingress codec-policy to add CN and telephone-event to A¹. The result contains the audio codec PCMU and the CN and telephone-event signaling codecs because the SBC excepts signalling codecs from the rule that the system can apply the add-codecs-on-egress parameter only by the egress codec policy.

Even with CN transcoding enabled, the SBC does not transcode the audio codec (PCMU). The SBC transcodes silence detection and generation, as well as CN packet detection and generation. The SBC transcodes telephone-events to in-band DTMF audio.

Ingress codec policy	Settings	Egress codec policy	Settings
allow codecs	*	allow codecs	*
add codecs on egress	CN telephone event	add codecs on egress	N/A
order codecs	N/A	order codecs	N/A
dtmf-in-audio	preferred	dtmf-in-audio	preferred