System Behavior With Comfort Noise Transcoding
The following scenarios show how the Oracle® Enterprise Session Border Controller (E-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 E-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 E-SBC adds the default payload type of 13 to the SDP m= line.
When the E-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 E-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 | Egress codec policy | ||
|---|---|---|---|
| allow codecs | * | allow codecs | * |
| add codecs on egress | add codecs on egress | PCMA CN | |
| order codecs | order codecs | ||
The side of the call that does not support CN, must use an audio codec that the E-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 E-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.
- The Offerer sends an offer O° to the E-SBC with an SDP m-line for an audio codec PCMU, but with no CN signaling codec
- The codec-policy for the ingress realm allows the audio codec, PCMU, and outputs O¹ which contains PCMU.
- The E-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.
- 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.
- The E-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 E-SBC does not transcode the audio codec (PCMU). The E-SBC transcodes silence detection and generation, as well as CN packet detection and generation.
| Ingress codec policy | Egress codec policy | ||
|---|---|---|---|
| allow codecs | * | allow codecs | * |
| add codecs on egress | add codecs on egress | CN | |
| order codecs | order codecs | ||
Comfort Noise and Telephone Event Codecs Both Transcoded Scenario
In the following scenario, the E-SBC transcodes both Comfort Noise (CN) and telephone event codecs.
- The Offerer sends an offer (O°) with an SDP m-line for an audio codec PCMU to the E-SBC, and includes both CN and telephone-event signaling codecs.
- The codec-policy for the ingress realm allows all codecs and outputs O¹. The E-SBC egress codec-policy takes O¹ as input, allows all codecs, and produces output O².
- The Answerer responds with the answer in A° containing the PCMU audio codec. The egress codec-policy produces answer A¹ with no changes.
- The E-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 E-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 E-SBC does not transcode the audio codec (PCMU). The E-SBC transcodes silence detection and generation, as well as CN packet detection and generation. The E-SBC transcodes telephone-events to in-band DTMF audio.
| Ingress codec policy | Egress codec policy | ||
|---|---|---|---|
| allow codecs | * | allow codecs | * |
| add codecs on egress | CN telephone event | add codecs on egress | |
| order codecs | order codecs | ||
| dtmf-in-audio | preferred | dtmf-in-audio | preferred |
