Transcoding Free Operation for Local Media Playback

You can configure the SBC to avoid using transcoding resources within certain local media playback scenarios. After establishing a RBT call that includes transcoding, the SBC can trigger this Transcoding Free Operation (TrFO) feature if the P-Acme-TrFO header is present. Having determined that the call can proceed without transcoding, the SBC originates a reINVITE towards the calling party containing the called side codec. Once the reINVITE is completed, the call can continue without transcoding. In addition, the negotiated codec on the called party side must have been included in the calling party's original offer (after ingress codec-policy execution).

The SBC retains the codec list sent by the calling party, usually within the initial offer with all supported codecs. It also monitors the result of the ingress codec-policy. The SBC triggers the TrFO function for RBT when the following conditions are true:

The P-Acme-TrFO header;
One of the following are true:
- The voice codecs are different on either side, or
- The voice codecs are the same, but telephone event is missing from one side.

In this case, the SBC originates a reINVITE towards the calling party containing the called side codec. Once the reINVITE is completed, the call can continue without transcoding.

Note:

If the SBC detects fax tone, and you have enabled RTCP generation, the TrFO call flow can proceed, but the DSPs will still be in use afterwards so the feature can still be provided.

Navigate to the initial INVITE ingress realm's realm-config and configure the feature-trfo parameter using the syntax below. This is the same realm on which you have configured the ringback-trigger.

ORACLE(realm-config)# feature-trfo rbt

This 'manual' trigger by the SBC to force SDP re-negotiation prevents the call from requiring transcoding.

Related Configuration

You must also configure the following on the SBC for TrFO to work correctly with RBT:

Transcoding enabled
realm-config, hide-egress-media-update enabled
media-manager, options +unique-sdp-id
media-manager, anonymous-sdp enabled

TRFO reINVITE generation rules

The following table describes the general TrFO reINVITE generation rules when you set the feature-trfo parameter to rbt on the ingress realm and the P-Acme-TrFO header is present in either the provisional messages or the final 200OK.

Ringback-trigger Configuration	SDP injected	Transcoding	Codec is present in the initial INVITE	Is TRFO reINVITE generated?	Examples
When not set to any of these values: 180-no-sdp 180-force 183 183-no-sdp	N/A	N/A	N/A	No	TrFO is not triggered. TrFO reINVITE is not triggered because the negotiated callee codec was not present in the original INVITE offer.
180-no-sdp 180-force 183 183-no-sdp	No	N/A	N/A	No	TrFO is not triggered. TrFO reINVITE is not triggered because SDP was not inserted for RBT.
180-no-sdp 180-force 183 183-no-sdp	Yes	No	N/A	No	TrFO is not triggered. TrFO reINVITE is not triggered because there is not transcoding
180-no-sdp 180-force 183 183-no-sdp	Yes	Yes	No	No	TrFO is not triggered.
180-no-sdp 180-force 183 183-no-sdp	Yes	Yes - different caller/callee audio codecs OR - same codec, telephone-event on one side	Yes	Yes	TrFO is Triggered.