Content for  TS 23.228  Word version:  19.0.0

When an IMS-ALG supporting OMR allocates a TrGW during forwarding of the SDP offer for proactive transcoding with resource reservation, if subsequent IMS-ALGs do not replace the transcoder and the answering side signals in the SDP answer that transcoding is required, then the IMS-ALG retains the TrGW for transcoding and anchors it in the user plane path. The call flow is the same as the usual call flow for TrGW insertion except for the addition of some SDP extension information to the SDP messages.

Figure Q.5 applies when IMS-ALG1 allocates a TrGW during forwarding of the SDP offer for proactive transcoding with resource reservation, IMS-ALG1 is the last signalling entity on the path before UA2, and UA2 signals in the SDP answer that transcoding is not required. UA2 ignores unrecognized OMR extension attributes. A second SDP offer/answer transaction is required to remove the transcoder from the path. As an alternative to the procedure shown, the IMS-ALG1 may forward connection information for a prior user plane segment without transcoding options while including an IP realm instance for the transcoding TrGW. This alternative avoids a second SDP offer/answer transaction if transcoding is not required, but does include a second SDP offer/answer transaction if transcoding is required.

Figure Q.6 applies when IMS-ALG1 allocates a TrGW during forwarding of the SDP offer for proactive transcoding with resource reservation, another IMS-ALG (IMS-ALG2) is the last signalling entity on the path before UA2, IMS-ALG2 must allocate a TrGW to provide NAT, and UA2 signals in the SDP answer that transcoding is not required. There may be additional IMS-ALGs between IMS-ALG1 and IMS-ALG2. This scenario avoids the need for a second SDP offer/answer transaction as required in clause Q.2.5.3 and clause Q.2.5.5. IMS-ALG2 signals to IMS-ALG1 in the SDP answer to bypass the transcoder.

Figure Q.7 applies when IMS-ALG1 allocates a TrGW during forwarding of the SDP offer for proactive transcoding with resource reservation, another IMS-ALG (IMS-ALG2) is the last signalling entity on the path before UA2, IMS-ALG2 does not need to allocate a TrGW to provide NAT, and UA2 signals in the SDP answer that transcoding is not required. There may be additional IMS-ALGs between IMS-ALG1 and IMS-ALG2. A second SDP offer/answer transaction is required to remove the transcoder from the path. The scenario allows IMS-ALG2 to initiate the second SDP offer/answer transaction rather than requiring IMS-ALG1 to do this, thus saving some messaging. As an alternative to the procedure shown, the IMS-ALG1 may forward connection information for a prior user plane segment without transcoding options while including an IP realm instance for the transcoding TrGW. This alternative avoids a second SDP offer/answer transaction if transcoding is not required, but does include a second SDP offer/answer transaction if transcoding is required.

Figure Q.8 applies when IMS-ALG1 allocates a TrGW during forwarding of the SDP offer for proactive transcoding with resource reservation, and a later IMS-ALG (IMS-ALG2) chooses to bypass the transcoding offered by IMS-ALG1 and optionally offer its own transcoding options. There may be additional IMS-ALGs between IMS-ALG1 and IMS-ALG2. The flow assumes that TrGW2 must remain to providing transcoding or NAT. The call flow variant if TrGW2 is not needed can be derived by combining this flow with one of the other transcoding flows. An IMS-ALG may remove codec options, or re-instate codec options removed by a previous IMS-ALG by bypassing that IMS-ALG's TrGW if allocated. The call flow continues to apply except that TrGW allocation is optional.

An IMS-ALG performing proactive transcoding without resource reservation provides an indication in the forwarded SDP that an address is unavailable if transcoding is selected. Subsequent IMS-ALGs can replace this transcoder when forwarding the SDP offer according to the procedure in clause Q.2.5.6, but cannot insert a transcoding TrGW on behalf of the IMS-ALG performing proactive transcoding if needed. Thus the procedures in clause Q.2.5.4 and clause Q.2.5.5 do not apply. If transcoding is required, the IMS-ALG performs a procedure very similar to clause Q.2.5.3 to allocate the transcoding TrGW and signal its address to the answering side.

An IMS-ALG performing reactive transcoding follows all OMR procedures when forwarding the initial SDP offer but does not allocate a transcoder. If the initial SDP offer is rejected due to lack of support for an offered codec, the IMS-ALG performing reactive transcoding will restart the OMR procedure with the answering side after allocating and anchoring a TrGW with transcoding. The IMS-ALG removes all prior visited realm and secondary realm instances from the SDP offer before forwarding.