Section: 4.13.6.3 Transfer of PDU session used for IMS voice from non-3GPP access to 5GS
Content: Figure 4.13.6.3-1: Transfer of PDU session used for IMS voice from non-3GPP access to 5GS When the UE has an ongoing IMS voice session via non-3GPP access using ePDG or N3IWF and the session is transferred to NG-RAN, depending on the selected RAT in 5GS (NR or E-UTRA) and the support of EPS/inter-RAT fallback in NG-RAN, either the IMS voice session continues over NG-RAN (E-UTRA) or EPS/inter-RAT fallback is triggered. Steps 1, 2 and 3 apply to either of the above two cases. 1. UE has ongoing IMS voice session via non-3GPP access using ePDG or N3IWF. UE is triggered to move to 3GPP access and camps in NG-RAN. 2. If the UE is not registered via 3GPP access, the UE shall initiate Registration procedure as defined in clause 4.2.2.2.2. 3. UE initiates PDU session establishment for the PDU session used for IMS voice service in order to initiate handover from EPC/ePDG to 5GS as defined in clause 4.11.4.1 step 2 or to initiate handover from N3IWF to 3GPP access in 5GC in step 2 of clauses 4.9.2.1 and 4.9.2.3. The SMF accepts the PDU session transfer from the UE. NOTE 1: If the UE is aware (e.g. implementation-dependent mechanisms) that voice over NR may not be natively supported in the current Registration area, the UE can attempt to move to E-UTRA to initiate a handover of the IMS PDU Session to EPC or 5GC to continue the IMS voice session. The remaining steps are not executed. 4. NG-RAN may decide to trigger EPS or inter-RAT fallback, taking into account UE capabilities, indication from AMF that "Redirection for EPS fallback for voice is possible" (received as part of initial context setup as defined in TS 38.413[ NG-RAN; NG Application Protocol (NGAP) ]  [10]), network configuration (e.g. N26 availability configuration) and radio conditions. NG-RAN may initiate measurement report solicitation from the UE including E-UTRA as target. If NG-RAN does not trigger EPS or inter-RAT fallback, then the procedure stops here and following steps are not executed. NOTE 2: If the AMF has indicated that "Redirection for EPS fallback for voice is not possible", then EPS fallback for IMS voice is not performed. If NG-RAN has not received indication "Redirection for EPS fallback for voice", the decision to execute EPS fallback for IMS voice or not is based on network configuration (e.g. based on N26 availability and other criteria). 5. NG-RAN responds indicating rejection either to set up the QoS flow for IMS voice or the entire PDU session used for IMS voice service as received in step 3 towards the SMF+PGW-C (or H-SMF+P-GW-C via V-SMF, in the case of roaming scenario) via AMF with an indication that mobility due to fallback for IMS voice is ongoing. The SMF+PGW-C reports the EPS Fallback event to the PCF if the PCF has subscribed to this event. If NG-RAN responds indicating rejection to set up the QoS flow for IMS voice, steps 5-7 from clause 4.13.6.1 are executed if EPS fallback is triggered, or steps 5-6 from clause 4.13.6.2 are executed if inter-RAT Fallback for IMS voice is triggered. The SMF+PGW-C executes the release of resources in non-3GPP AN as specified in step 3 of clauses 4.11.4.1, 4.9.2.1 and 4.9.2.3. NOTE 3: The timing of executing the release of resources in non-3GPP AN will depend on whether NG-RAN decides to trigger EPS or inter-RAT fallback but will take place at least after step 5. If NG-RAN rejects entire PDU session used for IMS voice service, the SMF should stop the ongoing procedure and keep the PDN connection / PDU Session at non-3GPP side. The NG-RAN performs AN release with redirection or handover procedure. In both cases, after receiving the AN release with redirection or completion of the handover procedure the UE initiates TAU procedure. In the case of inter-system redirection to EPS, if the UE decides to re-initiate handover of an IMS voice session over non-3GPP access to EPS after inter-system change from 5GS to EPS is completed, the UE includes active flag in the request. NOTE 4: Depending on UE implementation, the UE can re-initiate handover of an IMS voice session over non-3GPP access to EPS if the inter-system change from 5GS to EPS is triggered by the NG-RAN and the UE does not receive a response to the PDU session establishment request i.e. when the establishment of the entire PDU session used for voice is rejected by NG-RAN.