Section: 4.13.6.2 Inter RAT Fallback in 5GC for IMS voice
Content: Figure 4.13.6.2-1 describes the RAT fallback procedure in 5GC for IMS voice. When the UE is served by the 5GC, the UE has one or more ongoing PDU Sessions each including one or more QoS Flows. The serving PLMN AMF has sent an indication towards the UE during the Registration procedure that IMS voice over PS session is supported, see clause 5.16.3.10 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2] and the UE has registered in the IMS. Figure 4.13.6.2-1: RAT Fallback for IMS voice 1. UE camps on source NG-RAN in the 5GS and an MO or MT IMS voice session establishment has been initiated. 2. Network initiated PDU Session modification to setup QoS flow for IMS voice reaches the source NG-RAN (see N2 PDU Session Request in clause 4.3.3). 3. If source NG-RAN is configured to support RAT fallback for IMS voice, source NG-RAN decides to trigger RAT fallback, taking into account on UE capabilities, network configuration and radio conditions. Source NG-RAN may initiate measurement report solicitation from the UE including target NG-RAN. 4. Source NG-RAN responds indicating rejection of the PDU Session modification to setup QoS flow for IMS voice received in step 2 by PDU Session Response message towards the SMF (or 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 maintains the PCC rule(s) associated with the QoS Flow(s). 5. Source NG-RAN initiates Xn based Inter NG-RAN handover (see clause 4.9.1.2) or N2 based inter NG-RAN handover (see clause 4.9.1.3), or redirection to E-UTRA connected to 5GC (see clause 4.2.6). The SMF reports change of the RAT type if subscribed by PCF. 6. After completion of the Inter NG-RAN (inter-RAT) handover or redirection to E-UTRA connected to 5GC, the SMF re-initiates the PDU Session modification to setup QoS flow for IMS voice. The SMF reports about Successful Resource Allocation and Access Network Information if subscribed by PCF. The IMS signalling related to IMS voice call establishment continues after step 1 as specified in TS 23.228[ IP Multimedia Subsystem (IMS); Stage 2 ]  [55]. At least for the duration of the IMS voice call the target NG-RAN is configured to not trigger inter NG-RAN handover back to source NG-RAN.