Content for  TS 23.502  Word version:  19.1.0

UE camps on NG-RAN in the 5GS and an MO or MT IMS voice session establishment has been initiated.

Network initiated PDU Session modification to setup QoS flow for voice reaches the NG-RAN (see N2 PDU Session Request in clause 4.3.3).

NG-RAN is configured to support EPS fallback for IMS voice and decides to trigger fallback to EPS, taking into account UE capabilities, indication from AMF that "Redirection for EPS fallback for voice is possible" (received as part of initial context setup, handover resource allocation or path switch request acknowledge as defined in TS 38.413), network configuration (e.g. N26 availability configuration) and radio conditions. If NG-RAN decides not to trigger fallback to EPS, then the procedure stops here and following steps are not executed. NG-RAN may initiate measurement report solicitation from the UE including E-UTRAN as target.

NG-RAN responds indicating rejection of the PDU Session modification to setup QoS flow for IMS voice received in step 2 by PDU Session Modification Response message towards the SMF+PGW-C (or H-SMF+P-GW-C via V-SMF, in the case of home routed roaming scenario) via AMF with an indication that mobility due to fallback for IMS voice is ongoing. The SMF+PGW-C maintains the PCC rule(s) associated with the QoS Flow(s) and reports the EPS Fallback event to the PCF if PCF has subscribed to this event.

NG-RAN initiates either handover (see clause 4.11.1.2.1), or AN Release via inter-system redirection to EPS (see clause 4.2.6 and clause 4.11.1.3.2), taking into account UE capabilities. The SMF+PGW-C reports change of the RAT type if subscribed by PCF as specified in clause 4.11.1.2.1, or clause 4.11.1.3.2.

When the UE is connected to EPS, either 6a or 6b is executed: Step 6a. Step 6b.

After completion of the mobility procedure to EPS or as part of the 5GS to EPS handover procedure, the SMF+PGW-C re-initiates the setup of the dedicated bearer(s) for the maintained PCC rule(s) in step 4 including of the dedicated bearer for IMS voice, mapping the 5G QoS to EPC QoS parameters as specified in clause 4.11.1.2.1. The SMF+PGW-C reports about Successful Resource Allocation and Access Network Information if subscribed by PCF.

UE camps on source NG-RAN in the 5GS and an MO or MT IMS voice session establishment has been initiated.

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).

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.

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).

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.

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.

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.

If the UE is not registered via 3GPP access, the UE shall initiate Registration procedure as defined in clause 4.2.2.2.2.

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.

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), 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.

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. 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.

The UE uses SMS over NAS procedures in clause 4.13.3 to send an SMS to the AF. The service centre address points to the SMS-SC which contains the function described in this procedure, the destination SME address is set to short/long code of the AF and the Application Port ID element of the TP-User-Data field is set to the appropriate value. For MSISDN-less subscription, the dummy MSISDN is used. This MSISDN and the IMSI of the UE are sent to SMS-SC.

SMS-SC uses the destination SME address (long/short code of the AF) to identify the corresponding NEF based on a pre-configured mapping table. SMS-SC extracts the SMS payload, Application port ID and IMSI of the UE and delivers them to NEF along with the destination SME address (long/short code of the AF). The NEF acts as an MTC-IWF in this procedure.

The NEF invokes Nudm_SDM_Get (Identifier Translation, IMSI, Application Port ID, AF Identifier) to resolve the IMSI and Application Port ID to a GPSI (External Id).

The UDM provides a Nudm_SDM_Get response (GPSI). If the UE is not allowed to send an SMS payload to this AF, or there is no valid subscription information for this user, the flow proceeds to step 6.

The NEF provides a Nnef_MSISDN-less_MO_SMSNotify (SMS payload, GPSI and Application Port ID) message to the AF. The AF is identified with the destination SME address (long/short code of the AF) received from step 2. The payload is delivered directly to the AF, it is not processed by NEF.

The NEF, acting as an MTC-IWF, returns a success or failure delivery indication to SMS-SC.

SMS-SC indicates success/failure back to UE using existing SMS delivery report defined in TS 23.040.