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 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.
If AMF has indicated that "Redirection for EPS fallback for voice is not possible", then AN Release via inter-system redirection to EPS is not performed in step 5.
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 PGW-C+SMF (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 PGW-C+SMF 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 126.96.36.199.1
), or AN Release via inter-system redirection to EPS (see clause 4.2.6
and clause 188.8.131.52.2
), taking into account UE capabilities. The PGW-C+SMF reports change of the RAT type if subscribed by PCF as specified in clause 184.108.40.206.1
, or clause 220.127.116.11.2.6. When the UE is connected to EPS, either 6a or 6b is executed
In the case of 5GS to EPS handover, see clause 18.104.22.168.1
, and in the case of inter-system redirection to EPS with N26 interface, see clause 22.214.171.124.2
. In either case the UE initiates TAU procedure; or
In the case of inter-system redirection to EPS without N26 interface, see clause 126.96.36.199
. If the UE supports Request Type flag "handover" for PDN connectivity request during the attach procedure as described in clause 188.8.131.52 of TS 23.401
and has received the indication that interworking without N26 is supported, then the UE initiates Attach with PDN connectivity request with request type "handover".
In inter-system redirection, the UE uses the emergency indication in the RRC message as specified in clause 6.2.2 of TS 36.331
and E-UTRAN provides the emergency indication to MME during Tracking Area Update or Attach procedure. For the handover procedure see clause 184.108.40.206.1
, step 1.
After completion of the mobility procedure to EPS or as part of the 5GS to EPS handover procedure (see clause 220.127.116.11.1
), the SMF/PGW 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. The PGW-C+SMF behaves as specified in clause 18.104.22.168.1
. The PGW-C+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 the
At least for the duration of the voice call in EPS the E-UTRAN is configured to not trigger any handover to 5GS.