Content for TR 23.700-89 Word version: 18.0.0
6.6 Solution #6: Provide updated RFSP when UE is already moved to EPS with N26 interface
6.6.1 Description
6.6.2 Procedures
6.6.3 Impacts on services, entities and interfaces
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6.6 Solution #6: Provide updated RFSP when UE is already moved to EPS with N26 interface p. 20
6.6.1 Description p. 20
This paper reuse the concept of solution 1 to implement AM policy association maintenance timer to send any further updated RFSP and updated validity timer (as mentioned in solution 3) from AMF to MME when UE is already moved to EPS.
When UE moves to EPS, MME registers its address in UDM+HSS and then UDM+HSS shall maintain UE's 5G registration state temporarily for some duration i.e. AM Policy Association maintenance timer if the request from MME has some new indication (MME will provide this indication for only those UEs for which it has received validity condition along with RFSP Index In use from AMF). It provides this timer while sending deregistration notification to AMF so that both AMF and UDM+HSS will be in sync in holding this temporary registration state for the UE. AMF after receiving this deregistration notification shall behave as per the step 14d in clause 4.2.2.2.2 of TS 23.502 except for the termination of AM policy towards PCF for the duration mentioned by UDM. The AMF shall send the AM Policy Association maintenance timer to PCF while sending to delete the AM policy. PCF after receiving this AM Policy delete message shall defer the policy termination for the received time period and only send the updated policy related to UE's RFSP Index. If the implementation specific (guard timer) is running at AMF then AMF shall stop this timer and start the AM Policy Association maintenance timer received in deregistration notification from UDM.
During this time period based on some trigger (e.g. no more congestion in 5GC), the PCF may update UE's RFSP index or validity timer (to extend the timer if congestion still persist in 5G) in order to direct the UE back from EPC to 5GC. The update RFSP index is deliver to the AMF. AMF then will query the MME address from UDM+HSS and transfer the updated RFSP to the latest MME (e.g. If UE has been moved from one MME to another while in EPS) through N26 interface.
During this time period if UE moves back to 5G because of some reasons (e.g. mobility or radio conditions) and gets registered through a new AMF in 5G, then UDM+HSS will inform old AMF and remove old AMF address which was stored temporarily. When the AM Policy Association maintenance timer expires, the AMF clears the UE context, PCF terminates the AM Policy and UDM+HSS removes the AMF address which was stored temporarily.
If the AM policy Association maintenance timer expired while UE is in EPS then if there is a need for UE to bring back to 5GS, subscribed RFSP index can be modified in HSS and sent to MME so that MME redirect UE to 5GS.
6.6.2 Procedures p. 21
The detailed procedure is described in Figure 6.6.2-1.
Step 1.
The UE registers to 5GC via NG-RAN. AMF establishes AM Policy association with PCF as in clause 4.16.1.2 of TS 23.502.
Step 2.
The PCF decides to adjust the RFSP index to direct the UE from 5G to 4G. The PCF decision could be based on various inputs. For example:
Step 3.
- Based on various analytics and prediction result from NWDAF (e.g. network congestion in 5G).
- According to the request from AF.
UE moves to EPS via E-UTRAN
Step 4.
AMF sends the "RFSP Index in use" and validity timer to MME
Step 5.
The MME sends the ULR with single registration indication along with one new indication and register its address in UDM+HSS.
Step 6.
The UDM+HSS starts the "AM Policy Association maintenanceTimer" and maintain the UE's 5G registration state and provides this timer in the Nudm_UECM_DeregistrationNotification to AMF.
Step 7.
The MME stores and use the received "RFSP Index in use" and validity timer from AMF.
Step 8.
The AMF didn't clear the UE context for the time duration received from UDM+HSS. AMF provides "AM Policy Association maintenance timer" in the Npcf_AMPolicyControl_Delete to PCF. PCF shall defer the policy termination for the received time duration and update only policy related to UE's RFSP Index during this time period and implicitly removes the AM policy after expiry of this timer.
Step 9.
UE moves to new MME.
Step 10.
The new MME sends the ULR with single registration indication and registers its address in UDM+HSS
Step 11.
Based on some trigger, PCF decides to update the RFSP Index or validity timer and notify to the AMF according to current description in TS 23.503.
Step 12.
The AMF queries the new MME address from UDM+HSS and receive it.
Step 13.
The AMF queries the serving node address for the UE from UDM using existing Nudm_UECM_Get service operation and UDM provides serving MME (new MME) address. The AMF sends the "RFSP Index in use" or validity timer to new MME.
Step 14.
The new MME updates the received "RFSP Index in use" and validity timer and use it.
6.6.3 Impacts on services, entities and interfaces p. 22
This solution may have the following impacts to existing entities and interfaces:
AMF:
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When a UE is moving from 5GS to EPS, and HSS/UDM invokes Nudm_
UECM_ to notify the AMF associated with 3GPP access with reason as 5GS to EPS Mobility, the AMF should maintain the UE's registration context for the received time period. AMF provides "AM Policy Association maintenance timer" in the Npcf_DeregistrationNotification AMPolicyControl_ to PCF.Delete
- UDM+HSS should maintain the UE's 5G registration state temporarily for some duration based on the indication received from MME. Also it will provide this time period to AMF while sending deregistration notification.
- MME should be able to receive and apply the updated RFSP Index from AMF after the 5GS to EPS handover procedure or idle mode mobility complete. It will provide one indication for those UEs it has received validity condition along with RFSP Index In use from AMF.
- PCF should defer the policy termination for the received time duration and update only policy related to UE's RFSP Index during this time period and implicitly removes the AM policy after expiry of this timer.
