Content for  TR 23.700-89  Word version:  18.0.0

This solution is to allow MME to receive RFSP Index update from PCF when UE moves from 5G to 4G in N26-based interworking architecture. When MME is able to receive updated RFSP index from PCF as AMF, the RFSP index in use will be consistent when UE moves between 5G and 4G. This is a solution for N26-based interworking. When N26-based interworking architecture applies, AMF and MME use N26 interface to transfer mobility management message. The parameters, subscribed RFSP index and RFSP index in use, are transferred in:

• Forward Relocation Request as in 5GS to EPS handover procedures using N26 interface, or
• Context Response as in 5GS to EPS Idle mode mobility using N26 interface.

Therefore, this solution proposes that, when UE moves from 5G to 4G, the old AMF should keep or defer to terminate the AM association of the UE within a reasonable period, i.e. AM Policy Association maintenance timer. When the PCF is triggered to update the RFSP Index, the updated RFSP index is delivered to the old AMF in the existing AM Policy association, and the AMF will transfer it to new MME use the N26 interface in a GTPv2 message. For example, a UE may have subscribed RFSP Index of 5G prioritization in subscription data stored in HSS/UDM. If a UE initially attaches to 5GC with AM Policy association established:

1. If something triggers the PCF to change the RFSP Index as of 4G prioritization, the UE will moves from 5GC to EPC. In handover or idle mode mobility procedure, the old AMF sends the PCF-updated RFSP Index, i.e. the RFSP Index in use, to the new MME using N26 interface.
2. With the received RFSP Index in use, the new MME may, as local logic, ignore the subscribed RFSP Index and keep the RFSP index in use as 4G prioritization. The UE will stay in 4G and will not be send right back to 5G as the subscribed RFSP Index as 5G prioritization. The old AMF may keep the established AM association or defer the AM policy association termination for the UE to receive further update on RFSP Index update from PCF, just as the old AMF will keep UE context for some time after the UE deregister. The duration value of this timer should be pre-configured at the AMF. And the AMF should subscribe to HSS/UDM for UE's serving MME update notification in case the UE moves further to another MME.
3. Later but before the AM Policy Association maintenance timer runs out, based on some trigger, the PCF may update UE's RFSP index in order to direct the UE back from EPC to 5GC. The update RFSP index is deliver to the old AMF. Then the old AMF transfers it to the serving MME the UE register to through N26 interface. Then the serving MME updates the RFSP index in use and triggers the UE move back to 5G using current mechanism.
4. When the AM Policy Association maintenance timer runs out, the old AMF initiates the AM Policy Association termination and unsubscribe to the HSS/UDM for further change on serving MME. This should be no later than the old AMF to release the UE context.
5. As AM Policy Association maintenance timer running in the old AMF, an equal timer should runs in the serving MME. The initial value of the timer may be pre-configured at the MME and no less than the value of the AM policy Association maintenance timer at the AMF. When the MME aware that the timer runs out, the MME will be free to select the RFSP Index in use, e.g. the subscription value, as in current specification. Also, when handover happens between MMEs, the remaining value of the timer should also be transfer as UE context.

If a UE initially attaches to EPC, or the AM policy association is invalid, that is, no exist AM policy association in PCF for that UE, the MME may update UE's RFSP Index according to current mechanism. The MME may decide to direct UE to 5GC by changing the RFSP Index. In some cases, when the EPS to 5GS procedure is complete and soon after a new AM association is established, the PCF may decides the UE should be in 4G. Then the EPS to 5GS procedure as described above may take place. With the proposed mechanism, the PCF selected RFSP Index will take over and the UE will not ping-pong between 4G and 5G.

Figure 6.1.2 depicts the procedure of propose solution of providing PCF-updated RFSP Index update from 5GC to MME with N26 interface.

Step 1 and Step 2 follow the procedure already defined in TS 23.502. The change is: the AMF, where the UE originally register to, will not initiate the AM Policy Association termination procedure with the PCF when it received the Nudm_UECM_DeregistrationNotification from HSS/UDM as in clause 4.11.1.5.3 of TS 23.502 (TAU of handover) nor clause 4.11.1.3.2 of TS 23.502 (Idle mode mobility). But the AM Policy Association maintenance timer starts to run in AMF when the Nudm_UECM_DeregistrationNotification receives. A timer also start to runs in target MME1 to indicate it should wait for further input from AMF on RFSP Index update. Step 3, the old AMF subscribes to HSS/UDM for the notification in case of UE S1-handover happens to insure the old AMF always know the serving MME of the UE. Step 4 and Step 5, the PCF is trigger to update the RFSP Index and notify to the old AMF according to current description in TS 23.503. Step 6 and Step 7 are the proposed new mobility management messages using N26 interface, in which the AMF transfer the updated RFSP index to MME1 and MME1 reply with an acknowledgement. If before the AM policy association maintenance timer runs out, S1-handover or TAU happens in EPC: Step 8, the UE deregisters from MME1 and register to MME2 in EPC and the remaining of the timer is transfer from MM1 to MM2 as part of UE context. Step 9, HSS/UDM notifies the old AMF with MME2. The RFSP index in use will be sent from MME1 to MME2 as in current specification. For further RFSP Index update from PCF as in step 4, the old AMF transfer the updated RFSP index to MM2 as in step 5, 6 and 7. When the AM Policy Association maintenance timer runs out, the AMF initiates the AM Policy Association termination as in TS 23.502 and unsubscribe to HSS/UDM for further serving MME change notification. When the timer at the MME runs out, the MME could reselect an RFSP Index value as defined in current specification. Or, if the AMF receives from HSS/UDM that the UE is not registered to any MME, i.e. deregistered from EPC, the AMF should initiates the AM Policy Association termination before the timer runs out.

The solution has the following impacts: AMF:

• When a UE is moving from 5GS to EPS, and HSS/UDM invokes Nudm_UECM_DeregistrationNotification to notify the AMF associated with 3GPP access with reason as 5GS to EPS Mobility, the AMF should not initiate the AM Policy Association termination.
• The AMF should maintain a timer for AM Policy Association maintenance in case of 5GS to EPS Mobility. And within that timer, the AMF subscribes to HSS/UDM for UE's serving MME update notification and transfer the updated RFSP Index to UE's serving MME when it is receive from PCF. When the timer runs out, the AMF initiate the AM Policy Association termination and unsubscribe to the HSS/UDM for further notification.
• New procedure is needed for AMF to inform the MME of the updated RFSP after UE moves to EPS.

MME:

• The MME should maintain a timer after the 5GS to EPS handover procedure or idle mode mobility complete within which it is able to receive and apply the updated RFSP Index from AMF.
• When S1-handover or TAU happens at EPC, the remaining value of the timer should be transfer from the source MME to the target MME as part of UE context.
• New procedure is needed for the MME to receive AMF provided RFSP index updated if the MME does not know the AMF.

HSS/UDM:

• New service needs to be defined to allow AMF to get UE's serving MME information.
• When HSS and UDM are deploy separately, new services are needed to allow HSS to provide serving MME information to UDM, then UDM to provide serving MME information to AMF. In a combined UDM+HSS deployment, service between HSS and UDM is not needed or handled in implementation specific way.

N26:

• New mobility management message are defined to support the RFSP Index transfer from AMF to MME.

N8 (& UDICOM interface if used)

• New event exposed to repost the change of MME to AMF.