Currently, most operators run 4G and 5G merged network. To promote 5G service, the subscription data of UE may be set to 5G access first, i.e. the subscribed RFSP Index is of the value of "5G has higher priority than 4G". In some scenarios, the PCF may adjust the RFSP index to direct the UE from 5G to 4G according to dynamic network situations, for example:

• When network congestion prediction received from NWDAF, the PCF may move some of the "5G prioritized" UEs which consuming low value applications to 4G access.
• According to the request from AF, the PCF may move the requested UE(s) from 5G to 4G.

When UE registers to EPC, the MME chooses the RFSP Index in use based on the subscribed RFSP Index, the locally configured operator's policies and the UE related context information available at the MME. If the EPC decides the UE needs to go back to 5G, the MME provides "5G prioritized" RFSP Index to eNB. To avoid network issue, e.g. the ping-pong issue, where the 5GC keeps sending the UE to EPC based on authorized RFSP Index from PCF, while the MME only has the subscribed RFSP Index and kick the UE in the above scenarios back to 5G immediately, this key issue will study AM policy control on RFSP Index consistency when UE moves from 5GC to EPC:

• Whether the current interworking procedure supports MME received the RFSP Index in use from 5GC? If no, what enhancements is needed.
• If the MME get the RFSP Index in used in handover procedure or idle mode mobility procedure, how and when it resumes to the subscription RFSP Index.
• When UE is under EPC, should MME receive any update of RFSP Index from 5GC and how.