Content for  TR 23.700-89  Word version:  18.0.0

The solution address KI#1 for deployment scenario with N26. In this solution,

• The assumption is that PCF is aware of the mapping from RFSP index to cell selection and the RAT/Frequency Priorities which is typically known by RAN (see Annex I of TS 36.300 and Annex D in TS 38.300). Note that RFSP index is also known as "SPID" (Subscriber Profile ID) for RAT/Frequency Priority.
• When the PCF provides authorized RFSP Index, if the PCF determines that the change of authorized RFSP index value indicates a change from prioritizing 5G access to prioritizing 4G access for the UE, the PCF includes a validity time (working name "RFSPinUseExpiryTime") indicating a time by which this RFSP index will take effect if the UE moves to EPS. As specified in clause 6.1.1.3 of TS 23.503, PCF may use the network analytics on "Service Experience" for an Application Identifier, "any RAT type" and/or "any Frequency value" to determine the "authorized RFSP index" value for the UE when running this application. Furthermore, as specified in clause 6.4.3 of TS 23.288, the "Service Experience" analytics including a Validity period for the Application service experience analytics. So, it is possible for the PCF to determine the "authorized RFSP index" together with a validity time i.e. RFSPinUseExpiryTime.
• When the AMF receives the authorized RFSP Index from the PCF with the validity time, the AMF stores also the validity time.
• If the NG-RAN triggers handover before "RFSPinUseExpiryTime", the AMF will include the new parameter "RFSPinUseExpiryTime" in Forward Relocation Request to the MME. If the NG-RAN triggers release with redirect, the UE will perform Tracking Area Request procedure, and the AMF will include new parameter "RFSPinUseExpiryTime" in Context Response if time is before "RFSPinUseExpiryTime".
• When the MME receives RFSPinUseExpiryTime, the MME will use "RFSP index in use". When the time indicated in RFSPinUseExpiryTime is due, the MME will do RFSP index re-evaluation as specified in clause 4.3.6 of TS 23.401.

Clause 4.11.1.2.1 of TS 23.502 (5GS to EPS handover using N26 interface) is reused with the following addition:

• In step 3 when AMF sends Forward Relocation Request to the MME, the AMF additionally includes a timestamp indicating to the MME that "RFSP index in use" should be used before the indicated time is due. When the indicated time is due, the MME re-evaluates the RFSP index as specified in clause 4.3.6 of TS 23.401.

Clause 4.11.1.3.2 of TS 23.502 (5GS to EPS Idle mode mobility using N26 interface) is reused with the following addition:

• In step 6 when AMF sends Context Response to the MME, the AMF additionally includes a timestamp indicating to the MME that "RFSP index in use" should be used before indicated time is due. When the indicated time is due, the MME re-evaluates the RFSP index as specified in clause 4.3.6 of TS 23.401.

If the UE moves further from one MME to another MME before the time indicated in RFSPinUseExpiryTime is due, the old MME includes the RFSPinUseExpiryTime in Forward Relocation Request or Context Response from the old MME to the new MME.

Impact on interface: Impact on PCF:

• Include additional validity time when PCF determines that the change of RFSP index value indicates a change from prioritizing 5G access to prioritizing 4G access for the UE

Impact on AMF:

• When the AMF receives the additional validity time (in the form of due time), the AMF stores it in the UE Context. When mobility to EPS happens, the AMF also includes the validity time in the following GTP-C v2 messages informing the MME that "RFSP index in use" should be used before the indicated time is due:
  • Forward Relocation Request.
  • Context Response.

Impact on MME:

• When receiving the new timer in Forward Relocation Request, or Context Response, the MME will store it in the UE Context and use "RFSP index in use" received from the AMF or an old MME. When the indicated time is due, the MME will re-evaluate the RFSP index as specified in clause 4.3.6 of TS 23.401.