Content for  TR 23.700-89  Word version:  18.0.0

This solution is applicable only when N26-based interworking is used. 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.

During UE registration in 5GC, the AMF may report to the PCF the subscribed RFSP Index received from the UDM for further evaluation as described in clause 6.1.2.1 of TS 23.503. When receiving the "authorized RFSP Index" from the PCF, the AMF replaces the "subscribed RFSP Index" with the "authorized RFSP Index" as specified in clause 5.3.4.3.1 of TS 23.501. So the "subscribed RFSP Index" stored in the UE context in AMF may be different than the "subscribed RFSP Index" received from the UDM as the AMF may replace the "subscribed RFSP Index" with the "authorized RFSP Index" received from the PCF. Therefore, this solution proposes that, when UE moves from 5G to 4G, the MME shall compare the "subscribed RFSP Index" received from the AMF (which could be the "Authorized RFSP Index" provided by the PCF) with the "RFSP Index" received from the HSS. If they are different, the MME shall store the "subscribed RFSP Index" received from AMF in a new IE "5GC Authorized RFSP Index" and store the RFSP Index received from the HSS in "subscribed RFSP Index". The difference in "5GC Authorized RFSP Index" and "subscribed RFSP Index" is an indication to the MME that 5GC has dynamically assigned an RFSP Index to the UE and the MME shall, based on operator's policies, ignore the RFSP Index received from the HSS e.g. for a configured duration. This policy/duration could be operator determined e.g.:

• "5GC Authorized RFSP Index" is used only for the current registration in the MME and is not used when the UE moves to another MME due to mobility.
• "5GC Authorized RFSP Index" is used only for a configured duration (e.g. T). When the UE moves to another MME due to mobility (e.g. after a period "t"), the source MME then provides the remaining duration (i.e. T-t) for which "5GC Authorized RFSP Index" shall be used by the destination MME.
• Or it could be a combination of both of the above.

When the configured duration is reached, the MME may remove the "5GC Authorized RFSP Index" from its context for the UE and start using the "subscribed RFSP Index" that was received from the HSS.

There is no additional procedure needed to update the RFSP Index when the UE is served by the EPC. MME uses the 5GC authorized RFSP Index for a configuration duration as explained in clause 6.10.1.

The solution has the following impacts: PCF:

• No impact.

AMF:

• No impact.

MME:

• MME shall compare the "subscribed RFSP Index" received from the AMF (which could be the "Authorized RFSP Index" provided by the PCF) with the "RFSP Index" received from the HSS.
• If they are different, the MME shall store the "subscribed RFSP Index" received from AMF in a new IE "5GC Authorized RFSP Index" and store the RFSP Index received from the HSS in "subscribed RFSP Index".
• MME shall ignore the RFSP Index received from the HSS for a configured duration.
• When the configured duration is reached, the MME may remove the "5GC Authorized RFSP Index" from its context for the UE and start using the "subscribed RFSP Index" that was received from the HSS.

N26:

• No impact.