5.15.7.2 Idle mode aspects

In addition to the interworking principles documented in clause 5.17.2 the following applies for interworking with N26:

• When UE moves from 5GS to EPS, the UE context information sent by AMF to MME includes the UE Usage type, which is retrieved from UDM by AMF as part of subscription data.
• When UE moves from EPS to 5GS, then the UE includes the S-NSSAIs (with values for the Serving PLMN of the target 5GS, if available) associated with the established PDN connections in the Requested NSSAI in RRC Connection Establishment (subject to the conditions set out in clause 5.15.9) and NAS. The UE also provides to the AMF in the Registration Request message the mapping information as described in clause 5.15.6. The UE derives the S-NSSAIs values for the Serving PLMN by using the latest available information from EPS (if received in PCO) and from 5GS (e.g. based on URSP, Configured NSSAI, Allowed NSSAI). In the home-routed roaming case, the AMF selects default V-SMFs. The SMF+PGW-C sends PDU Session IDs and related S-NSSAIs to AMF. The AMF derives S-NSSAI values for the Serving PLMN as described in clause 5.15.5.2.1 and determines whether the AMF is the appropriate AMF to serve the UE. If not, the AMF reallocation may need be triggered. For each PDU Session the AMF determines whether the V-SMF need be reselected based on the associated S-NSSAI value for the Serving PLMN. If the V-SMF need be reallocated, i.e. change from the default V-SMF to another V-SMF, the AMF trigger the V-SMF reallocation as described in clause 4.23.3 of TS 23.502 [3].

In addition to the interworking principles documented in clause 5.17.2 the following applies for interworking without N26:

• When the UE initiates the Registration procedure and subject to the conditions set out in clause 5.15.9, the UE includes the S-NSSAI (with values for the Serving PLMN of the target 5GS) associated with the established PDN connections in the Requested NSSAI in the RRC Connection Establishment.
• The UE includes the S-NSSAIs (with values for the Serving PLMN of the target 5GS, if available) and the HPLMN S-NSSAI received in the PCO for the PDN connections as mapping information when moving PDN connections to 5GC using PDU Session Establishment Request message. The UE derives the S-NSSAIs values for the Serving PLMN by using, the latest available information from EPS (if received in PCO) and from 5GS (e.g. based on URSP, Configured NSSAI, Allowed NSSAI).