5.15.7.3 Connected mode aspects

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

• When a UE is CM-CONNECTED in 5GC and a handover to EPS occur, the AMF selects the target MME based on the source AMF Region ID, AMF Set ID and target location information. The AMF forwards the UE context to the selected MME over the N26 Interface. In the UE context, the AMF also includes the UE Usage type, if it is received as part of subscription data. The Handover procedure is executed as documented in TS 23.502 [3]. When the Handover procedure completes successfully the UE performs a Tracking Area Update. This completes the UE registration in the target EPS. As part of this the UE obtains a DCN-ID if the target EPS uses it.
• When a UE is ECM-CONNECTED in EPC and performs a handover to 5GS, the MME selects the target AMF based on target location information, e.g. TAI and any other available local information (including the UE Usage Type if one is available for the UE in the subscription data) and forwards the UE context to the selected AMF over the N26 interface. In the home-routed roaming case, the AMF selects default V-SMFs. The Handover procedure is executed as documented in TS 23.502 [3]. The SMF+PGW-C sends PDU Session IDs and related S-NSSAIs to AMF. Based on the received S-NSSAIs values the target AMF derives the S-NSSAI values for the Serving PLMN, the target AMF reselects a final target AMF if necessary as described in clause 5.15.5.2.1, the AMF reallocation procedure is triggered. For each PDU Session based on the associated derived S-NSSAI values if the V-SMF need be reallocated, the final target AMF triggers the V-SMF reallocation as described in clause 4.23.2 of TS 23.502 [3]. When the Handover procedure completes successfully the UE performs a Registration procedure. This completes the UE registration in the target 5GS and as part of this the UE obtains an Allowed NSSAI.