Section: 5.15.7 Network slicing and Interworking with EPS 5.15.7.1 General
Content: A 5GS supports Network Slicing and might need to interwork with the EPS in its PLMN or in other PLMNs as specified in clause 5.17.2. The EPC may support the Dedicated Core Networks (DCN). In some deployments, the MME selection may be assisted by a DCN-ID provided by the UE to the RAN (see TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [26]). Mobility between 5GC to EPC does not guarantee all active PDU Session(s) can be transferred to the EPC. During PDN connection establishment in the EPC, the UE allocates the PDU Session ID and sends it to the SMF+PGW-C via PCO. As described in clause 4.11.0a.5 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3], an S-NSSAI associated with the PDN connection is determined based on the S-NSSAI(s) supported by the SMF+PGW-C, the Subscribed S-NSSAI from UDM, whether interworking with EPS is supported for the DNN and S-NSSAI in the Session Management Subscription data and the operator policy by the SMF+PGW-C, e.g. based on a combination of SMF+PGW-C address and APN, and is sent to the UE in PCO together with a PLMN ID that the S-NSSAI relates to. In Home Routed roaming case, the UE receives a HPLMN S-NSSAI value from the SMF+PGW-C. If the SMF+PGW-C supports more than one S-NSSAI and the APN is valid for more than one S-NSSAI, the SMF+PGW-C should only select an S-NSSAI that is mapped to the subscribed S-NSSAI of the UE and this subscribed S-NSSAI is not subject to Network Slice-Specific Authentication and Authorization. The UE stores this S-NSSAI and the PLMN ID associated with the PDN connection. The UE derives Requested NSSAI by taking into account of the received PLMN ID. The Requested NSSAI is included in the NAS Registration Request message and, subject to the conditions in clause 5.15.9, the RRC message carrying this Registration Request when the UE registers in 5GC if the UE is non-roaming or the UE has Configured NSSAI for the VPLMN in roaming case. If the UE has no Configured NSSAI of the VPLMN, the UE includes the HPLMN S-NSSAIs in the NAS Registration Request message as described in clause 5.15.5.2.1. When UE moves from EPS to 5GS, AMF reallocation may happen as described in clause 5.15.7.2 and clause 5.15.7.3. NOTE: It is assumed that if a MME is configured with a N26 interface towards an AMF, the MME has N26 interfaces with all AMFs serving the same area than the initial AMF and that can serve UE subject to EPS to 5GS mobility.