5.15.7.1 General

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 [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 . A UE that supports selecting S-NSSAI based on URSP rules while in EPS and that is provisioned with URSP rules follows the procedures defined in TS 24.526 [110] clause 4.4.2 to select, if available, an S-NSSAI for the APN. Any selected S-NSSAI and the PDU Session ID shall be sent to the SMF+PGW-C via PCO.

If a UE supports NSSRG handling over EPS, then the UE shall ensure that:

• While determining the Requested NSSAI (e.g. over non-3GPP access), the S-NSSAIs in the Requested NSSAI share a common NSSRG with the S-NSSAIs of already established PDN connections (e.g. over 3GPP access). In addition, the UE shall only establish PDN connections in EPS that share a common NSSRG value with the Allowed NSSAI.
• PDN connections established in EPS shall share a common NSSRG value.

As described in clause 4.11.0a.5 of TS 23.502 [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 taking into account the S-NSSAI provided by the UE if any) 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 take into account the S-NSSAI provided by the UE and if UE has not indicated in PCO it supports the NSSAA over EPS feature defined in Annex L of TS 23.502 [3], 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. If the UE has indicated in PCO it supports NSSAA over EPC as defined in Annex L of TS 23.502 [3] and the SMF+PGW-C supports the feature in Annex L of TS 23.502 [3], the UE and SMF+PGW-C behave as specified in Annex L of TS 23.502 [3] and the SMF+PGW-C can also select an S-NSSAI that is subject to NSSAA. 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.

If the PCO received by the SMF+PGW-C contains an S-NSSAI that is not supported by the SMF+PGW-C, then the SMF+PGW-C may determine another SMF+PGW-C that supports the DNN and the S-NSSAI received in the PCO, based on operator policies and follows the PGW selection in case of PGW mismatch with network slice subscribed by the UE as specified in clause 7.2.1 and clause 7.2.2 of TS 29.274 [101].

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.