ePDG and 5GS Interworking for Handover

The following figure illustrates the non-roaming architecture for interworking between the ePDG or EPC and 5GS.

Non-Roaming Architecture for Interworking between ePDG or EPC and 5GS

The interworking between the ePDG and 5GS is similar to the interworking between EPC and 5GS without the N26 interface. In this interworking, the IP address preservation occurs on the UEs on inter-system mobility. Saving and fetching PGW-C+SMF and the corresponding APN and DNN information through the HSS+UDM makes interworking possible. In such networks, AMF also supports interworking with UEs without the N26 interface during the initial registration in 5GC. The AMF may support interworking with UEs without N26 in the Attach procedure in 5GS. In case of a non-3GPP untrusted Wi-Fi access, the ePDG does not communicate with the AMF because the N26 interface does not exist.

A 5GS supports network slicing and can interwork with the EPS in its PLMN or in other PLMNs. SMF+PGW-C performs UDM registration for each UE with PGW-C FQDN + NSSAI values. With this registration, the AMF or ePDG identify the PGW-C IP-address from the UDM or HSS as part of the subscription information after the UE authorization is completed.

The mobility between 5GC to EPC does not ensure that all the active PDU sessions 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 PGW-C+SMF through the PCO.

An S-NSSAI that is associated with the PDN connection is determined based on the operator policy by the PGW-C+SMF. For example, the combination of PGW-C+SMF address and APN is sent to the UE in PCO along with a PLMN ID to which the S-NSSAI relates. If the PGW-C+SMF supports multiple S-NSSAI and the APN is valid for multiple S-NSSAIs, the PGW-C+SMF selects only the S-NSSAI that is mapped to the subscribed S-NSSAIs of the UE. The UE saves this S-NSSAI and the PLMN ID that is associated with the PDN connection. The UE derives the requested NSSAI through the received PLMN ID. The requested NSSAI is included in the NAS registration request message and RRC that is carrying the registration request when the UE registers in 5GC. This scenario is applicable if the UE is non-roaming or the UE has configured NSSAI for the VPLMN in roaming case.