Feature Description
The SMF implements the 3GPP recommendations for interworking of Evolved Packet System (EPS) and 5G Core Network (5GC).
The UEs capable of supporting both 4G and 5G NAS connect to the Evolved Terrestrial Radio Access Network (E-UTRAN) and the 5GC network. The SMF with the EPS interworking capability acts as a PGW-C+SMF and uses the S5 or S8 interface with S-GW to receive the 4G Session Creation Request. All the other interfaces involved in the 4G Session Creation (for example, Gx, Gy, Gz, and so on) are replaced by the corresponding 5GC Service Based Interfaces (for example, Npcf and Nchf).
After a PDU session is created on the PGW-C+SMF with E-UTRAN, Mobility Management Entity (MME) and Serving Gateway (S-GW), the UE can hand over E-UTRAN to 5G New Radio (NR) and vice-versa.
The SMF currently supports interworking with EPS using the N26 interface. This interface is an inter-CN interface between the MME and 5GS AMF to enable interworking between Evolved Packet Core (EPC) and the NG core networks. Support of N26 interface in the network is optional for interworking. N26 supports a subset of the functionalities over S10 interface to enable interworking.
The UE uses EPC NAS or 5GC NAS procedures depending on the core network by which it is served.