4G Session Support

For UEs, the SMF supports both 5G and 4G NAS to connect to both 4G and 5G core networks. The SMF includes the EPS interworking support and acts as a PGW-C+SMF. The interfaces, such as the Gx, Gy, or Gz, which are used for a 4G session creation are replaced with the corresponding 5G core SBI interfaces, such as the Npcf and Nchf.

The SMF supports interworking with EPS by using the N26 interface (which is an inter-CN interface between the MME and the 5GS AMF) to enable interworking between the Evolved Packet Core (EPC) and the NG core networks. Support of the N26 interface in the network is optional for interworking. The N26 interface supports a subset of the functionalities over S10 interface to enable interworking. The UE uses the EPC NAS or 5GC NAS procedures that are based on the core network. The SMF supports QoS flow failures for access and mobility procedures.

The following features are related to this use case: