Architecture for interworking between 5GS and EPC/E-UTRAN

N26 interface is an inter-CN interface between the MME and 5GS AMF in order to enable interworking between EPC and the NG core. Support of the N26 interface in the network is optional for interworking. N26 supports a subset of the functionalities (essential for interworking) that are supported over S10.
PGW-C + SMF and UPF + PGW-U are dedicated for interworking between 5GS and EPC, which are optional and are based on UE MM Core Network Capability and UE subscription. UEs that are not subject to 5GS and EPC interworking may be served by entities not dedicated for interworking, i.e. by either by PGW or SMF/UPF.
There can be another UPF (not shown in the figure above) between the NG-RAN and the UPF + PGW-U, i.e. the UPF + PGW-U can support N9 towards an additional UPF, if needed.
The SMF and UPF need to support EPC PGW logic and functionality across the S5-C and S5-U interfaces. This means that the EPC SGW is unaffected.
The PCF need to support the necessary policy data parameters across the N7 interface in order to support the PGW-C functionality of the SMF .
Existing 4G users of course do not need to be migrated to the 5GC architecture. They should be completely unaffected by the introduction of 5G.