PDU session establishment with 4G/N26/IWF

3GPP has specified provisions for the UEs capable of supporting both 5G and 4G NAS to connect to the E-UTRAN and 5G core network. An SMF capable of the EPS interworking support would act as a PGW-C+SMF and would have the S5/S8 interface with SGW to receive the 4G session creation request.

Figure 7.7 – 4G interworking function

All the other interfaces involved in a 4G session creation (for example, Gx, Gy, Gz, etc.) are replaced by the corresponding 5G core SBI interfaces (Npcf, Nudm, and Nchf). The SMF+PGW-C provides information over N4 to the UPF+PGW-U related to the handling of traffic over S5-U.

This option will be used typically by network operators when most of their network has a 5G core deployed. In this case, it makes sense to slowly retire the legacy of the 4G-based core and leverage the interworking function of the 5G core to cater to 4G devices.

In Figure 7.7, we see how interworking is...