 send the MAC address and the DS-TT Ethernet port. This marks the end of the PDU session establishment procedure. In this section, we have gone through the details of the PDU establishment procedure and the call flow to set up a PDU session in 5G. We have also learned about the concept of a PDU and the role it has to play in establishing connectivity for a UE. In the next section, we will go through the call flow to establish a 4G call using the 5G packet core network. 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. PDU session establishment with 4G/N26/IWF HSS + SMF + PGW-C UPF + PGW-U S1-MME E-UTRAN NG-RAN 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 supported between the 5G and 4G cores. Another key feature that is enabled by this design is the fact that handovers between 4G and 5G and vice-versa would be seamless, which would be made possible by leveraging a common UPF that will serve both 4G and 5G sessions. Hence, continuity can be maintained on the user plane and the user traffic will not be disrupted during this transaction. 4G (E-UTRAN) initial attach In this section, we will go into detail on how to establish a 4G session using the interworking function within