 the 5G core. This is one of the key aspects that will enable service providers to later use parts of 5G core for operating their 4G network as well, thereby reducing the need to operate full-fledged 4G and 5G core networks. Packet Core Procedures eNodeB SMF+PGW-C UPF+PGW-U Attach Request Selection of SGW and SMF+PGW-C Create Session Request Create Session Request UDM Registration and Subscription Fetch SM Policy Association Establishment TFT and QoS mapping from 5G pcc rules to 4G Qos Charging Session Initialisation N4 Session Establishment Create Session Response Create Session Response Initial Context Setup Request(A Hach Accept) RRC Configuration Direct Attach Transfer Complete Modify Bearer Request Modify Bearer Request N4 Session Modification Modify Bearer Response Modify Bearer Request UL and DL data eNodeB SMF+PGW-C UPF+PGW-U Figure 7.8 - 4G initial attach with IWF (5G core) PDU session establishment with 4G/N26/IWF Some of the key features in the call flow are as follows: The PGW in the procedure is replaced by SMF+PGWC. The MME selects the PGWC/SMF by checking the N1 mode support from the UE capability - that is, within the registration request sent by the UE during the initial attach and the subscription from HSS. The MME sends the DNS query based on the service type to select the best match for PGWC/SMF according to the top one: = Service gap control: Not supported = N1 mode: Supported = Dual connectivity with NR: Supported = Control plane data backoff: Not supported Figure 7.9 -N1 - node in a packet capture The MME sets the 5GIWKS indication flag on CSReq: = 5GSIWKI (5GS Interworking Indication): : True = EEVRSI (Extended EBI Value Range Support Indication) Figure 7.10 - 5GS interworking indication in a packet capture The SMF/PGWC allows the 4G session. Either the PDU session ID is received on PCO/APCO or the CSReq will have a 5GIWKS bit set. During PDN connection establishment in the EPC, the UE allocates the PDU session ID and sends it to the PGW-C+SMF via PCO. An S-NSSAI associated with the PDN co