Improvements to core network functions

Interworking functions (IWF) 501, 503 allow a user equipment (UE) 111a attached to a 4G network 100 to make use of services of a 5G core network 200. Packet data flows from a Packet Data Network PDN 223 are encapsulated by an IW-UPF 505 into corresponding GTP-U tunnels each relating to a different bearer for the UE and transmitted over an S1-U interface to a base station (eNodeB) 113 serving the UE. Each bearer relates to a different quality of service. A control plane Interworking Function (IWF-C) 501 receives a create session request from a Mobile Management Entity (MME) 121. A create session management context request is generated by the IWF-C 501 and transmitted to a Session Management Function (SMF) 225. A create session management context request response is received from the SMF 225. A create session request response is generated by the IWF-C 501 and sent to the MME 121. The IWF-C may also convert a modify bearer request from the MME for transmission to the SMF. An IWF-C(P) 503 receives a Policy Control Function PCF request from a SMF and generates a Policy and Charging Rules Function (PCRF) request for transmission to a PCRF 13.