The UPF (User Plane Function) is one of the most important units in 5GC. It is a key unit that the Radio Network (RAN) interacts with during PDU data transmission. The UPF is also an evolution of CUPS (Control Plane and User Plane Separation), responsible for inspecting, routing, and forwarding packets within QoS flows in subscription policies. It uses SDF templates sent by the SMF through the N4 interface to enforce uplink (UL) and downlink (DL) traffic rules. When the service ends, it will allocate or terminate the QoS flow in the PDU session; the order of use of UPF interface session update and deletion is as follows; please refer to the order of use of UPF interface (protocol) and terminal call in 5G.
I. Session Modification Terminal-specific QoS flow is allocated through the PDU session modification process; additional dedicated QoS flow supports traffic with higher QoS requirements (such as voice, video, game traffic, etc.); the application of session modification (update) in UPF is shown in Figure (1);
Figure 1. UPF interface usage order of terminal session modification (update) in 5G
II. Session deletion When the terminal service session ends, QoS flow will be allocated or terminated in the PDU session. The session deletion usage order in the UPF interface is as follows:
Figure 2.5G Terminal deletion UPF related interface usage order
