In 5G, the order of UPF interface (protocol) usage with terminal calls

User Plane Function (UPF) is one of the most important Network Functions (NFs) in the 5G core network. It is the second network function that the NR RAN interacts with during PDU flows. UPF is an evolution of CUPS (Control Plane Separation from User Plane), specifically responsible for inspecting, routing, and forwarding packets within QoS flows in subscription policies. It also uses SDF templates sent by the SMF through the N4 interface to enforce UL (Uplink) and DL (Downlink) traffic rules; when the corresponding service ends, it allocates or terminates QoS flows in the PDU session.

Figure 1.5G SMF and its interface (protocol)

I. UPF Interfaces and Protocols include the following:

• N4[5] After the user plane is established, the session management context and necessary parameters are transmitted from the single-mode fiber (SMF) to the user plane function (UPF).
• PFCP[7] Any communication between the SMF and UPF is managed by the packet forwarding PFCP (control protocol); it is one of the main protocols separating the user plane and the control plane.
• GTP[3] The GPRS tunneling protocol (GTP) is responsible for providing seamless interconnection and carrying traffic between roaming or home users and key network interfaces in 4G, NSA (5G non-standalone), SA (5G standalone), and mobile edge computing architectures. In 5G, GTP tunnels are also used for the N9 interface.

II. Call Flow (Session Establishment and UPF Initialization) During PDU session establishment, the SMF connects to the UPF via PFCP (N4 interface). This PFCP session carries an SDF template containing information such as PDR, QFI, URR, and FAR. The UPF will allocate a default QoS (non-GBR) flow during the initial session establishment.

III. Terminal (UE) Call Interface Usage Sequence

• [6] N4 processes session establishment request
• [6] PFCP processes PDR creation
• [6] [12] Check existing PDI of PDR
• [6] [12] Check TEID
• [6] [12] Check source interface
• [6] [12] Check previous SDF filter ID
• [6] [12] Set all filter flags: BID, FL, SPI, TTC, FD
• [6] PFCP processes FAR creation
• [6] Create URR
• [6] Create BAR
• [6] Create QRR
• [6] Set N3 TEID and QFI
• [4] UPF Initialization
• [4] PFCP Context Initialization
• [1] Initialize UPF Context
• [1] Set User Plane Functional Characteristics: FTUP, EMPU, MNOP, VTIME, UPF Attribute Length
• [6] [7] Session Establishment Response
• [5] N4 Build Session Establishment Response
• [5] Node ID
• [5] PFCP Request Accepted
• [5] F-SEID
• [5] PDR Existence Checked
• [5] PFCP Build Message
• FTUP: The UP function supports the allocation/release of F-TEID.
• EMPU: The UP function supports sending end-of-file packets.
• MNOP: The UP function supports measuring the number of packets in the URR, which is performed via the "Measure Number of Packets in URR" flag. MNOP (Packet Count Measurement): When set to "1", it indicates that in flow-based measurements, in addition to measuring in bytes, the uplink/downlink/total number of packets transmitted is also requested.
• VTIME:UP functionality supports the quota validity period feature. If UP functionality supports the VTIME feature, it requests UP functionality to send a usage report after the validity period expires. After the quota validity period expires, if data packets are received on the UPF, the UPF should stop forwarding data packets or only allow forwarding of limited user plane traffic, depending on the operator's policy in the UP functionality.

Abbreviations:

• FL: Flow Tag
• TTC: TOS (Traffic Category)
• SPI: Security Parameter Index
• FD: Flow Description
• BID: Bidirectional SDF Filter