5G Call – UPF User Plane Establishment and Data Transmission

The User Plane Function (UPF) is one of the most important network functions (NFs) in the 5G core network. It is the second network functional unit that the Radio Network (RAN) interacts with during PDU flows in 5G (NR). As a key element in the evolution of Control Plane and User Plane Separation (CUPS), the UPF is responsible for inspecting, routing, and forwarding packets within QoS flows in subscription policies. It uses the SMF to send SDF templates via the N4 interface to enforce uplink (UL) and downlink (DL) traffic rules. When the corresponding service ends, the UPF allocates or terminates QoS flows in the PDU session.

I. User Plane Establishment When initially accessing the 5G system, the terminal (UE) needs to establish a user plane channel with the data center according to the control plane guidance for service data transmission. During this process:

• When the terminal (UE) wants to access the 5G network, it first undergoes a registration process. After completing all control plane procedures, the SMF processes all session-related information during the user plane establishment phase. The AMF requests the downlink DL TEID (Terminal Equipment Identifier) ​​of all PDU sessions passed to the SMF. The SMF then selects the best UPF for the UE within the specified range and sends a session establishment request containing all parameters for the default PDU session establishment.
• Afterwards, a session default QoS flow (non-GBR) is created to exchange with the data network (DN) for traffic. The service traffic includes a longer route for calculating latency and maintaining traffic.

Figure 1. 5G Terminal User Plane Establishment Process (Messages)

• [5] New UE establishment request, requires creating session context
• [1] Set UPF address
• [5] [10] Request to create session with UPF
• [3] Session context response
• [4] [5] Get default session update
• [3] Default QoS, AMBR
• [3] Add default downlink and uplink PDR rules for IMSI

II. First Uplink/Downlink Data Transmission When actual data transmission (i.e., uplink or downlink data) occurs, the AMF sends another SM context request to the SMF, in which:

• The SMF sends a session modification request containing information related to the requested session type.
• The UPF establishes a PDU session within the rules and regulations according to user requirements. The UPF then adds QoS flow mapping, sets the TEID, inserts various rules (such as PDR, FAR, URR, etc.), and some session-related policies to the PDU session. It also bills each packet exchange and adds a unique session ID to distinguish it from other PDU sessions. The UPF also adds an IMSI number to identify the UE to which the current session belongs.
• The session context is prepared by the UPF and sent to the AMF via the SMF, which then forwards it to the gNB. It contains information such as the UPF's local TEID, QoS context, and session release message.

Figure 2.5G Terminal User Plane First Data Transmission Flow (Message)

• [2] QoS Policy Management (Policy Type)
• [2] Dynamic Rule Setting
• [2] Static and Dynamic Rule Update
• [3] Mapping FDR, PDR, QDR, BAR, URR
• [3] Attaching Rules to Session
• [3] Creating a New TEID and Inserting it into the PDR
• [2] Setting the TEID to be Passed to UPF
• [2] QoS/Bearer Management
• [5] Creating a Session Request
• [9] Updating and Creating a Session
• [6] Handling Rule Scheduling
• [7] Receiving Charging Authorization
• [2] Initializing Charging Credits
• [2] Obtaining All Active Policies
• [10] Setting Up UPF Session
• [4] Reading, Creating, Updating, and Searching for Sessions
• [8] Reading and Writing Sessions, and Serializing and Deserializing All Session Vectors
• [5] Inactive State When PDU Session Moves to Idle State
• [6] Handling Session Update Response
• [5] Process setup messages from AMF (initial request or existing PDU session)
• [3] Update state change notifications sent to AMF
• [3] Prepare responses (session context) to send to AMF for forwarding to gNB
• [3] Send the UPF local TEID to AMF for use by gNB
• [3] Send the appropriate QoS context to AMF
• [5] Obtain the PDU session ID from the RAT context
• [5] Request AMF to send a message to release the session