UPF
User Plane Function - handles user plane traffic in 5G networks.
User Plane Function
The User Plane Function (UPF) is a key network function in the 5G Core network responsible for handling user plane traffic. It serves as the gateway between the radio access network and external data networks, providing packet routing, forwarding, and QoS enforcement.
UPF Overview
The UPF is the anchor point for intra- and inter-RAT mobility and the interconnection point to external data networks. It is controlled by the SMF via the N4 interface and handles all user data traffic.
The UPF combines functionality from multiple 4G network elements, including the SGW-U (for mobility anchoring) and PGW-U (for external network connectivity).
Key UPF Functions
The UPF performs the following key functions:
- Packet Routing and Forwarding: Routes and forwards packets between RAN and external networks
- Packet Inspection: Inspects packet contents for various purposes
- QoS Enforcement: Enforces QoS policies for different traffic flows
- Traffic Usage Reporting: Reports traffic usage for charging and monitoring
- Lawful Intercept: Supports lawful interception of user traffic
- Traffic Steering: Steers traffic to appropriate destinations
- Packet Marking: Marks packets for QoS and other purposes
- Mobility Anchoring: Serves as anchor point for UE mobility
UPF Interfaces
The UPF connects to other network elements through various interfaces:
| Interface | Connected To | Purpose |
|---|---|---|
| N3 | NG-RAN | User plane traffic between RAN and UPF |
| N4 | SMF | Control plane signaling for UPF management |
| N6 | Data Network | User plane traffic between UPF and external networks |
| N9 | UPF | User plane traffic between UPFs |
Packet Processing in UPF
The UPF processes packets based on rules configured by the SMF:
- Packet Detection Rules (PDR): Define how to detect and classify packets
- Forwarding Action Rules (FAR): Define how to forward packets
- QoS Enforcement Rules (QER): Define QoS enforcement actions
- Usage Reporting Rules (URR): Define usage measurement and reporting
- Buffering Action Rules (BAR): Define buffering actions for downlink data
UPF Deployment Models
The UPF can be deployed in various configurations:
- Centralized UPF: Located in central data centers
- Distributed UPF: Located closer to the edge of the network
- Edge UPF: Located at the very edge of the network for low latency
- Chained UPFs: Multiple UPFs in a chain for specific services
- Branched UPFs: Traffic split across multiple UPFs
UPF and Edge Computing
The UPF plays a key role in edge computing scenarios:
- Local Traffic Breakout: Directs traffic to local applications
- Application Function Interaction: Interacts with local applications
- Local Hosting: Hosts applications directly on or near the UPF
- Traffic Steering: Steers traffic to appropriate edge computing resources
QoS Support in UPF
The UPF supports 5G QoS through several mechanisms:
- QoS Flow Handling: Processes traffic according to QoS flow requirements
- QFI Marking: Marks packets with QoS Flow Identifiers
- Rate Limiting: Enforces maximum bit rates
- Traffic Prioritization: Prioritizes traffic based on QoS requirements
- Reflective QoS Support: Enables UE to apply QoS on uplink based on downlink
Network Slicing Support
The UPF supports network slicing through:
- Slice-specific UPF Instances: Dedicated UPF instances for specific slices
- Slice-aware Packet Processing: Different processing rules for different slices
- Slice Isolation: Ensuring traffic isolation between slices
- Slice-specific QoS: QoS enforcement based on slice requirements
Related Specifications
The UPF is specified in the following 3GPP documents:
- TS 23.501: System Architecture for the 5G System
- TS 23.502: Procedures for the 5G System
- TS 29.244: Interface between the Control Plane and the User Plane nodes
- TS 29.281: General Packet Radio System (GPRS) Tunnelling Protocol User Plane (GTPv1-U)
- TS 38.415: PDU Session User Plane Protocol