Traffic Plane Function

Description

The Traffic Plane Function (TPF) is a critical component within the 3GPP Policy and Charging Control (PCC) architecture, specifically defined in the context of the Flow Based Charging (FBC) system. It is the entity that resides in the user data path (the traffic plane) and is responsible for the real-time detection, processing, and action upon service data flows based on instructions from the Policy and Charging Rules Function (PCRF). In many implementations, the TPF is integrated into a Gateway node, such as the Packet Data Network Gateway (PGW) in EPC or the User Plane Function (UPF) in 5GC.

Architecturally, the TPF consists of several key sub-functions: the Service Data Flow Detection function, which identifies packets belonging to a specific flow using filters (e.g., based on 5-tuple); the Policy Enforcement function, which applies actions like gating (allowing/blocking), bandwidth control, and QoS marking; and the Charging function, which performs usage measurement and interacts with online and offline charging systems. The TPF communicates with the PCRF via the Gx reference point, receiving Policy and Charging Control (PCC) rules that define the treatment for each detected flow. These rules include parameters for QoS (QCI, ARP), charging keys, and applicable charging methods.

How it works involves a continuous cycle. When a user establishes a data session, the PCRF provisions initial PCC rules to the TPF. As user traffic passes through the gateway, the TPF inspects packets, matches them to the active service data flow templates, and applies the corresponding policy actions. It also meters the traffic volume or duration for charging. If an event triggers a change in policy (e.g., user runs out of quota), the PCRF can push updated rules to the TPF in real-time. The TPF's role is thus to be the enforcement point that translates high-level service policies into concrete packet-level actions, enabling differentiated services, fair resource usage, and sophisticated charging models like sponsored data or zero-rating.

Purpose & Motivation

The TPF was created to address the need for dynamic, flow-based policy control and charging in IP-based mobile networks. Earlier pre-IMS systems had static, subscription-based QoS and simple volume charging, which were insufficient for offering tiered services, application-specific policies, and real-time spending controls. The TPF, as part of the PCC framework, introduced a separation between the control plane (PCRF) that makes policy decisions and the user plane that enforces them.

The problems it solves include the inability to apply different policies to different types of traffic from the same user (e.g., prioritizing VoIP over web browsing) and the lack of integration between charging and policy enforcement. By deploying a TPF in the traffic path, operators gained the ability to create and monetize new service offerings, manage network congestion more intelligently, and provide subscribers with real-time notifications and controls over their data usage. Its introduction in Release 6 was motivated by the growth of data services and the requirement for an IMS-compliant charging and policy mechanism that could support complex service scenarios.

Key Features

• Real-time detection of service data flows using configurable packet filters
• Enforcement of gating (allow/deny), QoS marking, and bandwidth policing actions
• Integration of policy control with online and offline charging functions
• Communication with the PCRF over the Gx interface for dynamic rule provisioning
• Support for application-aware traffic management and sponsored data connectivity
• Often co-located with gateway functions (GGSN, PGW, UPF) in the user plane

Evolution Across Releases

Defining Specifications