Dynamic Policy and Charging Control Rule

Description

A Dynamic PCC Rule is a key concept in the 3GPP Policy and Charging Control (PCC) architecture, introduced in Release 8. It refers to a PCC rule whose definition is provided to the Policy and Charging Enforcement Function (PCEF) by the Policy and Charging Rules Function (PCRF) over the Gx reference point. Unlike predefined PCC rules, which are statically configured in the PCEF, dynamic PCC rules are generated by the PCRF in real-time based on session information, subscriber profile, and network policies. This allows the network to apply fine-grained policy and charging control to individual service data flows.

The PCC architecture consists of several functional entities: the PCRF, which makes policy decisions; the PCEF, which enforces those decisions; the Application Function (AF), which provides service-related information; the Subscription Profile Repository (SPR), which stores subscriber data; and the Online Charging System (OCS) and Offline Charging System (OFCS) for charging. The PCRF acts as the brain of the PCC, receiving inputs from the AF, SPR, and other sources, and then formulating PCC rules that are sent to the PCEF. The PCEF, typically located in the Gateway (e.g., P-GW in EPC), enforces these rules by installing them into the bearer binding and event reporting function.

Dynamic PCC rules are used when the policy decision cannot be predetermined, such as when a new service is requested, when the subscriber's usage exceeds a threshold, or when the network conditions change. For example, when a user initiates a VoIP call, the AF (e.g., IMS) signals the PCRF with the service information. The PCRF then creates a dynamic PCC rule that specifies the QoS parameters (e.g., QCI, bit rates), charging instructions, and other policy controls. This rule is sent to the PCEF, which enforces it by setting up the appropriate bearer with the requested QoS and applying the charging rules. Dynamic rules can be modified or removed during the lifetime of a session, allowing the network to adapt to changing conditions.

The Gx reference point is the interface between the PCRF and the PCEF. It uses the Diameter protocol and is specified in 3GPP TS 29.212 (for EPC) and TS 29.512 (for 5G). The PCRF sends PCC rules to the PCEF using Credit-Control-Request (CCR) and Credit-Control-Answer (CCA) messages. The PCEF acknowledges the installation of the rules and reports events back to the PCRF. In 5G, the architecture evolved: the PCRF is replaced by the Policy Control Function (PCF), the PCEF is replaced by the Session Management Function (SMF) and User Plane Function (UPF), and the Gx reference point is replaced by the N7 interface (between PCF and SMF). The definition of a dynamic PCC rule was updated in Release 20 to reflect this: 'a PCC rule, for which the definition is provided to the SMF by the PCF.'

Dynamic PCC rules are essential for enabling advanced services such as sponsored data, zero-rating, QoS differentiation, and policy-based charging. They allow operators to control how different applications and services are treated in the network, ensuring that critical services receive the necessary QoS and that charging is applied correctly. The dynamic nature of these rules means that the network can respond to real-time events, such as the start of a video stream or the exhaustion of a data quota, and adjust the policy accordingly. This flexibility is a cornerstone of modern mobile networks, enabling the delivery of differentiated services and new business models.

Purpose & Motivation

Dynamic PCC Rules were introduced to provide real-time, flexible policy and charging control in mobile networks. Before the PCC architecture, policy control was largely static and limited to basic QoS parameters. The PCRF and dynamic PCC rules were created to enable operators to apply sophisticated policies based on subscriber profiles, service types, and network conditions. This allows for differentiated services, such as giving higher priority to VoLTE traffic or applying zero-rating to specific applications.

The need for dynamic rules arises from the fact that many policy decisions cannot be made in advance. For example, when a subscriber starts a video call, the network needs to allocate appropriate QoS resources and apply the correct charging. A predefined rule might not cover all possible scenarios, so the PCRF dynamically generates a rule tailored to that specific session. This ensures that the network can adapt to the actual service being used and the current network state.

Dynamic PCC rules also enable new business models, such as sponsored data, where a third party pays for the data usage of a specific application. The PCRF can receive information from an Application Function (e.g., a sponsor's server) and create a dynamic rule that exempts that traffic from the subscriber's data cap. This level of control is not possible with static rules. The evolution to 5G continues this trend, with the PCF and SMF playing similar roles, and the N7 interface providing the same dynamic rule provisioning capabilities. Overall, dynamic PCC rules are a fundamental enabler of the flexible, service-aware networks that operators deploy today.