Policy control

Description

Policy control is a core network function defined in 3GPP specifications that enables the network to dynamically manage and enforce policies related to QoS, charging, and access control for subscriber sessions. It is the process whereby the Policy and Charging Rules Function (PCRF) in 4G or the Policy Control Function (PCF) in 5G indicates to the Policy and Charging Enforcement Function (PCEF) in 4G or the Session Management Function (SMF) in 5G how to control the IP-CAN bearer or QoS Flow, respectively. Policy control includes QoS control and/or gating control.

The architecture of policy control is based on the Policy and Charging Control (PCC) framework, which was introduced in 3GPP Release 7. The PCC framework consists of several functional entities: the PCRF (or PCF), which makes policy decisions; the PCEF (or SMF/UPF), which enforces these decisions; the Subscription Profile Repository (SPR) or Unified Data Repository (UDR), which stores subscriber policy information; and the Application Function (AF), which provides service-related information. The PCRF/PCF acts as the brain of the policy control system, receiving inputs from various sources such as the AF, the SPR/UDR, and network conditions, and then generating policy rules that are sent to the PCEF/SMF for enforcement.

In 4G networks, the PCRF communicates with the PCEF over the Gx interface (Diameter protocol). The PCEF is typically located in the P-GW (PDN Gateway) and is responsible for enforcing the policy rules on the IP-CAN bearer. The PCRF can also communicate with the AF over the Rx interface to receive application-level information, such as media type and required bandwidth, which can be used to make policy decisions. The policy rules include QoS parameters (e.g., QCI, ARP, MBR, GBR) and gating control instructions (e.g., allow/deny traffic to certain IP flows).

In 5G networks, the policy control architecture has evolved. The PCF replaces the PCRF, and the SMF replaces the PCEF for session management, while the UPF (User Plane Function) enforces the policies in the user plane. The PCF communicates with the SMF over the N7 interface (HTTP/2-based service-based interface). The PCF also interacts with the AF over the N5 interface. The policy rules in 5G are applied to QoS Flows within a PDU session, rather than to IP-CAN bearers. The PCF can provide QoS parameters such as 5QI, ARP, and Session-AMBR, as well as gating control and packet detection rules.

Policy control is essential for enabling differentiated services and ensuring that network resources are used efficiently. It allows operators to offer tiered services, apply fair usage policies, and dynamically adjust QoS based on network conditions or application requirements. For example, a video streaming service might require a higher QoS than web browsing, and policy control can ensure that the appropriate QoS is applied. Similarly, policy control can be used to block or allow certain types of traffic, such as parental controls or corporate security policies.

The evolution of policy control from Rel-7 to Rel-20 reflects the changing needs of mobile networks. In Rel-7, the focus was on basic QoS and gating control for IP-CAN bearers. Over time, the framework has been extended to support additional features such as application detection and control (ADC), sponsored data connectivity, and network slicing. In Rel-20, the definition was updated to reflect the 5G architecture, where the PCF indicates to the SMF how to control the QoS Flow, and the term "IP-CAN bearer" was replaced with "QoS Flow". This change aligns the definition with the 5G System (5GS) and its service-based architecture.

Purpose & Motivation

Policy control was introduced to address the need for dynamic, subscriber-aware, and application-aware management of network resources. Before its introduction, QoS and charging were largely static and based on subscription profiles. This limited the ability of operators to offer differentiated services and to adapt to changing network conditions or application requirements. The PCC framework, introduced in Rel-7, provided a standardized way to dynamically control QoS and charging based on real-time inputs from applications and network conditions.

The primary motivation for policy control was to enable operators to monetize their networks by offering tiered services and ensuring that premium services receive the appropriate QoS. For example, an operator could offer a "gold" tier with guaranteed bit rates and a "bronze" tier with best-effort service. Policy control allows the network to enforce these tiers dynamically, based on the subscriber's profile and the application being used.

Another key motivation was to support application-aware policies. With the rise of over-the-top (OTT) services, operators needed a way to detect and control application traffic. The ADC feature, introduced in later releases, allows the PCRF/PCF to detect specific applications and apply policies accordingly. This enables use cases such as zero-rating (sponsored data) and application-specific QoS.

Furthermore, policy control addresses the need for efficient resource utilization. By dynamically adjusting QoS based on network load and application requirements, operators can optimize the use of their spectrum and backhaul resources. For example, during periods of congestion, the network can reduce the QoS for non-essential traffic to ensure that critical services (e.g., emergency calls) are not affected.

In 5G, policy control has become even more important due to the diverse set of services and the need for network slicing. The PCF can apply different policies for different network slices, ensuring that each slice meets its specific SLA. The evolution of the definition in Rel-20 reflects this shift, emphasizing the control of QoS Flows in the 5G core.