Home Policy Control Function

Description

The Home Policy Control Function (H-PCF) is a critical component within the 5G Core (5GC) architecture, specifically defined within the Policy Control Framework. It operates as the policy decision point for subscribers roaming outside their home network. The H-PCF resides in the Home Public Land Mobile Network (HPLMN) and interfaces with the Visited PCF (V-PCF) located in the Visited PLMN (VPLMN) via the N24 interface. Its primary role is to provide home-routed policy control, ensuring that the home operator's policies, including Quality of Service (QoS), charging rules, and access control, are enforced even when the user equipment (UE) is attached to a foreign network.

Architecturally, the H-PCF is part of the Policy Control Function (PCF), which is the unified policy framework in 5G. For roaming scenarios, the PCF is logically separated into V-PCF and H-PCF. The V-PCF interacts directly with the Session Management Function (SMF) in the visited network to enforce session-level policies. When a policy decision requires input from the home network—such as for subscriber-specific service entitlements, spending limits, or home-defined QoS profiles—the V-PCF queries the H-PCF. The H-PCF then makes the authoritative policy decision based on subscriber data from the Unified Data Repository (UDR) and potentially other home network functions.

The H-PCF works by receiving policy requests from the V-PCF over the N24 interface, which is based on HTTP/2 and uses JSON-based service-based interfaces (SBIs). These requests contain information about the UE's session, such as requested QoS, data network name (DNN), and subscription identifiers. The H-PCF evaluates this against home network policies and subscriber profiles. It then returns policy decisions, which may include authorized QoS parameters, charging methods (online/offline), and service-specific rules. The V-PCF translates these decisions into instructions for the SMF and other functions. This separation allows the visited network to handle local policy enforcement while deferring to the home network for subscriber-specific and potentially sensitive policy logic.

Key components involved with the H-PCF include the N24 interface for V-PCF communication, the Npcf interface for internal PCF service-based interactions, and integration with the UDR via the Nudr interface for accessing subscription and policy data. Its role is pivotal in maintaining service consistency, enabling advanced charging models like sponsored data, and supporting regulatory requirements for roaming. By centralizing home policy decisions, the H-PCF simplifies the visited network's policy control complexity and ensures that home operator business rules are consistently applied globally.

Purpose & Motivation

The H-PCF was introduced in 3GPP Release 15 as part of the new 5G System (5GS) architecture to address the limitations of previous policy control frameworks in roaming scenarios, particularly those from the Evolved Packet Core (EPC). In 4G EPC, roaming policy control was handled by the Home PCRF (H-PCRF) interacting with a Visited PCRF (V-PCRF) via the S9 interface. While functional, this approach had complexities in service chaining, latency due to multiple signaling hops, and limited flexibility for new 5G services like network slicing and edge computing.

The creation of the H-PCF was motivated by the need for a more agile, cloud-native, and service-based policy framework in 5G. The 5GC architecture adopted a Service-Based Architecture (SBA) with HTTP/2 and JSON, replacing the Diameter-based protocols used in EPC. The H-PCF, as part of this new framework, is designed to support dynamic policy decisions required for 5G use cases, such as network slicing, where a roaming subscriber's slice access must be authorized by the home network. It also enables more efficient home-routed traffic models and supports integration with network exposure functions for third-party services.

Furthermore, the H-PCF solves the problem of consistent policy enforcement across administrative domains. It allows home operators to retain control over subscriber experiences and charging, which is crucial for service differentiation and revenue assurance. By decoupling the policy decision point (H-PCF) from the policy enforcement point (in the visited network), it facilitates smoother inter-operator agreements and reduces the visited network's burden in managing foreign subscriber policies. This design is essential for the global scalability of 5G services and the support of complex IoT and enterprise roaming scenarios.