Home Unified Data Repository

Description

The Home Unified Data Repository (H-UDR) is a core network function within the 5G System (5GS) and, by evolution, applicable to the 4G Evolved Packet Core (EPC). Introduced in 3GPP Release 15 as part of the broader Unified Data Repository (UDR) concept, it specifically refers to a UDR instance residing in the subscriber's Home Public Land Mobile Network (HPLMN). The UDR itself is a converged data storage function designed to hold subscription data, policy data, application data, and structured data for exposure in a single, unified repository, replacing the separate data stores of previous architectures like the Home Subscriber Server (HSS) and Policy and Charging Rules Function (PCRF) databases.

Architecturally, the H-UDR is a critical central node in the HPLMN's data management layer. It provides a standardized service-based interface (Nudr) to other Network Functions (NFs) that need to access, create, update, or delete data. Key consumers of the H-UDR include the Home Unified Data Management (H-UDR's managing counterpart, the UDM), the Home Policy Control Function (H-PCF), the Network Exposure Function (NEF), and the Network Slice Selection Function (NSSF). The H-UDR stores data in a structured, profile-oriented manner. For example, it holds the subscription data profile for a user (accessed by UDM), the policy data profile (accessed by PCF), and application-specific data (accessed by NEF or other Application Functions).

The H-UDR operates as a persistent storage backend. When the UDM needs to retrieve a user's subscription information to authenticate a registration request, it queries the H-UDR via the Nudr service-based interface. Similarly, the H-PCF retrieves policy rules associated with a user or a service from the H-UDR. A key feature is data independence; the H-UDR does not process the data but merely stores and retrieves it based on the requests from authorized consumer NFs. This separation allows for independent scalability of storage and logic functions. The data is organized into Data Sets (e.g., subscription data, policy data, application data), and access is controlled based on the identity of the requesting NF and the data subset it is authorized to read or modify.

Its role in the network is foundational for service delivery and policy enforcement. By centralizing 'home' data in the HPLMN, it ensures a single source of truth for subscriber profiles, service policies, and application context. This is especially crucial in roaming scenarios. When a user roams, the Visited Network (VPLMN) accesses policy and subscription information via the inter-PLMN interfaces (e.g., N8 between V-UDM and H-UDM, which in turn fetches data from H-UDR). The H-UDR enables consistent service experience across networks, supports network slicing by storing slice selection policies, and facilitates application exposure by safely storing application data that can be accessed by authorized third parties via the NEF.

Purpose & Motivation

The H-UDR was created as part of the 5G Core's move towards a cloud-native, service-based architecture (SBA). Prior to 5G, subscriber and policy data were stored in separate, monolithic network elements like the HSS (for authentication and subscription) and the SPR (Subscriber Profile Repository) or internal PCRF database (for policy). This siloed approach led to data duplication, consistency challenges, complex integration, and limited flexibility for introducing new services that required combined data sets. The UDR concept aimed to unify this storage.

The 'Home' designation of the H-UDR addresses the specific need for a centralized, authoritative data repository within the subscriber's home network. In roaming and multi-network scenarios, it is imperative that the master copy of a subscriber's profile, policies, and service data resides in and is controlled by the HPLMN. The H-UDR solves the problem of data sovereignty and consistency. It ensures that any network (visited or home) making decisions about a user's service derives those policies from the same source, preventing conflicts and enabling uniform service delivery globally. It also simplifies the data management architecture for the home operator by consolidating storage.

Furthermore, the H-UDR enables new 5G paradigms like network slicing and edge computing. Slice selection policies and service-specific parameters can be stored as part of a user's profile in the H-UDR. When a UE attaches, the UDM and NSSF can query this centralized store to determine the appropriate network slice instance. For edge computing, application context and preferences can be stored in the H-UDR and exposed to edge Application Functions via the NEF. Thus, the H-UDR is not just an evolution of legacy databases but a foundational enabler for flexible, service-aware 5G networks, addressing the limitations of fragmented data storage in previous generations.

Key Features

• Centralized storage for subscription, policy, and application data in the HPLMN
• Provides a service-based interface (Nudr) for access by UDM, PCF, NEF, and other NFs
• Organizes data into distinct Data Sets for access control and management
• Serves as the single source of truth for home network user profiles and policies
• Enables consistent policy enforcement and service experience during roaming
• Supports network slicing by storing slice selection and service profile data

Evolution Across Releases

Defining Specifications