Description
Home Units (HU) is a conceptual and categorical term used within 3GPP technical specifications to denote entities, equipment, or subscription profiles that are associated with a user's primary or home network. It serves as a label to differentiate between the home administrative domain and other domains, such as visited or serving networks, especially in scenarios involving roaming. The term itself is often used in requirement clauses, architectural descriptions, and testing contexts rather than naming a specific protocol or network node.
In practice, an HU can encompass a range of concepts. It may refer to the Home Subscriber Server (HSS) or Home Location Register (HLR) that stores the master subscription data. It can also represent the home network's policy control functions, like the Home Policy Control Function (H-PCF) in early architectures, or the user's subscription profile held within the Unified Data Management (UDM) in 5G. The key principle is that the HU embodies the authoritative source of subscriber information and home network policies. When a user is roaming, the serving network entities in the Visited Network (VN) will interact with the HU in the Home Network (HN) to authenticate the user, retrieve subscription data, and apply home network policies.
The role of the HU is foundational for mobility, roaming, and consistent service delivery. It ensures that regardless of the user's physical location or the radio access network they are attached to, their service experience is governed by the rules and data from their home network provider. This separation of home and visited responsibilities is a cornerstone of global cellular interoperability. Architecturally, interactions with HUs are defined through standardized interfaces, such as the Diameter-based interfaces between a Visited MME and a Home HSS, or the service-based interfaces in 5G Core.
Purpose & Motivation
The concept of Home Units exists to formally separate the administrative and functional responsibilities between a subscriber's home network provider and any other network the subscriber might temporarily use. This separation is essential for enabling global roaming, where a user can obtain service from a network operated by a different provider. Without a clear definition of the 'home' entity, managing subscriber data, authentication, billing, and policy enforcement across network boundaries would be chaotic and non-interoperable.
Historically, as cellular networks evolved from single-operator deployments to complex international federations, a standardized model was needed. The HU concept addresses the limitation of monolithic network architectures by defining a clear 'home' anchor point. It solves problems related to subscriber identity management, service portability, and consistent policy application. The home network retains control over the master subscription record and key policies, while the visited network provides the radio access and local routing. This model motivates the creation of clear interfaces (like Cx/Dx, S6a, N8/N10) and protocols specifically designed for communication between serving network functions and the home units, ensuring security and reliability in cross-network operations.
Key Features
- Represents the authoritative home network domain for a subscriber
- Stores master subscription data and permanent subscriber identities
- Anchors home network policy and charging rules
- Essential for authentication and authorization in roaming scenarios
- Provides a consistent reference point for service delivery
- Distinguished from Visited Units (VU) or Serving Network entities
Evolution Across Releases
Introduced the term 'Home Units' within the vocabulary of 3GPP specifications, particularly in the context of IP Multimedia Subsystem (IMS) and all-IP network architectures. It established the foundational model for separating home and visited network responsibilities for subscription and policy control.
Defining Specifications
| Specification | Title |
|---|---|
| TS 21.905 | 3GPP TS 21.905 |