User Identity

Description

User Identity is a conceptual representation of a user within a specific context, defined as information representing a user in a specific context. Unlike a User Identifier, which is the concrete data element, a User Identity is the abstract notion of who the user is in a given situation. A single user can have several User Identities—for example, one for professional use and another for private life—each serving different purposes and potentially associated with different identifiers.

In 3GPP networks, User Identity is realized through various identifiers. In the early releases, it was defined as the means to uniquely identify a User, such as an IMSI. This tied the identity directly to a subscription. However, as services diversified, the definition evolved to recognize that a user may have multiple identities across different contexts. For instance, in IMS, a user may have multiple Public User Identities (IMPUs) for different services or devices. In 5G, the concept of a user identity is further abstracted with the SUPI representing the permanent identity and various GPSIs for external use.

The architecture supporting User Identity includes databases like the HSS/UDM that store subscription-related identities, and application servers that manage service-specific identities. The identity is used during authentication, authorization, and accounting. For example, when a user registers, the network retrieves the user's identity and associated profiles to determine allowed services and QoS. In multi-access scenarios, the same user may be identified differently across access technologies, requiring mapping functions.

Key components include the identity itself (e.g., IMSI, IMPU, GPSI), the context in which it is used (e.g., IMS, MTC, private network), and the mechanisms for identity management (e.g., authentication, identity federation). The concept also encompasses privacy considerations: temporary identities are used to prevent tracking, and in 5G, the SUCI conceals the SUPI during transmission.

User Identity is crucial for personalization, charging, and security. It allows the network to tailor services to individual users, enforce policies, and ensure that only authorized users access resources. As networks move toward network slicing and edge computing, the ability to manage multiple user identities per user becomes even more important, enabling context-aware service delivery and seamless roaming across heterogeneous environments.

Purpose & Motivation

The concept of User Identity was introduced to address the need for a flexible representation of a user that goes beyond a single subscription identifier. In early mobile networks, a user was equated with a single IMSI, which sufficed for voice services. However, with the advent of data services, IMS, and multi-device usage, it became clear that a user could have multiple identities depending on context—professional, private, or service-specific. The User Identity concept solves the problem of mapping a single human user to multiple network identities, enabling personalized services and accurate charging across different contexts.

Historically, the term was first defined in Rel-6 as 'the means to uniquely identify a User (such as an IMSI).' This definition was narrow, focusing on uniqueness. As networks evolved, the definition was revised in Rel-16 to 'information representing a user in a specific context,' explicitly acknowledging that a user can have several identities. This change reflects the growing complexity of user-centric services, where a user might have different identities for different applications, devices, or roles.

The User Identity concept also addresses privacy and security. By allowing multiple identities, the network can use temporary or pseudonymous identities to protect user privacy. In 5G, the separation of SUPI and SUCI is a direct application of this principle. Furthermore, the concept supports identity federation and interworking with external networks, where a user's identity may be represented differently in different systems.