User Identifier

Description

The User Identifier is a fundamental concept in 3GPP networks, defined as a piece of information used to identify one specific User Identity in one or more systems. It serves as the binding element between a user's identity and the various network functions that need to recognize, authenticate, authorize, and charge that user. Unlike a User Identity, which represents the user in a specific context, a User Identifier is the concrete data element—such as an IMSI, MSISDN, or SIP URI—that carries that identity through signaling and management procedures.

Architecturally, User Identifiers appear across multiple domains. In the Core Network, the IMSI uniquely identifies a subscriber in the HSS/UDM, while the MSISDN is used for routing and charging. In IMS, SIP URIs and Tel URIs serve as public user identities. In charging systems, the Charging Identifier (e.g., IMS Charging Identifier, ICID) correlates records for a single session. The identifier may be permanent (e.g., IMSI) or temporary (e.g., TMSI, GUTI), and may be used across one or more systems—for example, a single user identifier can link charging records from EPC and IMS.

The operation of User Identifiers involves mapping, resolution, and privacy mechanisms. When a UE attaches, the network assigns temporary identifiers to protect privacy. During session establishment, the identifier is used to retrieve subscription data, apply policies, and generate charging records. In interworking scenarios, identifiers are translated between systems (e.g., from IMSI to SIP URI) using databases like the HSS or ENUM/DNS. The identifier also plays a critical role in lawful interception, where it is used to target specific users.

Key components include the identifier itself (e.g., IMSI, MSISDN, SIP URI, GPSI), the databases that store and resolve them (HSS, UDM, EIR), and the protocols that transport them (Diameter, HTTP/2, SIP). The identifier's scope can be local (within one PLMN) or global (across PLMNs). In 5G, the Subscription Concealed Identifier (SUCI) and Subscription Permanent Identifier (SUPI) are examples of user identifiers designed for privacy.

User Identifiers are essential for charging, billing, policy control, mobility management, and security. They enable the network to distinguish users, apply personalized services, and ensure accurate accounting. Without a robust identifier framework, functions like roaming, handover, and service continuity would be impossible. As networks evolve toward network slicing and multi-access edge computing, the role of user identifiers expands to support slice-specific identities and cross-slice correlation.

Purpose & Motivation

The User Identifier exists to provide a stable, unambiguous reference to a user across the diverse systems and procedures within a 3GPP network. Before its formalization, early mobile networks used simple subscriber numbers, but as services grew—from voice to data, IMS, and charging—the need for a flexible identifier that could represent a user in multiple contexts became critical. The User Identifier solves the problem of correlating user activity across different domains, such as circuit-switched and packet-switched networks, and later across EPC, IMS, and 5G core.

Historically, the term was introduced in Rel-6 with a focus on charging, where it was defined as an identifier of a user used, for example, in charging functionality. This reflected the growing complexity of billing in multi-service networks. Over time, the concept was generalized to cover any piece of information that identifies a User Identity in one or more systems, acknowledging the proliferation of identifiers like IMSI, MSISDN, SIP URI, and temporary identities. This broader definition supports inter-system mobility, roaming, and service personalization.

The User Identifier also addresses privacy and security requirements. Temporary identifiers (e.g., TMSI, GUTI) were introduced to prevent tracking and eavesdropping. In 5G, the SUCI was designed to conceal the SUPI during initial registration. Thus, the User Identifier framework balances the need for identification with the need for privacy, adapting to regulatory and technological changes.