IMS Network-Independent Public User Identity

Description

The IMS Network-Independent Public User Identity (INIPUI) is a type of public user identity defined in 3GPP specifications, notably 22.228 and 22.894, for use within the IP Multimedia Subsystem (IMS). It serves as a unique identifier for users that is decoupled from specific access networks, such as 5G NR, LTE, or non-3GPP access like Wi-Fi. Unlike network-dependent identities tied to a particular technology, the INIPUI remains constant regardless of how the user connects, facilitating consistent service delivery across heterogeneous environments. Architecturally, it is stored in the IMS core, specifically in the Home Subscriber Server (HSS) or equivalent databases, and is used in IMS signaling protocols like SIP for session establishment and management. When a user initiates a multimedia session, the INIPUI is included in SIP messages to route requests correctly and apply service policies. Key components involved include the Proxy-Call Session Control Function (P-CSCF), which receives the identity, and the Serving-CSCF (S-CSCF), which processes it for authentication and service triggering. The INIPUI works in tandem with private user identities and other identifiers to enable features like single sign-on and roaming. Its network independence is crucial for convergence scenarios, allowing operators to offer unified services over multiple access types without identity fragmentation. This supports advanced IMS applications, such as Voice over LTE (VoLTE) or Rich Communication Services (RCS), by ensuring that users can be reliably identified and charged regardless of their access method.

Purpose & Motivation

INIPUI was introduced to solve the challenge of identifying users consistently in IMS across diverse access networks, which became increasingly important with the proliferation of multi-access connectivity. Before its definition, public user identities in IMS were often tied to specific network types, complicating service delivery when users switched between, say, cellular and Wi-Fi. This could lead to service disruptions, inconsistent charging, or failed session handovers. The INIPUI addresses these issues by providing a network-agnostic identity that remains stable, enabling seamless mobility and service continuity. Its creation in Release 11 was motivated by the growth of IMS-based services like VoLTE and the need for convergence between fixed and mobile networks. By decoupling identity from access, it supports operator strategies for unified communications, reduces complexity in IMS core functions, and enhances user experience by allowing a single identity for all multimedia interactions. This evolution reflects 3GPP's move toward access-agnostic service architectures, where identities and services are independent of the underlying transport.