Public User Identity

Description

The Public User Identity (PUI) is a fundamental concept within the 3GPP IP Multimedia Subsystem (IMS) architecture, serving as the public-facing address of a user. It is the identifier used by other users or applications to initiate communication sessions, such as Voice over LTE (VoLTE) calls, video calls, or instant messaging sessions. A PUI is distinct from private identifiers and takes the form of either a SIP Uniform Resource Identifier (SIP URI, e.g., sip:[email protected]) or a TEL URI (e.g., tel:+1234567890), which maps to a traditional telephone number.

Architecturally, PUIs are stored in the IMS subscriber's profile within the Home Subscriber Server (HSS). A single IMS subscriber (represented by a private user identity) can be associated with multiple PUIs, allowing for one person to have several contact addresses (e.g., a business number and a personal number). During IMS registration, the UE registers its associated PUIs with the network via the Serving-Call Session Control Function (S-CSCF). The S-CSCF downloads the subscriber's service profile, including the list of PUIs, from the HSS to authorize and provide services for those identities.

In operation, when a session is initiated towards a user, the originating IMS network routes the request based on the target PUI. The interrogating-CSCF (I-CSCF) queries the HSS to determine which S-CSCF is serving that PUI, enabling the request to be forwarded correctly. PUIs are essential for service invocation, as IMS application servers (AS) trigger services based on the called or calling PUI. They are also used for identity presentation (Caller ID) and are a key element in IMS charging mechanisms. The management of PUIs, including their registration, deregistration, and barring, is a core part of IMS session control.

Purpose & Motivation

The PUI was created to solve the problem of addressing users in an all-IP, multimedia-centric network like IMS, which decouples services from the underlying access technology. Prior to IMS, telephony used primarily the Mobile Subscriber ISDN Number (MSISDN) as the sole address. This was insufficient for the rich multimedia and multi-identity scenarios envisioned for IMS, where a user might have email-like addresses (SIP URIs) alongside telephone numbers.

Its creation was motivated by the need for a flexible, Internet-standard compliant addressing scheme that could work globally across operator boundaries. The SIP URI format, aligned with IETF standards, allows for seamless interworking with non-3GPP IP-based communication services. The PUI provides the necessary abstraction layer, separating the user's public contact identity from their network-level private identity and their physical device. This solves the problem of service portability and enables advanced features like simultaneous ringing of multiple devices under one public identity, identity sharing (e.g., a department number), and the convergence of telephony and web-based communication paradigms.