Description
The International Mobile User Identity (IMUI) was a user identification concept defined in the very early phases of 3GPP standardization, primarily within Release 4 and Release 5. As detailed in specifications like TS 21.111 and TS 21.133, the IMUI was intended to be a unique identifier assigned to a mobile subscriber. Its purpose was to unambiguously identify a user within the global mobile telecommunications system, separate from the device identity (IMEI) or the subscriber's telephone number (MSISDN).
Architecturally, the IMUI would have been a data element stored within the subscriber's profile in the Home Location Register (HLR) or similar home network database. During procedures like registration (attach), location updating, or call setup, the IMUI could be used by the network to retrieve the subscriber's service profile and authenticate their right to access network services. It was part of a broader set of identifiers that included the International Mobile Subscriber Identity (IMSI), Temporary Mobile Subscriber Identity (TMSI), and Mobile Station International ISDN Number (MSISDN). The exact technical format and coding rules for the IMUI were defined, but its practical deployment and usage were limited.
The IMUI's role in the network was fundamentally for subscriber management and mobility. It served as a key for database lookups in the core network. However, the IMSI, which is a more well-known and permanently stored identifier in the Subscriber Identity Module (SIM), effectively fulfilled the same primary role. The IMSI is used for critical processes like authentication, where the network challenges the subscriber using secrets associated with the IMSI. The overlap in function between the IMUI and IMSI, coupled with the entrenched global use of the IMSI in GSM and its successor networks, led to the IMUI not being widely adopted or implemented in commercial networks.
From an operational perspective, the IMUI represented an early attempt to standardize a user-centric identity. However, the ecosystem rapidly converged on the IMSI as the primary, permanent subscription identifier. The IMUI was eventually phased out of active use in later 3GPP releases, with its requirements and definitions becoming obsolete. Its mention in specifications now serves primarily as a historical reference point in the evolution of mobile subscriber identification.
Purpose & Motivation
The IMUI was created to provide a standardized, internationally unique identifier for a mobile user, distinct from the device or the dialable phone number. In the early days of GSM and the transition to 3G (UMTS), there was a need to clearly define all the entities involved in mobile communication: the user (subscriber), the device (mobile equipment), and the subscription. The IMUI aimed to address the 'user' part of this equation.
The problem it intended to solve was the potential ambiguity between different identifiers. The MSISDN (phone number) could change or be ported. The IMEI identified the device, which could be swapped or shared. A user-centric identity like the IMUI would theoretically remain constant for the subscriber, providing a stable key for billing, service provisioning, and mobility management across different devices and numbers. This was particularly relevant as concepts like personal mobility (using your subscription on any device) and number portability emerged.
However, the IMSI, which was already deeply embedded in the GSM architecture for authentication (via the Authentication Center - AuC) and subscriber data storage (in the HLR), naturally extended to fulfill this user identification role. The SIM card, which stores the IMSI, became synonymous with the user's subscription. This made the introduction of a separate, parallel user identity (IMUI) redundant and operationally complex. Consequently, the IMUI was never widely implemented, and the 3GPP system evolved using the IMSI as the primary permanent subscriber identifier, rendering the IMUI obsolete.
Key Features
- Intended to be a unique global identifier for a mobile subscriber
- Separate from device identifier (IMEI) and phone number (MSISDN)
- Stored in the home network database (e.g., HLR)
- Used for subscriber identification and service profile retrieval
- Defined in early 3GPP releases (Rel-4, Rel-5)
- Superseded in practice by the IMSI
Evolution Across Releases
Defining Specifications
| Specification | Title |
|---|---|
| TS 21.111 | 3GPP TS 21.111 |
| TS 21.133 | 3GPP TS 21.133 |