Description
The National User Identifier (NUI), also referred to as the USIM Identifier, is a subscriber identity parameter defined in 3GPP specifications. It is stored within the Universal Subscriber Identity Module (USIM) application on a user's SIM card. The NUI serves as a nationally significant identifier, meaning it is unique and meaningful within the context of a single country's regulatory framework. Its primary technical role is to provide an unambiguous reference for a subscriber that is decoupled from the internationally roaming-capable IMSI (International Mobile Subscriber Identity).
Architecturally, the NUI resides in the USIM's file system, typically in the EF(NUI) elementary file. It is provisioned onto the USIM by the mobile network operator, often in compliance with national regulations. When required for purposes such as lawful interception, the network can request the NUI from the UE (specifically, from the USIM) during procedures like location update or at the initiation of a mobile-originated call. The network entity performing the interception (like the Lawful Interception Gateway) uses this identifier to correlate intercepted communications with a specific national subscriber record.
How it works is tied to specific network-triggered commands. The network can send a 'REQ NUI' command to the mobile equipment. This command is passed to the USIM application, which retrieves the NUI value from its secure memory and returns it to the network via the mobile equipment. The process is defined within the USIM application toolkit and related protocols. The NUI is not used in routine mobility or session management procedures; its use is specialized and triggered only for regulatory compliance functions.
Its role in the network is highly specific to security and regulatory domains. It acts as a key that links network activity (calls, messages, data sessions) to a user identity as defined by national authorities. This is crucial for investigations and surveillance operations conducted under legal mandates. The separation from the IMSI is intentional; the IMSI is used for international roaming and core network routing, while the NUI is kept for domestic regulatory purposes, providing a layer of administrative separation.
Purpose & Motivation
The NUI was created primarily to address national regulatory requirements for subscriber identification, specifically for Lawful Interception (LI) and national security purposes. Different countries have legal frameworks that mandate telecommunications operators to provide government authorities with the capability to identify and monitor communications of specific subscribers. Before the NUI, authorities might have relied on the IMSI or MSISDN (phone number), but these could be changed or have international implications.
The historical context involves the evolution of GSM and 3G networks into critical national infrastructure. Regulators sought a standardized, tamper-resistant identifier that was permanently associated with a subscription and under national control. The IMSI, while unique, is allocated by the operator and follows an international numbering plan (MCC, MNC), making it less suitable as a pure national administrative handle. The NUI solves this by being an operator-assigned identifier that is defined and used solely within a national context, as specified in national annexes of 3GPP specs.
It solves the problem of providing a reliable, network-retrievable identifier for a subscriber that is independent of the commercial identifiers used for routing and billing. This allows lawful interception systems to function based on a stable ID that is less likely to be changed or obscured. The motivation was to create a harmonized, 3GPP-standardized method for meeting these regulatory obligations, ensuring interoperability between different vendors' interception solutions and USIM cards across various operators within a country.
Key Features
- Stored securely on the USIM application
- Uniquely identifies a subscriber within a national jurisdiction
- Retrieved by the network via specific USIM command (REQ NUI)
- Used primarily for Lawful Interception and national security
- Independent of international identifiers like IMSI or MSISDN
- Defined in 3GPP TS 22.975 (Lawful Interception requirements) and TS 21.905 (vocabulary)
Evolution Across Releases
Introduced as the National User Identifier (NUI) in the context of Lawful Interception requirements for 3G networks. The initial architecture defined it as an identifier stored on the USIM, retrievable by the network, to uniquely identify a user for interception purposes within a country, separate from the IMSI.
Defining Specifications
| Specification | Title |
|---|---|
| TS 21.905 | 3GPP TS 21.905 |
| TS 22.975 | 3GPP TS 22.975 |