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| International Mobile Subscriber Identity | |
|---|---|
| Name | International Mobile Subscriber Identity |
| Abbreviation | IMSI |
| Introduced | 1980s |
| Used by | GSM, UMTS, LTE, 5G NR |
| Administered by | International Telecommunication Union (ITU), 3GPP |
International Mobile Subscriber Identity
The International Mobile Subscriber Identity is a unique numeric identifier assigned to mobile subscribers for use in GSM, UMTS, LTE and 5G NR networks. It enables roaming across networks operated by carriers such as Vodafone Group, AT&T, China Mobile, Deutsche Telekom and is referenced in standards developed by 3GPP, ETSI and coordinated with the International Telecommunication Union. The IMSI interacts with elements like the Home Subscriber Server, Visitor Location Register, and the Mobile Switching Center to support subscriber services and billing.
The IMSI serves as the primary identifier for a mobile subscription within public land mobile networks run by operators including T-Mobile US, Orange S.A., Telefonica, Telstra and Nippon Telegraph and Telephone. It is used alongside identifiers such as the MSISDN and the IMEI in systems deployed by vendors like Ericsson, Huawei, Nokia and Qualcomm. Administratively the IMSI is managed through national and international numbering plans coordinated by entities like the International Telecommunication Union and regional regulators such as the Federal Communications Commission, Ofcom, ARCEP, and ACMA.
The IMSI is composed of a country identifier and network/subscriber components defined in specifications by 3GPP and ETSI. It typically includes a Mobile Country Code assigned under the International Telecommunication Union and a Mobile Network Code allocated by national numbering authorities such as ANATEL, BNetzA, Ofcom or TRAI. The subscriber-specific portion is often termed the Mobile Subscriber Identification Number and is used by home systems like the Home Location Register and the Home Subscriber Server implemented by vendors such as Cisco Systems and Ericsson. Formatting conventions are described in technical releases from 3GPP TS 23.003 and related documents maintained by standards groups like 3GPP SA1.
National regulatory agencies including Federal Communications Commission, Ofcom, ARCEP, ANATEL and TRAI coordinate assignment of Mobile Network Codes within Mobile Country Codes allocated by the International Telecommunication Union. Mobile operators such as Verizon Communications, China Mobile, Vodafone, Telefónica and MTN Group assign subscriber ranges to commercial services, machine-to-machine platforms like those from Sierra Wireless and Telematics Wireless Solutions, and virtual network operators such as Virgin Mobile and Boost Mobile. Allocation practices are reflected in commercial agreements among carriers, roaming arrangements mediated by organizations like the GSMA and bilateral interconnect contracts used by infrastructure vendors like Nokia and Huawei.
IMSI is used in authentication procedures specified by 3GPP and implemented in authentication centers (AuC) and home subscriber servers (HSS) within architectures utilized by GSM, UMTS, LTE and 5G NR networks. Authentication algorithms such as A3, A8 and the MILENAGE family are standardized in documents produced by 3GPP and historically developed by contributors including GSM Association members and vendors like Ericsson and Nokia Siemens Networks. Operators such as AT&T, China Telecom and Deutsche Telekom rely on IMSI for roaming signaling between elements like the Visitor Location Register and the Mobile Switching Center during location updates, call setup and data session control.
Use of IMSI raises privacy concerns addressed by regulators like European Commission, Federal Communications Commission and privacy advocates associated with organizations such as Electronic Frontier Foundation and academic groups at institutions like MIT, Stanford University and ETH Zurich. Standards bodies including 3GPP and ETSI introduced protections such as temporary identifiers (e.g., TMSI) and encryption procedures influenced by cryptographers and research published by groups at University of Cambridge and Carnegie Mellon University. Security incidents affecting operators like T-Mobile, Vodafone and Telefónica have driven enhancements to subscriber confidentiality provisions and updates to protocol specifications in releases produced by 3GPP SA3.
Devices known as IMSI catchers (often called StingRay, Hailstorm or by vendor names) have been used by law enforcement agencies such as Federal Bureau of Investigation, Metropolitan Police Service, Bundespolizei and military organizations in contexts involving Operation Gladio-era surveillance debates and modern counterterrorism operations. Manufacturers and vendors implicated in reporting include Harris Corporation, Flir Systems and other suppliers in investigative coverage by outlets like The New York Times, The Guardian and Der Spiegel. Legal and policy scrutiny by courts in jurisdictions like United States Supreme Court cases, European Court of Human Rights decisions, and legislative hearings in bodies such as the United States Congress and the European Parliament has influenced disclosure, oversight and restrictions on use.
Regulation of IMSI allocation and use involves the International Telecommunication Union, regional standards groups like 3GPP and ETSI, industry organizations such as the GSMA, and national regulators including the Federal Communications Commission, Ofcom, ANATEL, ARCEP and TRAI. Technical specifications are published in 3GPP releases and ETSI documents and are implemented by equipment suppliers like Ericsson, Nokia, Huawei and ZTE. Oversight and compliance intersect with legal frameworks shaped by decisions from the European Court of Human Rights, rulings in the United States Supreme Court, and legislation enacted by parliaments and congresses including those of United Kingdom, Germany, United States and Australia.
Category:Mobile telephony identifiers