What is IMSI?
IMSI (International Mobile Subscriber Identity) is a globally unique number stored on the SIM/USIM card that identifies a mobile subscriber within the 3GPP system. The IMSI is used during network registration and authentication to associate the device with the subscriber’s profile, subscription, and service entitlements. In 5G, the IMSI is replaced by the SUPI (Subscription Permanent Identifier), and privacy is enhanced by transmitting only an encrypted version called the SUCI (Subscription Concealed Identifier) over the air.
How Does IMSI Work?
The IMSI is a maximum 15-digit number consisting of the MCC (Mobile Country Code, 3 digits), MNC (Mobile Network Code, 2–3 digits), and MSIN (Mobile Subscriber Identification Number, up to 10 digits). In 4G LTE, the UE sends the IMSI during initial attach if no temporary identity (GUTI) is available — this transmission is unencrypted, creating a well-known privacy vulnerability exploited by IMSI catchers. 5G addresses this by introducing SUCI, where the MSIN portion is encrypted using the network’s public key before transmission. The UDM/AUSF in the 5G core decrypts the SUCI to recover the SUPI for authentication.
Use Cases
Subscriber identification during network registration, roaming and inter-operator billing, lawful intercept and regulatory compliance, subscriber management and provisioning, and mobile number portability.
3GPP / Standards Reference
3GPP TS 23.003 (Numbering, addressing and identification — IMSI/SUPI), 3GPP TS 33.501 (Security architecture for 5G — SUCI privacy)
Related Terms
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This glossary entry is part of the 5GWorldPro Complete 5G Glossary. To go deeper into 5G architecture and technology, explore our 5G Training courses.
