USIM

USIM, or Universal Subscriber Identity Module, is the SIM application used in mobile networks to identify and authenticate a subscriber on 3G and later networks. It is implemented as an application on a universal integrated circuit card (UICC) and can reside on removable SIM cards as well as embedded SIMs (eSIM).

Function: The USIM stores subscriber identity information such as IMSI, authentication key Ki, and operator-specific profiles. It participates in the Authentication and Key Agreement (AKA) process to authenticate the subscriber and establish confidentiality and integrity of communications. The data and operations enabled by the USIM support network access, roaming, and provisioning of services.

Architecture and provisioning: In a UICC, the USIM is one of several applications; the card may also host other applications, for example ISIM for IP multimedia services. The USIM uses standardized file structures and security domains. Profiles are loaded or updated via over-the-air provisioning or through physical SIM replacement, allowing operators to manage subscriptions remotely.

Standards and evolution: The USIM application is defined by 3GPP specifications, notably TS 31.102. It builds on the GSM SIM concept but supports extended memory and capabilities for 3G/4G. With LTE and 5G, the same USIM framework remains in use, and devices may employ embedded or removable UICC to host USIM profiles and related services.

Relation to related terms: USIM is part of the UICC; ISIM is another application on the same card for IP multimedia identity; eSIM refers to an embedded UICC capable of storing USIM profiles.