SIM

SIM, short for Subscriber Identity Module, is a small smart card used in mobile devices to identify and authenticate a user to a cellular network. It stores the subscriber’s IMSI (International Mobile Subscriber Identity), authentication keys, and service-related data, enabling access to voice, SMS, data, and roaming services. The SIM also holds contacts and configuration information that the device uses to connect to networks.

In practice, several generations and formats of SIMs exist. The traditional SIM card is a physical removable card. For newer networks, the USIM (Universal Subscriber Identity Module) expands capabilities to support 3G, 4G, and 5G services, offering larger memory and stronger security. More recently, embedded SIMs (eSIM) are built into devices and can be remotely provisioned with carrier profiles, allowing users to switch carriers or plans without swapping physical cards. Remote SIM provisioning standards are overseen by industry bodies such as the GSMA.

Security is a core function of SIMs. The card performs mutual authentication with the mobile network using authentication keys and the AKA algorithm, helping prevent fraud. SIMs can be protected by a PIN and can be locked to a single device or carrier. Security considerations include PIN/PUK management, potential SIM cloning risks, and increasingly, SIM-based protections for devices beyond phones, such as tablets and IoT devices.

Historically introduced with GSM networks in the 1990s, SIM technology has evolved to support newer generations and the growing use of embedded profiles, while remaining a foundational element of mobile identity and service delivery.