SIM Application Toolkit

SIM Application Toolkit operates through a client–server model where the SIM card acts as the server and the mobile equipment (ME) as the client. The SIM initiates proactive commands, such as DISPLAY TEXT, GET INK, or SET UP MENU, which the ME executes. This allows the SIM to control the user interface and network interactions without requiring the handset's OS to have prior knowledge of the application. The commands are defined in 3GPP TS 31.111¹ and are transported over the SIM–ME interface using the ISO 7816-4 protocol.

SAT is widely used for mobile network operator (MNO) services, including SIM-based mobile banking, prepaid balance queries, and over-the-air (OTA) provisioning of SIM data. It also enables emergency call setup and network selection features. In many developing regions, SAT-based services provide access to financial services for unbanked populations, as seen in M-Pesa's early implementations². Additionally, SAT is used in machine-to-machine (M2M) communications for remote SIM management.

Originally defined in GSM 11.14, SAT was later incorporated into 3GPP specifications as TS 31.111, covering both GSM and UMTS. With the advent of LTE and 5G, SAT has been adapted to work with IP-based networks, enabling new services like SIM-based authentication for IoT devices. The standard also includes the Card Application Toolkit (CAT) for USIM, which extends SAT capabilities for 3G and beyond¹.

SAT has been used for unconventional purposes, such as remote SIM lock/unlock and device tracking. It also supports the 'SIM Alliance' framework for secure element access. A notable edge case is the use of SAT in satellite phones, where it enables network selection across different satellite constellations. Additionally, SAT commands can be used to trigger browser sessions, which was a precursor to mobile web services. Security researchers have demonstrated potential attacks via malicious SAT commands, leading to hardening measures in modern SIMs.

Proactive command

A command initiated by the SIM card to the mobile equipment, such as displaying text or sending a message.

USIM

Universal Subscriber Identity Module, the 3G/4G evolution of the SIM card.

OTA

Over-the-air provisioning, a method of remotely updating SIM data.