The SIM Toolkit (STK) is a set of commands or applications that define how a SIM card interacts with the outside world. The toolkit, which is usually programmed into the SIM card, enables the card to:
- Drive mobile equipment interface
- Build up interaction between the network application and end user
- Access or control the access to the network
Some of the common SIM Toolkit applications include displaying the text as instructed by the SIM Toolkit service, asking for user input, playing tones and launching applications such as the browser.
The SIM Toolkit is a standard of the GSM system and consists of a set of commands that can be activated by user actions or network events. The proactive commands are standard for all mobile equipment and defined by ETSI and 3GPP specifications. The standards allow additional value-added services and functionality to be integrated onto the SIM card to provide customized user interface and menus on the mobile handset.
The commands enable the SIM card to initiate actions and allow users to have direct access to services provided by the mobile network operators and other service providers such as utility companies, banks and entertainment organizations.
The SIM Toolkit provides a means for the applications in the SIM card to operate and interact with a compatible mobile device. The SIM Toolkit operation is independent of the phone make, design and manufacturer. It works as a client-server, where the SIM card platform is equivalent to the server side while the phone is like the client side.
The STK is implemented in three layers:
- The RIL, which is low-level software provided by the vendor
- The SIM Toolkit service, which is a simple machine code that converts the raw messages from the RIL to application-level messages
- The user interface (UI)
The SIM Toolkit features are grouped into the following functional categories:
- Controlling the user interface
- Communication services
- Menu management
- Application control
- Accessory management
The SIM Toolkit UI applications are usually protected by either the SIM pin, phone lock pin or both. The applications are not visible whenever the phone is locked, when there are no SIM applications in the SIM or when the phone does not have the physical SIM card.
The SIM Toolkit is used for the Android and the Windows 10 mobile operating systems, while the equivalent, SIM Application Toolkit, is used on the iPhone.