> For the complete documentation index, see [llms.txt](https://sim-cards.learn-telecom.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://sim-cards.learn-telecom.com/technology/sim-cards-internals.md).

# SIM Cards Internals

A SIM (Subscriber Identity Module) card is a small, removable smart card that is inserted into a mobile device, such as a smartphone, tablet, or modem, to securely store information related to the user's mobile network subscription. 

SIM cards are used to authenticate the user and the device to the mobile network, as well as store data such as contact information, SMS messages, and network-specific settings.