How do SIM cards work? | Explained

In 2021, there were more than 14 billion cellular devices in the world even though there were only 7 billion people. When someone wishes to contact a subscriber in this city, the network uses the subscriber’s SIM card to find them and confirm their identity. When a subscriber dials a recipient’s number, the phone sends data via the network – signed by the key on the SIM card – to a telephone exchange. The July 1996 edition says it “defines the interface between the SIM and the Mobile Equipment for use during the network operation phase of GSM as well as those aspects of the internal organisation of the SIM which are related to the network operation phase, within the digital cellular telecommunications system.” This is everything from its physical features – including operating temperature and “contact pressure” – to authentication and data access characteristics. This changed with the advent of the Universal Mobile Telecommunications System with 3G networks, when ‘SIM’ became only the software; the hardware was called the Universal Integrated Circuit Card.