SIM card working

October 27, 2023

‘SIM’ stands for ‘subscriber identification module’.
Specifically, it is an integrated circuit, or a microchip, that identifies the subscriber on a given network.
In order for a mobile phone to connect to any cellular network that follows the Global System for Mobile Communications (GSM) standard, a SIM card is mandatory.
This relationship is established using a unique authentication key – a piece of data that a user needs to ‘unlock’ access to the network.
Every SIM card stores this data and it is designed such that the user can’t access it through their phone.
Instead, signals sent by the phone into the network are ‘signed’ by the key, and the network uses the signature to understand whether the phone’s connection is legitimate.
- It is possible to duplicate a SIM card by accessing its key and storing it in multiple cards.
SIM cards also store information about its own ID number (the integrated circuit card identifier), the IMSI, the subscriber’s location area identity (i.e. their current location), a list of preferred networks (to whom the subscriber can connect when roaming), emergency numbers, and – depending on the space available – the subscriber’s contacts and SMS messages.
SIM cards are designed according to the ISO/IEC 7816 international standard maintained by – as its name indicates – the International Organisation for Standardisation and the International Electrotechnical Commission.
- It applies to electronic identification cards, including smart cards.
- In this standard, the card itself consists of the integrated circuit, which is glued to a silicon substrate on the top side.
- On the other side of the substrate are metal contacts, which form the gold-coloured side of the SIM card.
- Wires connect the integrated circuit from its bottom side to the metal contacts on the top side, and the contacts interface with the phone’s data connectors.