Subscriber Identification Module (SIM) Card:
The Subscriber Identification Module (SIM) cards are the ID cards of the cellular world, and they have evolved in step with cellular networks.
- Subscriber Identification Module (SIM) card 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.
- 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), and, emergency numbers.