A Subscriber Identity Module, or Subscriber Identification Module (SIM) card is a Smartcard that gives GSM phones their user identity. They store data such as contacts and information about the owner’s available services.
Internally the card is an integrated circuit that securely stores the international mobile subscriber identity (IMSI) and the related key used to identify and authenticate subscribers on mobile telephony devices (such as mobile phones and computers). A SIM circuit is embedded into a removable plastic card. This plastic card can be transferred between different mobile devices.
A SIM card follows certain smart card standards. The first cards were made the same size as a credit card (85.60 mm × 53.98 mm × 0.76 mm). The development of physically smaller mobile devices prompted the development of smaller SIM cards where the quantity of card surrounding the integrated circuit is reduced. Now the most modern micro-SIM cards are a fraction of the size of those first produced and will probably get smaller still as wearable devices reduce space still further.
The SIM “card” is inserted into the mobile phone, and each IMSI uniquely identifies the mobile station, its home wireless network, and the home country of the home wireless network, while the former is the number used for routing calls to the subscriber.
A SIM has a unique IMSI that does not change, while the MSISDN can change in time, i.e., different MSISDNs can be associated with the SIM.
A SIM card contains its unique integrated circuit card identifier (ICCID), international mobile subscriber identity (IMSI), security authentication and ciphering information, temporary information related to the local network, a list of the services the user has access to and two passwords: a personal identification number (PIN) for ordinary use and a personal unblocking code (PUK) for PIN unlocking.
SIM cards can be fitted in devices other than phones. We are aware that some wind power generators are fitted with SIM cards. Diagnostic software running constantly can use the SIM card to text out a message to a Virtual Mobile Number on detecting a fault in the windmill.