The SIM is installed in every GSM phone and identifies the terminal. Without the SIM card, the terminal is not operational. The SIM cards used in GSM (Global System for Mobile Communications) phones are smart processor cards. These cards posses a processor and a small memory. The SIM card may be protected against unauthorized use by a password or personal identity number. Typically SIM cards contain 32 K bytes of memory. Applications are developed and stored in SIM cards using SAT (SIM Application Toolkit).
The SIM card is approximately the size of a small postage stamp. The SIM securely stores the service-subscriber key (IMSI - International Mobile Subscriber Identity) used to identify a subscriber on mobile telephony device. The SIM card allows users to change phones by simply removing the SIM card from one mobile phone and inserting it into another mobile phone.
SIM cards are available in three standard sizes. The first is the size of a credit card (85.60 mm × 53.98 mm x 0.76 mm). The newer, most popular miniature version has the same thickness but a length of 25 mm and a width of 15 mm, and has one of its corners truncated (chamfered) to prevent misinsertion. The newest incarnation known as the 3FF or micro-SIM has dimensions of 15 mm × 12 mm.
There are three operating voltages for SIM cards: 5 V, 3 V and 1.8 V. The operating voltage of the majority of SIM cards launched before 1998 was 5 V. SIM cards produced subsequently are compatible with 3 V and 5 V or with 1.8 V and 3 V.
Most SIM cards will orthogonally store a number of SMS messages and phone book contacts. The contacts are stored in simple 'Name and number' pairs. The SIM also stores specific data such as the SMSC (Short Message Service Center) number, Service Provider Name (SPN), Service Dialing Numbers (SDN), Advice-Of-Charge parameters and Value Added Service (VAS) applications.