SIM re-directs here; for alternate uses see Sim (disambiguation)

A subscriber identity module (SIM) is a smartcard securely storing the key identifying a mobile subscriber. SIMs are most widely used in GSM systems, but a compatible module is also used for UMTS UEs (USIM) and IDEN phones. The card also contains storage space for text messages and a phone book.

From the technical point of view the SIM is a microcomputer, which performs all necessary operations based on informations stored inside it. It consists of a CPU, ROM, RAM, EEPROM and I/O circuits. Early versions consisted of the whole fullsize (85 x 54 mm) smart card. Soon the race for smaller telephones called for a smaller version of the card. The card was cropped down to 25 x 15 mm (as illustrated).

Since the SIM card slot is standardized (by the GSM11.11 standard), a subscriber can easily move their wireless account and phone number from one handset to another. This will also transfer their phone book and text messages. Similarly, in theory, a subscriber can change carriers by inserting a new carrier's SIM card into their existing handset. However, this is not always possible, because most carriers SIM-lock the phones that they sell, preventing rival carrier's cards being used.

The use and content of the card can be protected by use of PIN codes. One code, PIN1, can be defined to control normal use of the phone. Another code, PIN2, can be set, to allow the use of special functions (like limiting outbound telephone calls to a list of numbers). PUK1 and PUK2 is used to reset PIN1 and PIN2 respectively.

The SIM is also a database — it stores network state information such as its current location area identity (LAI). If the handset is turned off and back on again it will take data off the SIM and search for the LAI it was in. This saves time by avoiding having to search the whole list of frequencies that the telephone normally would.

Japan's PDC system also specifies a SIM, but this has never been implemented commercially. The specification of the interface between the Mobile Equipment and the SIM is given in the RCR STD-27 annex 4.

External links

• GSM Tutorial
• SMS Tutorial
• SMS Gateway