What is a SIM

A subscriber identity module (SIM) is a smart card 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. The card also contains storage space for text messages and a phone book. By using a SIM card, a subscriber can easily change the phone itself without losing his or her phone book and most importantly, without having to change her phone number.

Early versions consisted of the whole fullsize (85 x 54 mm) smart card. Soon the race for a 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 change carriers and use his current phone with a new provider's SIM card. However, this is difficult in the United States; almost all U.S. GSM providers SIM-lock phones that they sell—i.e., electronically lock their phones so that they can only be used with the provider's own SIM cards.

Some providers will unlock a customer's phone once he or she has fulfilled the service contract. Others, such as T-Mobile USA, have been known to unlock phones after a few months. Others, such as AT&T Wireless, will not unlock phones under any circumstances. AT&T Wireless not only locks its phones against its direct competitors, but even locks them against non-AT&T Wireless providers that have partnership agreements with the company.

The use and content of the card is protected by several levels of access codes. PIN is the every day access code for normal use of the phone. PIN2 is reqired to use 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 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. http://en.wikipedia.org/wiki/SIM

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