Subscriber Identity Module
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 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 on the SIM. 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 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 easily move their wireless account from one phone to another without losing his or her phone book and, more importantly, without having to change her phone number.
Additionally, in theory, a subscriber can change carriers and use his current phone with the new carrier's SIM card. However, this is not always possible, at least without technical intervention. Almost all U.S. GSM providers SIM-lock the phones that they sell — i.e., electronically lock their phones so that they can only be used with that 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. Still 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 also locks them against non-AT&T Wireless providers that have partnership agreements with the company. This problem can also occur if a subscriber attempts to move the same service to a different phone that was sold by a different carrier than their own.
As a result of customer frustration, numerous unlocking tools and methods exist for many different brands and models of GSM phones, but they are not sanctioned by the wireless carriers or the phone manufacturers. Some carriers will unlock old phones upon request via an over-the-air method if possible.
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 (http://www.visualgsm.com/gsm_index.htm)
- SMS Tutorial (http://www.visualgsm.com/wire_sms_index.htm)
- SMS Gateway (http://www.visualgsm.com)
de:SIM-Karte ja:SIMカード