Glossary definition of Subscriber Identity Module

Also known as: SIM

SIM (Subscriber Identity Module) is a smart card chip for use in GSM equipment, that makes a Mobile Station when inserted into a handset. A mobile phone cannot make or receive any calls except emergency calls without a SIM.

A SIM comprises a microcontroller, ROM for programs, and EEPROM for information. Information stored on the SIM includes the IMSI, plus various network specific user and security data. The memory on a SIM consists of a master file (MF), which has two dedicated files (DF) as its children. These contain elementary files (EF) which contain actual GSM data-each EF contains one record, which could be information such as a phone book or the IMSI. Record sizes are measured in words, with one word containing 8 bits (1 byte). These records also contain information set by the operator that controls what features are enabled, such as SMS, ISDN, and fixed dialling.

The first edition of the GSM SIM is the Phase 1 SIM. This only contained the rudimentary information for communicating over a GSM network, plus the IMSI and a cipher key for encryption. As networks grew, so did the demand for features; the Phase 2 SIM was introduced, which incorporated customisation features and more space for items such as recently called numbers. This personalisation also created a need for further security in the form of a PHONECODE, which controls access to changing the personalisation of a handset. In the event of a SIM change, the handset will require the PHONECODE of the last SIM before personalisation functions can be accessed.

The Phase 2 SIM is a very established standard, and therefore not feasible to change. Instead, new features can be added to the Phase 2+ specification. The Phase 2+ SIM enables the locking of a handset to a particular network by specifying group identifiers, voice call group services for unlimited multiparty calls, voice broadcast calls, the setting of different priorities for various incoming and outgoing calls, custom emergency call numbers, locking of a handset to multiple networks, shortcut dialling to services such as directory enquiries or pizza, barred numbers, and some other more technical options. The biggest change however in Phase 2+ is the ability of the SIM to take action, allowing it to control to handset rather than the other way around, by the use of ten proactive SIM features.

Since its introduction, the SIM has grown much further than is humble beginnings as a chunk of memory with an interface to the outside world.The full capabilities of a SIM can be exploited using a SIM application toolkit.