A subscriber identity module or subscriber identification module (SIM) is an integrated circuit that securely stores the International Mobile Subscriber Identity (IMSI) and the related key used to identify and authenticate subscribers on mobile telephony devices (such as mobile phones and computers).
A SIM is embedded into a removable SIM card, which can be transferred between different mobile devices. SIM cards were first made the same size as a credit card (85.60 mm x 53.98 mm x 0.76 mm). The development of physically-smaller mobile devices prompted the development of a smaller SIM card, the mini-SIM card. Mini-SIM cards have the same thickness as full-size cards, but their length and width are reduced to 25 mm x 15 mm.
A SIM card contains a unique serial number (ICCID), international mobile subscriber identity (IMSI), security authentication and ciphering information, temporary information related to the local network, a list of the services the user has access to and two passwords: a personal identification number (PIN) for ordinary use and a personal unblocking code (PUK) for PIN unlocking.
The SIM was initially specified by ETSI in the specification number TS 11.11, which describes the physical and logical behaviour of the SIM. With the development of UMTS, the specification work was partially transferred to 3GPP. 3GPP is now responsible for the further development of applications like SIM (TS 51.011) and USIM (TS 31.102) and ETSI for the further development of the physical card UICC.
The first SIM card was made in 1991 by Munich smart-card maker Giesecke & Devrient, who sold the first 300 SIM cards to the Finnish wireless network operator Radiolinja.
SIM Chip Structure and Packaging
There are three operating voltages for SIM cards: 5 V, 3 V and 1.8 V (ISO/IEC 7816-3 classes A, B and C, respectively). 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. Modern cards support 5 V, 3 V and 1.8 V.
SIM - Subscriber Identity Module
The microcontrollers used for SIM cards come in different configurations. The typical ROM size is between 64 KB and 512 KB, typical RAM size is between 1 KB and 8 KB, and typical EEPROM size is between 16 KB and 512 KB. The ROM contains the operating system of the card and might contain applets where the EEPROM contains the so called personalisation, which consists of security keys, phone book, SMS settings, etc., and operating system patches.
Modern SIM cards allow that applications can be loaded when the SIM is in use by the subscriber. These applications communicate with the handset or a server using SIM application toolkit, which was initially specified by ETSI in TS 11.14. SIM toolkit applications were initially written in native code using proprietary APIs. In order to allow interoperability of the applications Java Card was taken as the solution of choice by ETSI.
SIM cards store network-specific information used to authenticate and identify subscribers on the network. The most important of these are the ICCID, IMSI, Authentication Key (Ki), Local Area Identity (LAI) and Operator-Specific Emergency Number. The SIM also stores other carrier-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. (Refer to GSM 11.11)
SIM cards can come in at least two capacity types: 32 KB and 64 KB. Both allow a maximum of 250 contacts to be stored on the SIM, but while the 32 KB has room for 33 mobile network codes (MNCs) or "network identifiers", the 64 KB version has room for 80 MNCs. This is used by network operators to store information on preferred networks, mostly used when the SIM is not in its 'home' country but is roaming. The network operator that issued the SIM card can use this to have a SIM card connect to a preferred network in order to make use of the best price and/or quality network instead of having to pay the network operator that the SIM card 'saw' first. This does not mean that a SIM card can only connect to a maximum of 33 or 80 networks, but this means that the SIM card issuer can only specify up to that amount of preferred networks, if a SIM is outside these preferred networks it will use the first or best available network.
SIM cards are available in four standard sizes; full-size, mini-SIM, micro-SIM and embedded SIM. The first to appear was the full-size and is the size of a credit card (85.60 mm x 53.98 mm x 0.76 mm). A newer, more popular, version has the same thickness but has a length of 25 mm and a width of 15 mm, and has one of its corners truncated (chamfered) to prevent misinsertion. It is known as a mini-SIM or 2FF (2nd Form Factor). The newest incarnation, known as the micro-SIM or 3FF (3rd Form Factor), has dimensions of 15 mm x 12 mm.
The mini-SIM card has the same contact arrangement as the full-size SIM card and is normally supplied within a full-size card carrier, attached by a number of linking pieces. This arrangement (defined in ISO/IEC 7810 as ID-1/000) allows for such a card to be used in a device requiring a full-size card, or to be used in a device requiring a mini-SIM card after cleanly breaking the scorings manufactured in the outline of a mini-SIM card.
For use in even smaller devices, the 3FF card or micro-SIM cards have the same thickness and contact arrangements, but the length and width are further reduced as above.
SIM cards for M2M applications are available in a surface mount SON-8 package which may be soldered directly onto a circuit board.
The logical functions of a SIM card are independent from its format.
Usage in Mobile (Cell) Phone Standards
The use of SIM cards is mandatory in GSM devices.
The satellite phone networks Iridium, Thuraya and Inmarsat's BGAN also use SIM cards. Sometimes these SIM cards work in regular GSM phones and also allow GSM customers to roam in satellite networks by using their own SIM card in a satellite phone.
Japan's current and next generation cellular systems are based on W-CDMA (UMTS) and CDMA2000 and all use SIM cards.
CDMA-based devices originally did not use a removable card, and the service for these phones bound to a unique identifier contained in the handset itself. This is most prevalent in operators in the Americas. The first publication of the TIA-820 standard (also known as 3GPP2 C.S0023) in 2000 defined the Removable User Identity Module (R-UIM). Card-based CDMA devices are most prevalent in Asia.
The equivalent of a SIM in UMTS is called the Universal Integrated Circuit Card (UICC), which runs a USIM application. The UICC is still colloquially called a SIM card.
You can find out more about the Subscriber Identity Module and its history here: http://en.wikipedia.org/wiki/Subscriber_identity_module.