Subscriber Identity Module

Subscriber Identity Module

A Subscriber Identity Module (SIM) is part of a removable smart card ICC (Integrated Circuit Card), also known as SIM Cards, for mobile, telephony devices (such as computers) and mobile phones. SIM cards securely store the service-subscriber key (IMSI) used to identify a subscriber. The SIM card allows users to change phones by simply removing the SIM card from one mobile phone and inserting it into another mobile phone or broadband telephony device.

SIM cards are available in two standard sizes. The first is the size of a credit card (85.60 mm × 53.98 mm x 0.76 mm). The newer, more popular miniature-version has a width of 25 mm, a height of 15 mm, and a thickness of 0.76 mm. However most SIM cards are supplied as a full-sized card with the smaller card held in place by a few plastic links and can be easily broken off to be used in a phone that uses the smaller SIM.

The first SIM Card was made in 1991, with Munich smart card maker Giesecke & Devrient selling the first 300 SIM cards to Finnish wireless network operator Elisa Oyj (formerly Radiolinja).

The exact format of the SSN differs according to the mobile operator, however the following is constant:

  • The first 3 digits represents the Mobile Country Code (MCC).
  • The next 2 digits represent the Mobile Network Code (MNC, i.e. the mobile operator).
  • The next 12 digits is the number, from which some operators use the two left most to indicate the Home Location Register.
  • The 20th digit then is a checksum.

The numbering of the ICC-ID is based on International Standard ISO/IEC 7812. The maximum length of the visible card number is 20 characters; 19 digits are preferred, but telecommunication network operators who are already issuing Phase 1 SIM cards with an identification number length of 20 digits may retain this length. The number is composed of the following subparts:

Issuer Identification number (max. 7 digits)

  • Major Industry Identifier (MII), 2 digits, 89 for telecommunication purposes.
  • country code, 1-3 digits, as defined by ITU-T recommendation E.164.
  • issuer identifier, variable.

Individual account identification

  • individual account identification number.
  • parity check digit.

W-SIM is a SIM card which also integrates core cellular technology into the card itself.

A Virtual SIM is a mobile phone number provided by a wireless carrier which does not require a SIM Card to terminate phone calls on a user's mobile phone.

Usage in mobile phone standards

The use of SIM cards is mandatory in GSM devices. The equivalent of a SIM in UMTS is called the Universal Integrated Circuit Card (UICC), which runs a USIM application, whereas the Removable User Identity Module (RUIM) is more popular in CDMA-based devices. Many CDMA-based standards do not include any such card, and the service is bound to a unique identifier contained in the handset itself.

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.

The SIM card introduced a new and significant business opportunity of mobile telecoms operator/carrier business of the MVNO (Mobile Virtual Network Operator) which does not own or operate a cellular telecoms network, but which leases capacity from one of the network operators, and only provides a SIM card to its customers. MVNOs first appeared in Denmark, Hong Kong, Finland and the UK and today exist in over 50 countries including most of Europe, USA and Canada, and Australia and parts of Asia and account for approximately 10% of all mobile phone subscribers around the world.

On some networks the mobile phone is locked to its SIM card such as on the GSM networks in the USA. This tends to happen only in countries where mobile phones are heavily subsidised, but even then not all countries and not all operators; such as in the UK, typically, most phones with subsidies are not SIM-locked. In countries where the phones are not subsidised, such as Italy and Belgium, all phones are unlocked. Where the phone is not locked to its SIM card, the users can easily switch networks by simply replacing the SIM card of one network with that of another while using only one phone. This is typical for example among young users who may want to optimise their telecoms traffic by different tariffs to different friends on different networks. It is called the "SIM card switch"

Operating systems

SIM operating systems come in two main types: Native and Java Card. Native SIMs are based on proprietary, vendor specific software whereas the Java Card SIMs are based on standards, particularly Java Card which is a subset of the Java programming language specifically embedded devices. Java Card allows the SIM to contain programs that are hardware independent and interoperable.


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. (look to GSM 11.11)


Each SIM is Internationally identified by its ICC-ID (Integrated Circuit Card ID). ICCIDs are stored in the SIM cards and are also engraved or printed on the SIM card body during a process called personalization. The ICCID is defined by the ITU-T recommendation E.118. The number is up to 18 digits long and in addition is often associated with a single check digit calculated using the Luhn algorithm.


SIM cards are identified on their individual operator networks by holding a unique International Mobile Subscriber Identity. Mobile operators connect mobile phone calls and communicate with their market SIM cards using their IMSI.

Authentication key (Ki)

The Ki is a 128-bit value used in authenticating the SIMs on the mobile network. Each SIM holds a unique Ki assigned to it by the operator during the personalization process. The Ki is also stored on a database (known as Authentication Center or AuC) on the carrier’s network.

The SIM card is designed not to allow the Ki to be obtained using the smart-card interface. Instead, the SIM card provides a function, "RUN GSM ALGORITHM", that allows the phone to pass data to the SIM card to be signed with the Ki. This, by design, makes usage of the SIM card mandatory unless the Ki can be extracted from the SIM card, or the carrier is willing to reveal the Ki. In practice, the GSM "crypto" algorithm for computing SRES_2 (see step 4, below) from the Ki has certain vulnerabilities which can allow the extraction of the Ki from a SIM card and the making of a duplicate SIM card.

Authentication process

  1. When the Mobile Equipment starts up, it obtains the IMSI from the SIM card, and passes this to the mobile operator requesting access and authentication. The Mobile Equipment may have to pass a PIN to the SIM card before the SIM card will reveal this information.
  2. The operator network searches its database for the incoming IMSI and its associated Ki.
  3. The operator network then generates a Random Number (RAND) and signs it with the Ki associated with the IMSI (and stored on the SIM card), computing another number known as Signed Response (SRES_1).
  4. The operator network then sends the RAND to the Mobile Equipment, which passes it to the SIM card. The SIM card signs it with its Ki, producing SRES_2 which it gives to the Mobile Equipment along with encryption key Kc. The Mobile Equipment passes SRES_2 on to the operator network.
  5. The operator network then compares its computed SRES_1 with the computed SRES_2 that the Mobile Equipment returned. If the two numbers match the SIM is authenticated and the Mobile Equipment is granted access to the operator's network. Kc is used to encrypt all further communications between the Mobile Equipment and the network.

Location area identity

The SIM stores network state information, which is received from the Location Area Identity (LAI). Operator networks are divided into Location Areas, each having a unique LAI number. When the device changes locations, it stores the new LAI to the SIM and sends it back to the operator network with its new location. If the device is power cycled, it will take data off the SIM, and search for the previous LAI. This saves time by avoiding having to search the whole list of frequencies that the telephone normally would.

SMS messages and contacts

Most SIM cards will orthogonally store a number of SMS messages and phonebook contacts. The contacts stored are in simple 'Name and number' pairs - entries containing multiple phone numbers and additional phone numbers will usually not be stored on the SIM card. When a user tries to copy such entries to a SIM the handset's software will break them up into multiple entries, discarding any information that isn't a phone number. The number of contacts and messages stored depends on the SIM; early models would store as little as 5 messages and 20 contacts while modern SIM cards can usually store over 250 contacts.

SIM digits

A typical SIM (19 digits) example 89 91 10 1200 00 320451 0, provide several details as follows:

  • The first two digits (89 in the example) refers to the Telecom Id.
  • The next two digits (91 in the example) refers to the country code (91-India).
  • The next two digits (10 in the example) refers to the network code.
  • The next four digits (1200 in the example) refers to the month and year of manufacturing.
  • The next two digits (00 in the example) refers to the switch configuration code.
  • The next six digits (320451 in the example) refers to the SIM number.
  • The last digit which is separated from the rest is called the check digit.

Universal Subscriber Identity Module

A Universal Subscriber Identity Module is an application for UMTS mobile telephony running on a UICC smart card which is inserted in a 3G mobile phone. There is a common misconception to call the UICC card itself a USIM, but the USIM is merely a logical entity on the physical card.

It stores user subscriber information, authentication information and provides storage space for text messages and phone book contacts. The phone book on a UICC has been greatly enhanced.

For authentication purposes, the USIM stores a long-term preshared secret key K, which is shared with the Authentication Center (AuC) in the network. The USIM also verifies a sequence number that must be within a range using a window mechanism to avoid replay attacks, and is in charge of generating the session keys CK and IK to be used in the confidentiality and integrity algorithms of the KASUMI block cipher in UMTS.

Equivalents on 2G

  • The equivalent of USIM on GSM networks is SIM.
  • The equivalent of USIM on CDMA networks is RUIM.


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. The Subscriber Identity Module Expert Group was a committee of specialists assembled by the European Telecommunications Standards Institute (ETSI) to draw up the specifications (GSM 11.11) for interfacing between smart cards and mobile telephones. In 1994, the name SIMEG was changed to SMG9.


In July 2005, the Finnish government announced that a Citizen Certificate - a government-guaranteed 'electronic identity' included in a SIM card - would be made available to every individual resident in Finland before the end of 2005, allowing mobile phone users to access e-services on the move. The Citizen Certificate has been described as "basically an e-ID card that will be compatible with several hardware devices, such as mobile phones, PDAs, personal computers, digital TV sets, and public web kiosks". It is based on open standards and secured Public Key Infrastructure (PKI)


  • GSM 11.11 - Specification of the Subscriber Identity Module - Mobile Equipment (SIM - ME) interface.
  • GSM 11.14 - Specification of the SIM Application Toolkit for the Subscriber Identity Module - Mobile Equipment (SIM - ME) interface
  • Installing the SIM Card, Dell Latitude D830 Service Manual (documentation)
  • ITU-T E.118 - The International Telecommunication Charge Card. 2001 Feb. ITU-T.

See also

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