How SIM Card works?

SIM Card:

SIM4SIM card technology is one of the most popular technologies which is used in Mobile phones to activate the connection and to communicate and for making links with the server system and also used in various electrical and electronic projects. It is the Subscriber Identity Module that contains the integrated circuit to store the International Mobile Subscriber Identity or IMSI and the keys to identify and authenticate the subscribers on the communication system. The SIM is embedded in a smart card that can be removed and transferred to different mobile phones. SIM card provides security system to users. The first SIM card was made in 1991 by Giesecke and Deviant of Sagem communications in France.


The data stored in the SIM card includes a unique serial number called ICCID, International Mobile Subscriber Identity or IMSI, Security Authentication information, temporary information about the network, a Personal Identification Number or PIN and a Personal Unblocking code or PUK for unlocking. SIM card contains its internal memory in which stores the data, personal and financial information, identity for GSM/CDMA. Modern SIM cards allow the storage of application data that communicate with the handset or server using the SIM application tool kit. The SIM card stores network-specific information to authenticate the identity of the subscriber in the network. Out of the many keys, the most important keys are ICCID, IMSI, Authentication key or Ki, Local Area Identification or LAI, and an operator-specific emergency number. Micro sim has been invented for the latest mobile phones. The SIM also contains other data like Short Message Service Centre number or SMSC, Service Provider Name or SPN, Service Dialing Number or SDN, Value Added Service or VAS, etc. The SIM comes in various data capacities ranging from 32KB to 128K and can store 250 contacts.

Keys of SIM Card:

1. Integrated Circuit Card Identifier or ICCID – It is the Primary account number that has 19 digits long. The number has sections like Issuer Identification Number or IIN, Individual Account Identification, Check digit, etc.

2. International Mobile Subscriber Identity or IMSI – It is used to identify the individual operator’s network. Normally it has 109 digits. Its first 3 digits represent Mobile Country Code or MCC, the next 2 to 3 digits represent the Mobile Network Code or MNC, The next digits represent the Mobile Subscriber Identification Number or MSIN.


3. Authentication Key or Ki – It is a 128 bit used to authentication of the SIM card on the Mobile Network. Each SIM has a unique Authentication key assigned by the operator during personalization. The Authentication Key is also stored in the database of the carrier’s network. When the mobile phone first activates using the SIM card, it gets the International Mobile Subscriber Identity or IMSI from the SIM card and transfers it to the mobile operator for authentication. The database in the operating system then searches for incoming IMSI and the associated Authentication key. The operator database then generates a Random Number or RAND and signs it with the IMSI and gives another number called Signed Response 1(SRES_ 1). The RAND will be sent to the mobile phone and the SIM then signs it with the Authentication Key and produces the SRES_ 2 which then passes into the operator network. The operator network then compares the SRES_1 it produced and the SRES_2 from the mobile phone. If both match, the SIM is authenticated.

4. Location Area Identity or LAI– This the information stored in the SIM about the local network available. The operator network is divided into different small areas each having an LAI.


5. SMS messages – SIM card can store many SMS

6. Contacts – SIM can store around 250 contacts.

Functions of SIM card:

 The SIM card performs the following functions:

 1) It identifies the subscriber: The IMSI programmed on the SIM card, is the identity of a subscriber. Each IMSI is mapped to a mobile number and provisioned on the HLR to allow a subscriber to be identified.

2) Authenticate the subscriber: This is a process, where, using the authentication algorithm on the SIM card, a unique response is provided by each subscriber based on IMSI (stored on SIM) and RAND (provided by network). By matching this response with values computed on the network a legal subscriber is logged on to the network and he or she can now make use of the services of the mobile service provider. SIM card is becoming a feature of mobile work.

3) Storage: To store phone numbers and SMS.

4) Applications: The SIM Tool Kit or GSM 11.14 standard allows creating

Applications on the SIM to provide basic information on demand and other

Applications for m-commerce, chatting, cell broadcast, phonebook backup,

Location-based services etc.

Microprocessor-based SIM cards:

The most important part of the SIM card is its Microcontroller. It is a paper sized chip which is a typical ROM with a size between 64 KB to 512 KB. The RAM size ranges between 1KB to 8KB while the EEPROM size is in between 16KB to 512 KB. The ROM contains the OS or operating system for the card, while the EEPROM contains data called personalization that includes security keys, phone book, SMS settings, etc. The operating voltage of SIM maybe, 1.8V, 3V or 5V but the operating voltages of most of the modern SIM support 5V, 3V, and 1.8V.

There are two types of microprocessor cards.  These cards take the form of either contact cards, which require a card reader, or contactless cards, which use radio frequency signals to operate.


Types of SIM Card:

There are two types of SIM cards that are GSM and CDMA:


GSM technology stands for Global System for Mobiles and its foundation can be credited to Bell Laboratories in 1970.  It uses a circuit-switched system and divides each 200 kHz signal into 8  25 kHz time slots and operates in 900 MHz, 800 MHz, and 1.8GHz bands. It uses a narrow band transmission technique- basically Time Division Access Multiplexing. The data transfer rates vary from 64kbps to 120kbps.


CDMA means code division multiple access which explains about communication channel principle that employs spread-spectrum technology and a special coding scheme which are time-division multiplexing scheme and frequency division multiplexing scheme.

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