How SIM Card works?

SIM Card:

SIM is the popular Smart card used in Mobile phones to activate the connection and to make link with the server system. 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. The first SIM card was made in 1991 by Giescke and Devrient 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 . 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. 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.

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 range between 1KB and 8KB while the EEPROM size is between 16KB and 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 voltage of most of the modern SIM support 5V,3V and 1.8V.

Keys of SIM Card:

1. Integrated Circuit Card Identifier or ICCID – It is the Primary account number that has 19 digit 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 net work. Normally it has 109 digits. Its first 3 digits represent Mobile Country Code or MCC, the next 2 to 3 digits represents the Mobile Network Code or MNC, The next digits represents 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 data base 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 data base in the operator system then searches for incoming IMSI and the associated Authentication key. The operator data base 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 send 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 net work 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 a LAI.

5. SMS messages – SIM card can store many SMS

6. Contacts – SIM can store around 250 contacts.