How SIM works in GSM

The SIM card contains its internal memory, in which the data, personal and financial information as well as the identity for GSM / CDMA are stored. The SIM card stores network-specific information in order to authenticate the identity of the subscriber in the network.

SIMs were first launched in 1992 with the first digital “GSM” cell phones. The Nokia 1011 was the first GSM cell phone to hit the market.

SIM is used to provide storage on subscriber related information as following :

  • IMSI (International Mobile Subscriber Identity).
  • Temporary network data like TMSI, LAI, Location update status.
  • Subscriber Authentication Key (Ki) and Ciphering Key (Kc) which are used for security purposes.
  • BCCH information : List of carrier frequencies to be used for cell selection.
  • Forbidden PLMN.
  • Language preference.
  • PIN number (Personal Identification Number) and PIN error counter.
  • PUK number (Personal Unlock Key) and PUK error counter.

PIN management

The PIN number consist of 4 to 8 digit and it is loaded by the service activator an subscription time. Afterwards the PIN number can be changed as many times an user wishes including the length of the PIN number.

The user can disable the PIN function but again can be inhibited at subscription time by a authorized person. If an incorrect PIN is entered, an indication is given to the user. After 3 consecutive entries the SIM is blocked, even if if the SIM is removed or the mobile is switch off and on.

If the SIM card is blocked the user cannot access the network. The unblocking of the SIM card can only be done by keying in the PUK (Personal Unlock Key). PUK is 8 digit and is given to the user at subscription time.

If an incorrect PUK is entered more than 10 times then the PUK will not work anymore and the SIM card will continue to be blocked until taken to the mobile vendor service center.

Two physical types of SIM are specified :

  • ID – 1 SIM – Looks like a Credit card
  • Plug in SIM – Look like a small chip is installed semi permanent in the mobile equipment.

In order for a GSM phone to work, a SIM card must be inserted so that you can access the local mobile network to make or receive calls or send SMS.


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