The full form of SIM is Subscriber Identity Module. SIM card is small memory card containing the information about various network settings and information such as the mobile number and its Key for GSM and CDMA. It also holds text messages and contact details. It has to be inserted in the GSM/3G/LTE handset to work. Depending on the size of the chip there are different types.

Types of SIM Card

  1. Full-size SIM card
  2. Mini SIM cards
  3. Micro SIM card
  4. Nano SIM cards


            Subscriber Identity Module contains its unique serial number (ICCID), international mobile subscriber identity (IMSI) number, security authentication and ciphering information, temporary information related to the local network. It also has 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.

Subscriber Identity Module card


PIN and PUK:


PIN –Personal Identification Number 2 PINs exist (PIN 1 and PIN2) Limited attempts on PIN access.

 PUK –PIN Unblocking Code Resetting PUK, resets PIN and the attempt counter too many attempts on PUK blocks use permanently.

The SIM card performs the following valuable functions:


  1. Identification of a subscriber: The IMSI programmed on the Subscriber Identity Module card, is the identity of a customer. Each IMSI is mapped to a mobile number and provisioned on the HLR to allow a subscriber to be identified.
  2. Authentication of a subscriber: This is a process, where, using the authentication algorithm (COMP128V3 for 2/2.5 G GSM, CAVE for CDMA and MILENAGE for 3G) on the Subscriber Identity Module, a unique response is provided to each subscriber based on IMSI, Ki and RAND (provided by the network). By matching this response with values computed on the network, a right customer is logged on to the network, and he or she can now make use the services of the mobile service provider.
  3. Storage: To store phone numbers and SMS.
  4. Applications: The SIM Tool Kit or GSM 11.14 standard allows creating applications on the Subscriber Identity Module to provide necessary information on demand and other requests for m-commerce, chatting, cell broadcast, phone book backup, location-based services, etc.

Read it also: Mobile number portability guide

Read it also: Basic information about 4G technology

SIM Card and its functions

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