SIM Full Form | What is the Full Form of SIM?

SIM is the abbreviated form of Subscriber Identity Module or Subscriber Identification Module.


The SIM is a portable chip with an integrated circuit that helps in safely storing the international mobile subscriber identity (IMSI) number and its related key. It is used to identify and authenticate subscribers of the mobile devices.

SIM cards are always used in GSM phones. There are certain types of phones such as CDMA, TDMA, and AMPS that do not use a SIM card as the required data is programmed directly into the phone.

The circuits in the physical smart card, SIM, form part of a universal integrated circuit card. A SIM card is made up of PVC with contacts and semiconductors embedded within it. The first UICC smart cards were similar to the size of bank cards; sizes were reduced gradually over the years.

The first SIM card was developed by Munich smart-card maker Giesecke & Devrient in 1991. The SIM got its first specification from European Telecommunication Standards Institute.

This article discusses the full form of SIM, the information a SIM card stores, and the working of SIM.

A SIM card stores the following data:

  • Unique serial number (ICCID),
  • International mobile subscriber identity (IMSI) number,
  • Security authentication and ciphering information,
  • Temporary information related to the local network,
  • Two passwords: a Personal Identification Number (PIN) for protecting data, and a Personal Unblocking Code (PUC) for PIN unlocking.
  • SIM cards store SMS messages and the user’s contacts.
  • Up to 250 name/number pairs and up to 50 SMS text messages can be stored in SIM cards.

Replaceable SIM cards have four standard sizes:

  • Full-size – 85.6mm × 53.98mm × 0.76 mm
  • Mini-SIM – 25mm x 15mm x 0.76mm
  • Micro-SIM -15mm x 12mm x 0.76mm
  • Nano-SIM -12.3mm × 8.8mm × 0.67mm

The eSIM or Embedded SIM, an irremovable one comes in one size:

  • 6mm x 5mm x <1mm

A phone without a SIM is like a body without a soul. Without a SIM card, mobile won’t be able to connect to any mobile network. Various SIM cards can hold a different amount of data ranging between 16KB and 64KB. A SIM card stores the following data:

  • Cell phone number
  • Rate plan
  • LAI (Location Area Identity)
  • Service features and preferences
  • Text messages
  • Address book (info for about 250 contacts)
  • PIN lock

Working of a SIM Card

  • The phone obtains and relays the IMSI from the SIM card to the network.
  • The network looks up the IMSI in its internal database for the IMSI and checks its known authentication key.
  • The network generates a random integer and signs it with the authentication key to create a new number. This number is the response it uses to check if the SIM card is legitimate.
  • The phone receives the random integer from the network and forwards it to the SIM which signs it with its authentication key to create a new number. This number is sent to the network.
  • If the generated number sent by the network and the SIM card match, then the SIM card is declared legitimate and access is given.

Security concerns related to SIM

Hackers can target a person’s SIM card as the SIM card has indirect access to a person’s email, banking information, and accounts on various social media platforms. Generally, to recover a password of any of these accounts, an OTP via text message or SMS is sent on the mobile phone of users. If a hacker gains access to the information stored on a SIM card, they can easily transfer the data to another SIM card. Although, SIM cards have a security code to prevent their usage on separate devices but still it can be bypassed by changing the SIM card’s PIN code. Other security features of a SIM card are authentication and encryption to safeguard the data stored in the SIM and prevent eavesdropping of calls or messages.


We hope that you have understood the full form and the meaning of a SIM card, the data that SIM card stores safely, the functioning of a SIM card, and the security concerns related to a SIM card. Still, if you have any doubts or queries, feel free to write to us in the comments section below.

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