A subscriber identity module, commonly known as a SIM card, is a removable smart card used in mobile phones. Its main purpose is to store essential information that identifies the mobile device and enables voice encryption to enhance call privacy, making it extremely difficult to eavesdrop (unless the wireless carrier itself engages in eavesdropping activities).
By associating the customer ID and personal number with the SIM card rather than a specific mobile phone, seamless interchangeability of the same SIM card among different GSM mobile phones becomes possible.
In addition to identification data, SIM cards also serve as a storage medium for SMS messages and the user’s contact information. A modern SIM card can store approximately 250 name/number pairs and up to 50 SMS text messages.
However, SIM cards have certain limitations. They are unable to store multiple numbers per contact or handle more complex information. Therefore, when transferring contacts from the phone’s memory to the SIM card’s memory, the contacts are divided into separate entries for each phone number, while discarding other associated details.
However, there exist specific phone types such as CDMA, TDMA, and AMPS that do not utilize a SIM card. Instead, the essential data is directly programmed into these phones.
Replaceable SIM cards are available in 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 comes in one size:|
|eSIM||(6mm x 5mm x <1mm) Non Removable|
Over the years, the size of SIM cards has significantly reduced. The initial version, known as the full-size or 1FF (1st Form Factor), resembled a credit card in dimensions (85.60 mm × 53.98 mm × 0.76 mm). It was succeeded by the mini-SIM or 2FF (2nd Form Factor), which had the same thickness but measured 25 mm in length and 15 mm in width, with one corner cut to prevent misinsertion. Following that, the micro-SIM or 3FF (3rd Form Factor) was introduced, featuring dimensions of 15 mm × 12 mm.
In 2012, the nano-SIM or 4FF (4th Form Factor) was introduced, measuring 12.3 × 8.8 × 0.67 mm. Nano-SIM cards can be made compatible with devices having Micro-SIM and Mini-SIM slots using adapters. Likewise, a Micro-SIM card can be inserted into a Mini-SIM slot with an adapter.
In 2016, the eSIM (Embedded SIM), also known as embedded Universal Integrated Circuit Card (eUICC), was introduced. It is soldered onto a device’s motherboard during manufacturing and provides the same functionality as a removable SIM.
A SIM card is associated with two numeric passwords. The first one is the Personal Identification Number (PIN), which the user must input each time they start the device. This PIN can be disabled through the phone settings if desired.
When entering the PIN number, the user has only three attempts. If all three attempts are incorrect, the card becomes locked, and a PUK (Personal Unblocking Key) must be entered to restore functionality. The user is allowed ten attempts to enter the PUK before the card is permanently locked and rendered unusable.