A SIM card (also known as a Subscriber Identity Module or Subscriber Identification Module) is an integrated circuit (IC) that stores a user’s international mobile subscriber identity (IMSI) and its keys. It helps to authenticate and verify a subscriber’s identity on mobile devices and computers.
Specifically, a SIM card will contain the following information: its unique serial number (ICCID), IMSI, information about the local network, authentication codes and ciphering information, and the list of services the subscriber has access to. It will also contain two main passcodes: a personal identification number (PIN) for normal use and a personal unblocking key (PUK) to unblock the PIN.
SIM cards are often used in GSM phones, LTE-ready handsets, satellite phones, computers, smartwatches, and cameras.
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SIM Card Formats
Full-size SIM (1991)
The first SIM to be introduced to the public was the full-size SIM. It had the size of a credit card (85.6mm x 53.98mm x 0.76mm). Later, smaller SIMs would be embedded on a full-size card from which they can be removed.
Mini-SIM (1996)
The mini-SIM (also called the regular SIM or the standard SIM) has the same contact arrangement as the full-size SIM and is usually supplied within a full-size card carrier with linking pieces that hold it into place. This SIM can be used on devices that require a full-SIM card and mini-SIM card-compatible devices.
Micro-SIM (2003)
A micro-SIM card has the same contact arrangements and thickness as the cards above but with reduced length and width. This SIM card format was first introduced in 2003 by the European Telecommunications Standards Institute (ETSI) and other telecommunications groups and organizations. Its primary purpose was to fit into devices too small for a mini-SIM card format.
Micro-SIM cards were introduced commercially by different service providers for the launch of the first iPad and various smartphones in 2010. The iPhone 4 was the first smartphone to support a micro-SIM card format.
Nano-SIM (2012)
The nano-SIM card was introduced in October 2012, when mobile service providers from different countries started to produce it for smartphones that supported this SIM format. It has the same contact arrangements as the previous SIM cards and has an isolating material put around the contact area to prevent short circuits. The first smartphone to support a nano-SIM card format was the iPhone 5 in 2012.
eSIM (2016)
An Embedded SIM (eSIM) is a programmable SIM directly embedded in a device. The surface mount format has the same electrical interface as a full-size, mini, and micro-SIM card. However, it’s soldered to a circuit board as a part of the manufacturing process.
eSIM cards can be provisioned remotely; they can add or remove operators without physically switching a SIM from the host device. The release of Apple’s Apple Watch Series 3 in 2017 featured eSIM support.