SIM card

What is a SIM card?

A SIM card, also known as a subscriber identity module, is a smart card that stores identification information that pinpoints a smartphone to a specific mobile network.

Data that SIM cards contain include user identity, location and phone number, network authorization data, personal security keys, contact lists and stored text messages. SIM cards allow a mobile user to use this data and the features that come with them.

Without a SIM card, some phones would not be able to make calls, connect to internet services such as 4G LTE and 5G or send SMS messages. SIM cards are removable and have anywhere from 32 KB to 128 KB.

Not all phones with SIM cards work the same, however. There are two distinct technologies used; GSM (Global System for Mobile communication) and CDMA (Code Division Multiple Access). GSM is the most widely adopted technology digital mobile network. Network carriers such as AT&T and T-Mobile use GSM. If a carrier uses GSM, users can remove their SIM card from a device and move it to another mobile device with all the same data and contacts on it. The network carrier will still be able to identify the user as well.

CDMA enabled phones do not need a SIM card; instead, the mobile device will use an electronic serial number (ESN). Users that have a phone with an ESN cannot switch between devices as easily as users would need permission from their network carrier. Network carriers such as Sprint and Verizon use CDMA.

Even though carriers such as Sprint and Verizon do not need SIM cards, SIM cards are can still be found in devices under those networks. This is because mobile devices need the use of SIM cards to use 4G LTE.

A device called a SIM card reader can be used to upload data from a SIM card to a computer or other device.

Types of SIM cards

SIM cards have come in a variety of different sizes over time. Types of SIM cards include:

  • Standard SIM cards measure 25x15mm and are used in older and basic phones.
  • Micro SIM cards measure 15x12mm and are more likely to be found in phones from the 2010s and later.
  • Nano SIM cards measure 12.3x8.8mm and are used in newer smartphones.
  • Embedded SIMs, or eSIMs, measure 6x5mm, and have the SIM card installed in the phone already. ESIMs are activated remotely by the network carrier.

Some phones now offer support of dual-SIM, meaning a user can have two different SIM cards activated in the same device. For example, the iPhone 10s supports dual-SIM; one SIM card is removable and the other is an eSIM. Dual-SIM cards are useful for people who want to have two phone numbers for one device. For example, one work and one private contact number.

Benefits of SIM cards

A user can easily switch a SIM card from one phone to another and this portability of data offers several benefits. For example, a user that buys a new phone can install the current SIM card to associate the new phone with the same number and user preferences as the old one.

In another common situation, if a phone's battery runs out of power, the user can easily install the card to another subscriber's phone to borrow it without running up that user's minutes. Some vendors offer prepaid SIM cards that can provide travelers with local numbers as long as their cell phones are not locked to a specific carrier.

SIM card security issues

An individual's SIM card can be a target for hackers because the SIM card has indirect access to a person's email, banking information or social media accounts. One option to recover a password is sent through text or SMS. If a hacker gains access to the information stored on a SIM card, they could transfer the data to another SIM card.

SIM cards do have a security code to prevent it from being used in a separate device so users can go into their phone's settings and change the PIN code for the SIM card to something more complicated. Other security features include authentication and encryption to protect data and prevent eavesdropping.

This was last updated in December 2022

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