What is a SIM Card and How Does It Work?
A SIM card, also known as a subscriber identity module, is a smart card that stores the information necessary for a smartphone to connect to a mobile network. This tiny device contains data including user identity, location, phone number, network authorization, personal security keys, contact lists and stored text messages.
SIM cards enable a device to be identified and authenticated on the network, allowing users to make calls, connect to internet services such as 4G Long-Term Evolution (LTE) and 5G, or send Short Message Service (SMS) messages. Without a SIM card, some phones could not access these essential features.
SIM cards are removable and can store from 32 to 128 kilobytes of data. Two distinct technologies formed the foundation for 2G and 3G networks, but have been phased out in recent years in favor of newer technologies such as 4G LTE and 5G: Global System for Mobile Communication (GSM) and Code Division Multiple Access (CDMA).
A Brief History of SIM Cards
GSM, which was used by network carriers such as AT&T and T-Mobile, enabled users to remove their SIM cards from their devices and move them to other mobile devices, preserving their data and contacts. CDMA-enabled phones did not need a SIM card because they used an electronic serial number (ESN).
However, users who had a phone with an ESN could not switch between devices. Network carriers such as Sprint and Verizon used CDMA for their 2G and 3G networks, but phased out these technologies in 2022 in favor of 4G LTE and 5G.
How Does a SIM Card Work?
Every SIM card has its own unique ID number -- an IMSI, or International Mobile Subscriber Identity, which associates the device with its user. SIM cards also include an ICCID, or Integrated Circuit Card Identifier, which serves as a kind of serial number.
Information about a user's network plan, such as the plan type, data limits and voice minutes limit, is stored on the SIM card. The IMSI is also associated with the user's account to facilitate charges, fees, and other billing and usage.
When a person makes a call or sends a text, the device sends an access request to the network; using the ICCID, the SIM card verifies that the device is authorized to make the call or send the text on that network. The network will deny an unauthorized SIM card request.
Security Features of SIM Cards
SIM cards also store a user's personal data, including contacts and text message history. This is one of the primary ways of migrating that information from one device to another.
However, SIM cards have their downsides, including being a target for hackers because it is a key to their digital identity. A person's phone number is often linked to their email, banking and social media accounts. If a hacker gains access to a user's personal information, they could transfer the user's phone number to another SIM card using a SIM swapping attack.
The Future of SIM Cards
Although several industry observers have suggested that SIM cards are an outdated technology, there is not yet a clear successor to replace them. The dual SIM paradigm noted above -- two phone numbers for one device -- is likely to grow in popularity.
It is also likely that eSIMs will become more prevalent than physical SIMs. Smartphones are being used to address global health concerns such as sleep apnea and hearing loss. Learn how groundbreaking technology is creating smartphone-based diagnostic tools.