A SIM card is the portable memory chip that serves as the “brain” of your cell phone. Its main function is to connect to your carrier’s wireless network, while also holding your phone number, address book, settings, and text messages.
The “SIM” in SIM card stands for “Subscriber Identity Module.” Essentially, it identifies which cell phone user is using a particular device and allows you to access your phone’s communication features, so that you can make calls, send SMS messages, and use mobile internet services like 4G.
| Did You Know? Globally, there are over 7 billion devices that use SIM cards to connect to cellular networks. |
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You may have wondered what the long code printed on your SIM card's microchip is all about. That code contains information about the carrier, country of origin, and a unique user ID. It’s also how a phone bill can be attributed to a particular device, based on usage. If a particular user hasn’t paid his or her phone bill, the carrier can stop service to the device.
SIM cards are tied to a particular service provider (carrier) and can only be used with a service plan from that carrier, either with a long-term contract or on a prepaid or month-to-month basis.
Prepaid SIM cards are a good option for people who don’t want to be tied to one carrier with a long-term contract. They are especially useful for frequent travelers who want to use their phone abroad without paying international roaming charges.
SIM Cards, Then and Now
GSM vs. CDMA: What's the Difference?
The history of the SIM card is linked to the development of the GSM (Global System for Mobile Communications) network, which is the global standard for telecommunications. It’s the main network used around the world, especially in Europe, Asia, and Africa. Your SIM card is what allows your phone to connect with a GSM network.
Phones on the GSM network (such as T-Mobile and AT&T) always contain SIM cards, which can be inserted directly into other GSM phones.
The United States also has the CDMA (Code-Division Multiple Access) network, which is mainly used by Verizon and Sprint.
Historically, phones that operated on the CDMA network saved the phone number and other identifying information to the handset itself. While arguably less convenient for users, CDMA handsets have often been heavily subsidized, giving users less incentive to switch phones frequently. As technology changes, however, cell phone providers have upgraded to Long Term Evolution (LTE), which is based on GSM technology. This means that some type of SIM card is likely to be required for devices that use this network.
An increasing number of smartphones support both GSM and CDMA, making it easier to switch networks while keeping the same device.
Why Do SIM Cards Come in So Many Different Sizes?
When SIM cards were first introduced in 1991, they were huge -- about the size of a credit card! Thankfully, they have been getting smaller and smaller as technology develops. Depending on the age of your device, you may have a Standard, Micro, or Nano SIM inside your phone.
| Full SIM: The original SIM of the 1990s was rectangular and roughly the size of a credit card. Size: 3.37 inches by 2.13 inches (85.6 mm by 54 mm) |
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| Standard (Mini) SIM: Significantly smaler than the Full SIM, Standard (Mini) SIM cards were used in older mobile phones, such as the iPhone 3G and flip phones. Size: 0.98 inches by 0.59 inches (25 mm by 15 mm) |
| Micro SIM : Micro SIMs are very similar to Standard (Mini) SIMs. The size of the contact is the same, but with less plastic casing around the edges. In fact, Standard (Mini) SIMs can be cut to turn them into Micro SIMs. The Micro SIM became the accepted size with the introduction of the iPhone 4 in 2010, and other phone models quickly followed suit. Size: 0.59 inches by 0.47 inches (15 mm by 12 mm) |
| Nano SIM : Released in 2012, the Nano SIM is the most common size of SIM card in smartphones since the iPhone 5. Size: 0.48 inches by 0.35 inches (12.3 mm by 8.9 mm) |
| Embedded SIM (eSIM) : The latest version of SIM card technology comes embedded in the device and is the tiniest of the lot. You’ll find it in the latest iPhone and Google Pixel models. Size: 0.24 inches by 0.20 inches (6 mm by 5 mm) |
These days, many SIM cards are pre-cut so that you can easily pop out the size you need for your device. There are also adaptors that can be used to make Nano SIMs suitable for older phones designed to hold Standard SIMs. It’s also possible to “trim” down Standard SIM cards to fit the Micro SiM slot on more up-to-date phones, although this can potentially destroy the SIM card.
However, if you get a new phone designed for a different size SIM, it’s easy to port your number to a new SIM card. All you need is a PAC code from your carrier.
If you find yourself temporarily without a SIM card, or one that isn’t compatible with your cell phone, you will still be able to use your phone to make emergency calls, and possibly use certain features such as the camera or apps over WiFi.
Upgrading Your Phone, Swapping Your SIM
One of the biggest advantages of SIM cards is how easily they can be removed from one cell phone and inserted into another, compatible handset.
This makes it very straightforward to activate a new phone by inserting your existing SIM card. Your phone number and personal information is stored on the SIM card, so there's no need to do anything else to transfer this information. However, it’s important to note that most apps are actually stored in the phone's memory or secure digital (SD) card, so they won’t automatically be transferred to the new handset by simply swapping the SIM card.
It used to be incredibly common (especially in the U.S.) for carriers to lock their handsets, so that they would only work with SIM cards provided by that carrier. Service providers would often give customers a discount on their handset in exchange for signing a long-term contract. Handsets can be unlocked with the right code, however, although the exact process varies by manufacturer and model.
It is now standard practice for phones to be sold unlocked, or for the carrier to unlock them at the end of a contract period.
SIM Cards and Security
SIM cards can be locked using a personal identification number (PIN), so that only someone who has the PIN can use the card. If the phone is stolen, it will be unusable without the PIN.
In addition, the SIM card has an authentication code and an encryption key that protect the phone's transmissions. Because of the way the encryption information is transmitted to the carrier, it's usually necessary to have physical access to the SIM card in order to copy it.
The Future of SIM Cards
The future of SIM card technology has already begun to make the tangible “card” obsolete, instead replacing it with a chip embedded on the device's circuit board that carries out the functions of the Nano SIM, and more. The eSIM is an example of this futuristic technology that allows users to switch carriers without having to get a new SIM card. The iPhone 11 and the Google Pixel 4 already use eSIM technology, in additional to having a Nano SIM.
Many of the latest smartphones also feature
Dual Simtechnology, which allows you to insert two SIM cards into the same devices (or have one eSIM and one Nano SIM), allowing the same device to serve two different phone numbers, either on the same network or on two different networks. Dual SIM handsets are particularly popular with people who use the same device for both personal and business use, and want to keep their calls and phone bills separate, or international travelers who have a domestic SIM and a foreign SIM.
