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What Is A SIM Card?  

You’ll find them in many smartphones, tablets and smartwatches. But what are SIM cards and how do they help you stay connected? 
A lady enjoying her phone with a SIM card.

A SIM (Subscriber Identity Module) card is kind of like a key to a cellular network—storing vital information like your unique identification number, contacts, and other data. So, what are they exactly, what are the different types, and why do some phones not have them? We’ll take a closer look. 

What is a SIM card? 

In the early days of cell phones, SIM cards could store about 20 contacts and five SMS messages—We've come a long way since then. First developed in 1991 by German Smart Card Maker Giesecke+Devrient for a Finnish Network provider1, the SIM card was based on guidelines provided by the European Telecommunications Standards Institute. It was a way to connect a device to a mobile network and store its phone number.  

These days, SIM cards do so much more than that. These small, transferable cards are inserted into mobile devices and are still used to both identify and authenticate the device to a cellular network, just like the original SIM cards. However, they also store a wealth of data, like contacts, text messages, and a lot of other information that makes swapping devices a cinch. Because of SIM card technology, you are able to keep your phone number from device to device, or even carrier to carrier, while easily transferring hundreds of text messages and other data simply by inserting your current SIM into your new device. 

How does a SIM card work? 

Each SIM card is assigned a unique identification number that stores information about your network plan, such as type of plan, available data, voice minutes, and text messages. Carriers use this information to verify your account status and to apply charges and fees.  

Whenever you send a text message or make a call, the device sends a signal to the network to request access. The SIM card is used to verify that your device is allowed to call or text within a network. If the SIM card is unauthorized, then the request is denied.  

In addition to network data, personal information is stored on a SIM card. Things like your phone number, contacts, and text message history are stored on a SIM, which makes it easy to transfer information from one device to another.  

Evolution of SIM card sizes 

Today, SIM cards are tiny, but that wasn’t always the case. Here are the different types of SIM cards and the devices you’re most likely to find them in.  

  • Full-size SIM: The original SIM card was the size of a credit card and was used in the earliest mobile phones. These SIM cards are no longer available.  
  • Mini-SIM: These were much smaller than the original SIM cards, measuring measures 25 x 5 mm with a thickness of 0.76 mm. This card was usually stored behind the battery of a mobile phone, back when removable batteries, like those on mid-to-late 90s Nokia phones, were much more common. 
  • Micro-SIM: Mini-SIM cards were actually the standard until 2010, when SIM card technology suddenly got much smaller (and more powerful). The micro-SIM, which was about the size of a thumbnail at 15x12 mm, came standard in many early mobile devices, including the iPhone 4 and 4S, the first-generation iPad, Samsung Galaxy S III and S III mini, Samsung Galaxy Note and Note II, HTC One, Nokia Lumia 820 and 920, and Sony Xperia S, T, and Z.  
  • Nano-SIM: In 2012, an even smaller SIM card was introduced for modern smartphones and other mobile devices. The nano-SIM measures at just 12.3 x 8.8 mm, or about the size of a pinkie nail. The iPhone 5 was the first phone to use a nano-SIM, but since then, they’ve been standard for most mobile devices.  

SIM card capacity 

The capacity of the original SIM card was just 32 KB, enough to store about five text messages and 20 contacts. The Micro SIM card, though smaller in size, has a memory capacity of 64 KB. A Nano SIM card is the smallest type of SIM card and usually has a memory capacity of 128 KB but can hold up to 250 contacts. It’s also possible to buy SIM cards with 256KB of memory storage. However, that is usually not necessary, since most newer devices offer much more storage and cloud capabilities.  

One thing to note about SIM card memory capacity is that it differs from storage capacity. While newer SIM cards do have more memory capacity, nearly everything stored on a phone, such as photos, apps, and music, are stored with cloud technology and/or on an SD card.  

Emerging SIM technology 

Soon, it’s likely that devices won’t even require physical SIM cards. In fact, some of the latest smartphones do not even include SIM card trays. Here are the two most common types of embedded and cloud-based SIMs.  

  • Embedded SIM (eSIM): As we adopt more and more connected devices, from home appliances to drones, we also need more flexibility for connectivity. That’s where eSIMs come into play. The eSIM isn’t a device that can be removed. Instead, it is integrated into your device’s circuitry and can be programmed directly by a mobile network operator, so that devices no longer require a physical SIM. The iPhone 14 series no longer has a physical SIM card tray and has switched exclusively to eSIM.  

eSIM technology is convenient, since information can be transferred to a new device without requiring manual transfer of a SIM card. For example: 

  • International travel. If you’re traveling out of the country, you can simply add an international plan to your current device that starts working from the moment you land in another country, giving you hassle-free flexibility. 
  • Dedicated devices. If you have two separate devices—one for work, one for home—eSIM eliminates the need since you can have both your work and personal phone numbers on the same device, and switch back and forth as needed.  
  • Lost or stolen devices. They’re also great in the event that your device is lost or stolen, since all the information stored in a phone will also be stored electronically.  

However, eSIMs are also more complex than physical SIM cards, which are fairly easy to swap. They require additional setup and configuration. 

  • Soft SIM: Also known as a virtual SIM, these SIMs are cloud-based and usually used in tablets and smartwatches. They also work on devices that do not have physical SIM card slots.  

Soft SIMs have the same benefits as eSIMs but come with a few additional drawbacks. First of all, since a virtual SIM is cloud-based, it may not offer the same level of security as SIMs that are tethered to a device. Additionally, a soft SIM requires internet access to work, which can be frustrating in areas with poor connectivity.  

From early SIM cards the size of a credit card, to SIMs that exist electronically, SIM technology has come a long way. And it’s very likely that soon, most of the connected devices we use will require some type of embedded or cloud-based SIM. But no matter which type you use— SIMs play a vital role in mobile technology—a role that is likely going to grow in importance as connected devices become common in more facets of day-to-day life.  

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