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The struggle with SIM cards can be an annoyance when upgrading to a new cell phone or reverting to a backup. Haven’t we come far enough with technology that such a thing shouldn’t matter anymore? What is a SIM card and why is it so important? Is there a way to use a mobile phone without requiring one? Keep reading to find out.

What Are SIM Cards?


In the world of mobile phones, there are two primary phone types that are available to consumers: GSM (Global System for Mobile) and CDMA (Code Division Multiple Access). GSM phones are the ones that utilize SIM cards while CDMA phones do not.

SIM cards are the small cards which contains a chip that must be inserted into GSM phones before they will work. Without a SIM card, a GSM phone won’t be able to tap into any mobile network. The card is what holds all of the critical information.

For comparison sake, CDMA carriers keep a list of all phones that are allowed to use their network. Phones are tracked by their ESN (electronic serial number) so they do not require SIM cards. Once activated, a CDMA phone is tied directly to that particular carrier’s network.

In the United States, most mobile carriers provide CDMA phones. The two exceptions are AT&T and T-Mobile, who both provide GSM phones. Internationally, GSM is the more popular technology by a landslide. Why? Mostly due to legislature and industry influence that nudged providers towards using GSM.


How Do SIM Cards Work?


What sort of information does a SIM card hold? The most important bits of data include the IMSI (International Mobile Subscriber Identity) and the authentication key that validates the IMSI. This authentication key is provided by the carrier. If you’re interested in the nitty-gritty, SIM authentication goes like this:

  • On startup, the phone obtains the IMSI from the SIM card and relays it to the network. Think of this as the “request for access.”
  • The network takes the IMSI and looks in its internal database for that IMSI’s known authentication key.
  • The network generates a random number, A, and signs it with the authentication key to create a new number, B. This is the response it would expect if the SIM card is legitimate.
  • The phone receives A from the network and forwards it to the SIM card, which signs it with its own authentication key to create a new number, C. This number is relayed back to the network.
  • If the network’s number A matches the SIM card’s number C, then the SIM card is declared legitimate and access is granted.

Long story short: this data not only determines which network to connect to but also acts as the “login credentials” which allow a phone to use said network.

For this reason, SIM cards are actually quite convenient when it comes to switching phones. Since your subscriber data is on the card itself, you can plug the SIM into a different phone and all will be well. On the other hand, switching phones with a CDMA carrier is more difficult since the phone itself is the entity that’s registered with the network.

Each SIM card has a unique identifier called the ICCID (Integrated Circuit Card Identifier), which is stored in the card and engraved upon it. The ICCID contains 3 numbers: an identifying number for the SIM card issuer, an identifying number for the individual account, and a parity digit that’s calculated from the other two numbers for extra security.

SIM cards are also capable of storing other information, such as contact list data and SMS messages. Most SIM cards have a capacity between 32 to 128 KB. Transferring this data mainly involves removing the SIM card from one phone and inserting it into another, though this has become less important with the advent of backup apps Don't Lose Your Data: 5 Top Backup Solutions For Android Don't Lose Your Data: 5 Top Backup Solutions For Android Everybody needs to back up their data in case something happens to their device. Protect yourself with these amazing backup apps. Read More . However, SIM card storage is now dwarfed by internal phone storage capabilities, so SIM cards really have no use other than to grant access to specific networks now.

What About Locked SIMs?


Technically, the GSM phone is the entity that’s locked. Not the SIM card.

In practice, GSM carriers can implement software on phones such that a particular phone will only accept a designated SIM card from a particular network. If the phone and SIM card do not match up, the phone won’t operate. This is what it means when a phone is “locked.”

Unlocking a phone, then, is the process of removing this limitation such that a phone can accept SIM cards from other networks. If you ever plan on selling your phone, this is an important consideration since the buyer won’t be able to use it unless it’s been unlocked. Similarly, if you’ve bought (or been gifted) a locked phone, it will most likely need to be unlocked before you can use it.

There’s also one other important point to consider: pre-paid SIM cards The 8 Best SIM Cards for the Nexus 5: Get The Cheapest Prepaid Plan For You The 8 Best SIM Cards for the Nexus 5: Get The Cheapest Prepaid Plan For You Read More . These pay-as-you-go SIM cards do not require a subscription or a contract and tend to be cheaper, especially if you purchase them from a MVNO The Future Is Prepaid: How To Save Hundreds On Your Mobile Phone Bill In 3 Easy Steps The Future Is Prepaid: How To Save Hundreds On Your Mobile Phone Bill In 3 Easy Steps Cutting your mobile phone bill in half takes only three simple steps - first, find a carrier that offers discounted plans, such as an MVNO. Second, get an unlocked phone. Third, and optionally, you can... Read More . These can be immensely useful if you travel internationally and want to avoid expensive roaming charges.

A guide to unlocking would be outside the scope of this article, but you may be interested in buying unlocked phones Never Buy a Phone From Your Carrier! Buy Unlocked Phones and Save Hundreds Never Buy a Phone From Your Carrier! Buy Unlocked Phones and Save Hundreds Unlocked phones can cost a fortune, but the savings more than make up for it. You can save thousands by not signing a contract. We've got six phones that come contract free! Read More which can save you hundreds of dollars.


In the end, SIM cards are both a blessing and a curse. They grant freedom to customers to move from phone to phone as long as those phones are compliant with GSM standards, but can prove annoying if the card itself is somehow lost or damaged since they hold so much crucial data.

Image Credit: Professor teaching via Shutterstock, SIM Card via Shutterstock, Mobile Network via Shutterstock, SIM Locked via Shutterstock

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