What Is a SIM Card and What Does It Do?
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 what does it do?
Is there a way to use a mobile phone without requiring one? Keep reading to find out.
What Is a SIM Card?
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 use SIM cards while CDMA phones do not.
SIM cards are the small cards which contain a chip that you need to insert into a GSM phone before it will work. Without a SIM card, a GSM phone won’t be able to tap into any mobile network. The card holds all the critical information.
By comparison, CDMA carriers keep a list of all phones that can use their network. Phones are tracked by their ESN (electronic serial number) so they don’t need 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 main exceptions are AT&T and T-Mobile, which both provide GSM phones. Internationally, GSM is the more popular technology by a landslide, due to legislature and industry influence that nudged providers towards using GSM.
What Does a SIM Card Do?
What 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. The carrier provides this key.
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.
Switch Phones With a SIM Card
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 three numbers. There’s 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 of between 32KB to 128KB. 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.
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 Is a Locked SIM?
When your phone is locked to a particular network it’s actually the phone itself 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, removes this limitation so a phone can accept SIM cards from other networks. If you ever plan on selling your phone, this is an important consideration since you’ll need to unlock it before the buyer can use it. The same applies if you’ve bought a locked phone.
There’s also one other important point to consider: pre-paid SIM cards. These pay-as-you-go SIM cards do not require a subscription or a contract. They also tend to be cheaper, especially if you purchase them from an MVNO. These can be immensely useful if you travel internationally and want to avoid expensive roaming charges.
How much is a SIM card like this? At most it’s a nominal fee. But you’ll often get it for free when you pay for your first chunk of airtime allowance.
A guide to unlocking is outside the scope of this article, but you may be interested in buying unlocked phones which can save you hundreds of dollars. We’ve also shown how to fix the “SIM not provisioned MM2” error .
The Rise of eSIM
At some point, the industry is likely to move to the eSIM. This is where the SIM is embedded in the phone and activated remotely by the carrier, meaning you no longer need to fiddle with the tiny cards. It’s already in use for the second SIM in some dual-SIM devices, like the iPhone XS. Although you still need a card for your main SIM.
But it will take some time before this becomes standard. Until then, take a look at some apps that can help you manage your SIM card data for additional help. Also, learn about ways in which your SIM card can get hacked to protect yourself from such scenarios. And if you need a new phone plan, check out the best unlimited phone plans.
If you’re curious about eSIMs, read our introduction to eSIM and how it works .