It's not that complicated....

How do SIM Cards Work?

How do SIM Cards work?

SIM Card Video Explanation



Back in the good old days telephones had physical wires linking one end to another. This was regardless of whether were making a call across the continent or just using a couple tin cans attached to a string.  Whenever you’ve dialed a number our phone company could route your call exactly where it needed to go using those cables.  But in this brave new world of not only phones calls, but vines and snapchats and other stuff that kids are doing being blasted out from cell towers everywhere.  These signals have to know where they’re going every time.

This is where SIM cards come in. Despite the name, SIM cards isn’t a new casino building game from Maxis although that would actually be pretty sweet.

SIM stands for Subscriber Identity Module which gives a pretty big hint as to what it does.  It identifies what mobile phone subscriber is using a particular iPhone, Galaxy, or Windows phone (if that’s your thing).

Although SIM cards are quite low capacity with many of them holding well under a megabyte of data, the small amount of data that they do hold is extremely important if you want your phone to be of any use when you don’t have a WiFi signal.  SIM cards store a 64-bit number that serves as a unique identifier and although that only takes up 8 bytes of storage, there are over 9 quintillion unique numbers that can be stored in this way on a SIM card, so this method should work no matter how many people sign up for wireless service.

When you turn on your phone and first connect to the cell network, your phone will pass your SIM card ID number along with an authentication key also found on the SIM card to your cell phone provider.  Your provider then generates a random number and uses the key to spit out a response number at the same time.  That random number is sent back to your phone and the same calculation is done with the authentication key to generate another response number.  If the two numbers match, your provider will recognize this and connect your phone to the network, both ensuring that it’s actually you that’s connected to keep your data safe and to see who you are so your provider can block access if you haven’t been paying your bills.

Other than just identifying information, SIM cards can also store phonebook information. That might seem like a dated concept in an age where phones can store almost anything you can think of on separate internal memory or expansion cards this was important when things like flip phones were more popular so you wouldn’t lose all of your contact info when you upgrade it or if you’re not storing your phonebook with a Google or Apple cloud service.  If you are moving SIM cards between phones, just make sure that they’re the same size as there are different SIM form factors and that your new phone even has a SIM card slot at all.  Some phones, especially in the US, have authentication information on built-in internal memory instead.

Of course, while SIM cards are quite useful when you want to switch phones, this is isn’t always possible because of something call SIM locking.  You see phones you get from a major carrier with a service contract are often locked – meaning they’ll only work on that carrier’s network service.  Providers do this so they can subsidize the cost of the phone, allowing you to get higher-end equipment for lower price because they know they’ll be getting money from you for a certain time because of your subscription contract.  The good news is that some carriers nowadays have stopped locking, while many others will give you an unlock code once your contract expires so that you can put your phone on another network.  In fact, the US government passed a law in 2014 making it legal to unlock your own phone.  Since before that it was technically considered circumventing a copyright safeguard. Hmm…