How do SIM cards work?

Back in the good old days telephones had physical wires linking one end to another. This was regardless of whether or not you were making a call across the continent or just using a couple of tin cans attached to a string.

In the past whenever you have dialed a number your phone company could route your call exactly where it needed to go using cables.

But in this brave new world of not only phone calls but Twitter & Instagram and other stuff that the people 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. 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 in particular iPhone, Galaxy or a Windows phone.

And although SIM cards are quite a low capacity with many of them holding well under a megabyte of data, the small amount of information that they do hold is extremely important if you want your phone to be of any use.

SIM cards store a 64-bit number that serves as a unique identifier code and although that only takes up 8 bytes of storage, there are over a 9 quintillion unique numbers that can be stored in this way on a single SIM card. So this method should work no matter how many people register with a particular telecom service provider.

So how does it work?

When you turn on your phone and first connect to the cell network, your phone will pass your SIM card’s ID number along with an authentication key found on the SIM card to your telecom service provider.

Your service 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 authentication key to generate another response number.

If the two numbers match, your service 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.

But other than just identifying information, SIM cards can also store phonebook information and although that might seem like a dated concept in an age where phones can store almost anything you can think of, on internal memory or separate expansion cards.

This was important when things like basic mobile phones/flip phones were more popular so you wouldn’t lose all of your contact info when upgrading 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 are the same size, as there are different SIM form factors.

Some mobile phones have their authentication information on built-in internal memory instead.

Of course, while SIM cards are quite useful when you want to switch phones, this isn’t always possible because of something called SIM locking.

You see phones you get from a major carrier with a service contract are often locked, meaning they will 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 a much lower price because they know they will be getting money from you for a certain time due to your subscription contract.

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