How Do Sim Cards Work And Why Do We Need One?

By Adam | How To
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If you're living outside the US or in Europe and you've got a smartphone or any kind of mobile phone, you'll probably have a SIM card in it.

Any if you've ever wondered what that small piece of plastic that sits inside your phone is used for or how it works, you've come to the right place.

What You Will Learn

  • What a SIM card is
  • How SIM cards work

What is s SIM card?

In the mobile phone world, there are two basic phone types.  These are GSM (Global System for Mobile) and CDMA (Code Division Multiple Access).

In short, GSM phones use SIM cards, whilst CDMA phones don't.

A SIM card is short for Subscriber Identity Module and it's basically a piece of plastic that slots into your smartphone (or any mobile phone).

It's basically a unique identifier, that's unique to you and your phone and it allows you to connect to a mobile phone network, make calls and get charged for the calls that you make.

If you didn't have a SIM card, then you can't connect to a mobile phone network, with the exception of being able to connect to the emergency services such as 999.

CDMA phones on the other hand don't require SIM cards.  Instead, CDMA carriers keep a list of phones that are allowed to connect to their network and they are tracked by their ESN (Electronic Serial Number).

Most mobile carriers in the US provide CDMA phones, but there are two exceptions, AT&T and T-Mobile, which provide GSM phones too.

The SIM card itself is comprised of the Subscriber Identity Module (SIM) and a shaped plastic surround.

The small integrated chip contained on the card is the really important bit though because it contains sensitive data on it such as a unique identification number, the phone number and other data specific to the user it is registered to.

The SIM card also stores a 'key' (essentially a password) which is needed to decrypt encrypted voices at the other end of your calls.

Sim Cards

What else is stored on a SIM card?

The actual chip on a SIM card doesn't have much memory (between 32 to 128 KB) but it is able to store enough information that is required to connect to a network.

One piece of information that is stored on the SIM is the IMSI (International Mobile Subscriber Identity) number which is used to connect the person calling your mobile number to your phone rather than somebody else's.

SIM cards also store up to 250 contacts, some SMS messages and other information used by the carrier who supplied the card.

They can also be transferred from one handset to another (as long as it's compatible) without losing your contacts data, your phone number or start a new contract.

These days though, we tend to store contact details etc in cloud-based services or on the phone itself, but it was a big improvement on back in the day when you had to manually type in contact details using fiddly 'multitap' mobile phone keys.  I do miss the indestructible nature of my Nokia 3210 though!

As a result, SIM cards have no other use than to grant access to mobile phone networks.

How do SIM cards work?

So we've said that the SIM card stores the IMSI number but it also stores an authentication key that validates the IMSI.

This authentication key is provided by the carrier.

So what happens when you plug in your SIM card and turn your phone on?

  1. Well firstly it obtains the IMSI from the SIM card and sends it to the network.  At this stage, your phone is requesting access to the network.
  2. The network looks up the IMSI that the phone sent against its internal database and retrieves an authentication key for that IMSI.
  3. The network then generates a random number, let's call it A.  It then signs it with the authentication key which produces another number, let's call it B.
  4. The network then sends number A back to the phone which then forwards it on to the SIM card.  The SIM then signs number A with its own authentication key and creates another number, let's call it C.
  5. The network then compares number A with the SIM cards's number C.  If they match, then the SIM is deemed legitimate and the phone is then granted access to the network.

So this process is not only used to determine which network to connect to but it is effectively a "login" process for your mobile.

Because this data is stored on the SIM itself, it comes in handy when you decide to switch phones.  Switching phones with a CDMA carrier is however more difficult, because the phone itself is registered with the network.

One final piece of information that is stored on the SIM card is what's called the ICCID or Integrated Circuit Card Identifier.

This is actually stored on the card and engraved on it too.

There are three numbers in this identifier: the ID of the SIM card issuer, an ID for the individual account and a parity digit which is calculated from the other two identifiers for extra security.

What's a locked SIM?

So when you hear that a SIM is locked, it's not actually the SIM that is locked, it's actually the phone itself that's locked.

GSM carriers can actually install software on phones that will only accept SIM cards for a particular network.

So if the software detects that the SIM card is from another network, the phone will refuse to operate, hence the phone is "locked".

You can unlock phones however allowing the phone to accept SIM cards from any network, and you should definitely do this if you plan to sell your phone.

You should probably do this if you've just just bought a new phone too as it will most likely need unlocking before it can be used.

Prefer a video explanation of SIM cards?  Check the awesome video below by Techquickie.

Wrapping it up

So there you go, SIM cards can be a pain, but they do offer some benefits too and I think they have definite advantages over CDMA.

They allow you to hop from phone to phone whilst keeping the same number and contacts (if you store them on there).

But should you damage or loose them, your phone will be practically unusable – well to make calls at least.

What do you think?  Let me know in the comments!