How does a smartphone SIM card work? The technology behind SIM cards explained

If you’ve ever wondered what that small piece of plastic that slots into your smartphone is for, we’ve got the answers to all your questions.

Mobile phones need SIM cards to connect the user to the network of their choice.

A SIM - short for Subscriber Identity Module - is a piece of plastic that slots into your smartphone (or mobile phone) that acts as your unique ID, so that you can connect to a mobile phone network. This lets you make calls.

[Read more: Getting started with BT Mobile's SIM-only plans]

Without a SIM card, you can't connect to a mobile phone network, although for safety reasons, you can still dial 999.

When did SIMs first appear?

SIMs first appeared with the first digital 'GSM' mobile phones back in 1992. The Nokia 1011 was the first GSM mobile phone to go on sale. Unlike the analogue mobiles before it, GSM mobiles used digital technology for (relatively) interference-free communication — among other benefits.

Why do we use SIMs at all?

The SIM was part of a European telecommunications standard that separated mobile phones from the networks they connected to by moving all the necessary security and identification data onto a chip embedded into a removable piece of plastic the size of a credit card.

Since GSM technology also converted the voices at either end of a call into encrypted digital data before it was sent over the airwaves, the SIM also stored the ‘key’ (essentially a password) needed to decrypt this data.

What else do SIMs store?

The computer chip on a SIM doesn’t have much memory, but it stores a host of information needed to connect to a mobile phone network. This includes a unique ID called the IMSI (International Mobile Subscriber Identity) which is used to connect someone calling your mobile number to your phone, rather than to someone else’s.

SIMs can also store names and telephone numbers, which makes it much easier to switch phones without losing your contact details. This isn’t much of an issue in these days of cloud-based backup, but this innovation was a huge relief to anyone who remembers typing in contact details using the fiddly ‘multitap’ mobile phone keys of the 1990s.

Why are SIMs getting smaller?

The original Full-size SIM was fine for early digital mobile phones, but as mobile communications technology improved and the phones started to shrink, the SIM was soon too large.

The first solution was the thumbnail-sized Standard-SIM. These were still provided as part of a full-size SIM for compatibility with older models, but it could be snapped out for use in newer phones.

Standard-SIMs stuck around for many years, but once smartphones started to get smaller and thinner, space was in short supply. As a result the Micro-SIM standard was created in 2003, but manufacturers didn’t adopt it until 2010.

The Micro-SIM was only a little smaller than the Standard-SIM and most of the missing size was useless plastic. That meant many Micro-SIMs could be cut down to size using widely available tools (or a steady hand and pair of scissors) without otherwise damaging them.

Inevitably it wasn’t long before a still-smaller SIM was needed. In 2012 we got the Nano-SIM, which did away with all redundant plastic. It was essentially just a tiny chip.

[Read more: Discover how easy it is to swap to BT Mobile]

What size SIM do I need?

Most modern smartphones use a Micro or Nano SIM, but you can usually use a smaller SIM in a larger slot with a suitable adaptor. Join BT Mobile and you'll get a triple SIM, which includes standard, micro and nano sizes, so you can use the correct one for your phone.

What if I want more than one SIM?

Most smartphones only have one SIM slot, but there’s nothing to stop you swapping SIMs if you want to use a cheap pay-as-you-go SIM when you’re abroad, for example. But dual-SIM smartphones are available. They're a good option if you want to use two different mobile phone numbers on different networks from a single phone.

Can I use my smartphone SIM in my tablet, or vice versa?

It depends on the mobile network. 

BT Mobile customers can their SIM in an iPad or tablet, but not all BT Mobile features will be available. For instance, some tablets can't receive texts, which means you won't get messages warning you about usage and will have to use bt.com/mymobile to track data use.

My SIM is blocked — have I broken it?

SIM cards come with built-in security features to protect your account if your smartphone is stolen. First among these is a four-digit SIM PIN that, if enabled, must be entered whenever you restart your phone to ‘unlock’ it.

Get this PIN wrong (you normally get three tries) and your SIM will be blocked. But you can unblock it with a call to your mobile operator. SIMs can also be blocked if you don’t pay your bill or if your phone is lost or stolen - but you will need to inform your mobile phone network of the loss or theft for that to happen.

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