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

​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, make calls over and be charged for using a particular mobile phone network.

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.

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