IT'S EASY WITH AN eSIM

After more than three decades, the physical SIM card is on its way out. Darien Graham-Smith finds out why we should all welcome the change

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In a fast-moving world, SIMs have been a remarkably consistent part of our digital lives.

Today's high-end phones still support the exact same SIM technology and interfaces as the very first GSM mobiles, launched 35 years ago.

Short for “subscriber identity module”, the SIM is basically a token that identifies you to your mobile service provider, using a fixed code called an IMSI (international mobile subscriber identity) plus a secret key that can be used to cryptographically authenticate the card itself.

Here’s how it works: whenever your phone, tablet or mobile router tries to connect to a mobile network, it sends the SIM card’s IMSI to the operator. The operator checks that the IMSI is assigned to a valid subscriber, and issues a cryptographic challenge-and-response request to confirm that your SIM is the one associated with that IMSI. If all is in order, your phone is allowed to access voice and data services, and charges are applied to your subscriber account. If you don’t have a SIM, or if the secret key doesn’t check out - or if the IMSI isn’t accepted by the provider for any reason (such as if you don’t have an account, or haven’t paid your bill) - you won’t normally be able to use the network, except to make free emergency calls.

On the operator's side, the IMSI is linked to your phone number, so when someone tries to call you or send you a text message, they'll get through to whichever phone your SIM is installed in. Unlike the IMSI, however, the phone number isn’t tied to the card - for example, when you move from one mobile provider to another, the incoming operator can take over your old number from your previous operator and assign it to your new IMSI. In effect, the SIM as a whole serves as a unique identity module that declares and confirms who you are across the entire global mobile network.

The size of a SIM