How does the SIM card in your mobile work?

A SIM card stands for a Subscriber Identity Module, and it’s actually a small computer, but much, much smaller. The SIM card has a number of jobs that are essential to how your mobile phone works.

Identity verification

One of the most important jobs of the SIM card is to identify you as the genuine owner of the subscription. So if you make a call, send an SMS, or share a photo or video, the SIM tells the phone which network to use. And it tells your network to bill you for the services you use.

It can do this thanks to the IMSI (International Mobile Subscriber Identity) number and an authentication key that validates the IMSI. Think of this as login details.

When you turn your phone on, it obtains the IMSI from the SIM and sends this to your network. Your network then looks up the IMSI in its internal database, and also searches for your IMSI’s authentication key.

The network then generates a random number we’ll call “X” and signs it with your authentication key to generate a new number “Y”.

The network then sends “X” to your phone, which in turn forwards it to the SIM. The SIM then signs it with its authentication key to create a new number, “Z”.

“Z” is then sent back to the network.

If the network’s number “Y” matches the SIM card’s “Z”, then the SIM card is declared legitimate and you are given access to the network to make calls and use data.

Though this is complicated, it only takes a few seconds and has the added benefit of you being able to take your SIM with you if you want to change phones (if they aren’t SIM locked), and carry on using your network services.

Technical breakdown

SIM card

In the above picture of a SIM, you’ll see that it is split into multiple sections. Did you know that each individual part called a Pin has its own job?

  • Pin 1 is for connecting to the phone’s power supply
  • Pin 2 is to reset the SIM if required
  • Pin 3 provides the SIM with a clock signal for its processor
  • Pins 4 and 8 are reserved for future uses
  • Pin 5 is to ground the chip
  • Pin 6 can be used to communicate with an NFC enabled mobile phone
  • Pin 7 controls the interaction between the phone and the SIM

The processor handles the applications like authenticating your identity, and making sure you’re on the right network. Also it can store contact and SMS information in the SIM memory.

It used to be common for people to save their contacts on their SIM, and is still the case for basic feature phones. Standard SIMs can save around 250 contacts while there are larger ones that can hold to up to 500 or 600 contacts. It also used to be common for people to save their SMS texts on the SIMs too. However, as the storage is limited – often to only 10 or 20 messages – this is now done by the phone’s memory instead.

SIM cards are very complex devices, so we hope we’ve given you a little insight into how they work. If you have any questions, please don’t hesitate to get in touch.