How Does a SIM Card Work

9 May 2022  |  Admin

How Does a SIM Card Work


We are all familiar with the small piece of plastic that we insert into our mobile phones, but what does it do?

The SIM, an acronym of Subscriber Identity Module first appeared in the early 1990s when mobile phones moved to the new GSM networks for mobile communications. The SIM is a small piece of plastic that has a computer chip embedded into it, and it acts as a unique identifier when a mobile device is connecting to the GSM network.

By storing this unique identifier, the SIM allows the mobile device to connect to a particular MNO (mobile network operator) to make and receive calls, send and receive messages and to access the internet. Without the SIM, a mobile device it is unable to access any of these services via the GSM networks, although you are still able to make emergency calls to 999.


The IMSI (International Mobile Subscriber Identity) is a profile that is loaded onto the SIM by the MNO who issues the SIM. The IMSI is a numeric value, an authentication key to validate the IMSI with the MNO. This tells the MNO what services you are allowed to access, and what service to bill you for. The IMSI is also the thing that is used to connect your mobile telephone number to your device and ensure that all calls or messages intended for you reach the correct devices - after all, it is possible to take a SIM out of one device and put in into another, so the network needs to know which device to send your traffic to.

When you first turn your phone on, it connects to the network that the SIM tells it to and the network then uses the information on the SIM to authenticate it as legitimate and to allow the device to access services.

So why not just use a username and password?

The chip on a SIM also carries out quite a few other roles when inserted into a device.

Each individual part, called a Pin, of the SIM, has its own job:

·       Pin 1 connects to the device’s power supply

·       Pin 2 is used to reset the SIM

·       Pin 3 provides an internal clock for the chip processor

·       Pins 4 and 8 are not currently used

·       Pin 5 grounds the chip electrically

·       Pin 6 is used for Near Field Communication (NFC) enabled devices

·       Pin 7 allows the device to interact with the SIM

Historically SIMs were also used to store messages and contact details (names and telephone numbers), but with the introduction of smartphones and cloud-based systems, this is no longer used by most devices.

So, what is an eSIM?

Many devices are now being shipped with an eSIM already built into them, but while you cannot physically see an eSIM, it does exactly the same job as a physical SIM. The only main difference is that you are able to download the IMSI profile over the air (OTA) to the SIM.

What Are The Benefits Of The eSIMs?

The inclusion of the SIM in the device is being driven by the Internet of Things (IoT). The majority of smart devices need to have the ability to centrally communicate or the ability to communicate with each other. Due to the fact that the devices are often required to be very portable (think Smart Watch), mobile networks are seen as the best method of doing this.

The designers of such devices have embraced the eSIM as it reduces the space that is taken up by a traditional physical SIM and allows them to be far more creative and flexible when designing such devices.

Additionally, having the ability to download new IMSI profiles gives users much more flexibility when it comes to changing services.