Last updated December 2023
A SIM card stands for a Subscriber Identity Module, and it is actually a small computer, but much, much smaller. The SIM card has several jobs that are essential to your mobile phone's work.
Identity verification
One of the most important jobs of the SIM card is to identify you as the genuine subscription owner. Therefore, if you call, send an SMS, or share a photo or video, the SIM tells the phone which network to use. In addition, it means 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 searches for your IMSI's authentication key.
The network then generates a random number we will call "X" and signs it with your authentication key to generate a new number "Y".
The network then sends "X" to your phone, which 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 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
In the above picture of a SIM, you will 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
What is stored on a SIM card?
The SIM processor handles the applications like authenticating your identity and ensuring you are on the right network. In addition, 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, which is still true for basic feature phones. Standard SIMs can save around 250 contacts, while larger ones can hold up to 500 or 600 contacts. It was also common for people to keep their SMS texts on SIMs. However, as the storage is limited – often to only 10 or 20 messages – this is now done by the phone's memory instead.
SIMs can also store text messages and recent network information, such as which local networks you were last connected to, which can be used to work out your approximate location.
What is the difference between IMSI and IMEI?
IMSI (International Mobile Subscriber Identity) and IMEI (International Mobile Equipment Identity) are unique identification numbers that serve different purposes in the realm of mobile telecommunications:
IMSI:
- The IMSI is a unique number associated with a mobile phone subscriber.
- It is stored on a SIM card and identifies the subscriber to the network.
- The IMSI is used by the network to authenticate the subscriber and can be used to track user activity and billing information.
- It typically consists of a 15-digit code that includes a mobile country code (MCC), a mobile network code (MNC), and a unique subscriber number.
IMEI:
- The IMEI is a unique number assigned to a mobile device, such as a smartphone or tablet.
- It identifies the physical device itself and is used for identifying and tracking the device, not the subscriber.
- If a phone is lost or stolen, its IMEI can be blacklisted to prevent it from being used maliciously.
- The IMEI is usually a 15-digit number that can be found by dialling *#06# on the phone or by checking the device's settings or behind the battery.
In essence, the IMSI is tied to a mobile subscriber and their account, while the IMEI is tied to the physical hardware of the mobile device. They are both crucial for the operation of mobile networks, with the IMSI being key for security and billing purposes and the IMEI being important for device tracking and authentication.
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