How sim cards work ? A very general question but only few people have idea about it. Welcome to the world of communication, unlike days ago when we used heavy and large  devices for communication, now they have become compact, small and thin.

Importance of Sim card

Nowadays people use mobile devices for communication but these mobile devices won’t work without a SIM card. Everybody knows about SIM cards and why they don’t, it’s become the essential part of our daily life. Without this mobile devices are useless, without this you can’t connect to the network and you won’t be able to make a phone call. 

How sim cards work

Have you ever wondered how this SIM card works ? What’s the process/technique behind this SIM card and how can it make possible for devices to connect the network.

In the world of mobile phones, there are two primary phone types that are available to consumers: GSM (Global System for Mobile) and CDMA (Code Division Multiple Access). GSM phones use SIM cards while CDMA phones do not.

How SIM cards work?

Journey of sim card

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 interference-free communication among other benefits.

Engineering of sim card

Subscriber Identity Module(SIM) is an integrated circuit which stores many keys and data.

It contains a unique serial number, Integrated Circuit Card Identifier (ICCID) the ICCID contains three numbers. There’s an identifying number for the SIM card issuer, an identifying number for the individual account, and a parity digit that’s calculated from the other two numbers for extra security, International Mobile Subscriber Identity (IMSI), security authentication and ciphering information, temporary information related to the local network a list of the services the user has access to, and two passwords: a personal identification number (PIN) for ordinary use, and a personal unblocking key (PUK) for PIN unlocking.

You can read more information about it on here:

Concept of IMEI

There is one more thing which is worth mentioning IMEI number. The International Mobile Equipment Identity (IMEI) is a number, usually unique, to identify mobile phones. It is usually found printed inside the battery compartment of the phone, and you can also find it by dialing *#06# and it will be displayed on device. GSM networks use the IMEI number to identify valid devices, and can stop a stolen phone from accessing the network. Devices without a SIM card slot usually don’t have the IMEI code


On startup, the phone obtains the International Mobile Subscriber identity (IMSI) from the SIM card and relays it to the network. So how sim cards work? The network takes the IMSI and looks in its internal database for that IMSI’s known authentication key. If you want to know more about the IMSI you can read about it here: The network generates a random number, A, and signs it with the authentication key to create a new number, B. This is the response it would expect if the SIM card is legitimate. The phone receives A from the network and forwards it to the SIM card, which signs it with its own authentication key to create a new number, C. This number is relayed back to the network. If the network’s number A matches the SIM card’s number C, then the SIM card is declared legitimate and access is granted.

SIM cards are capable of storing information, such as contact list data and SMS messages. Most SIM cards have a capacity of between 32KB to 128KB. Transferring this data mainly involves removing the SIM card from one phone and inserting it into another, though this has become less important with the invention of backup apps. But imagine when these backup apps are not invented and we switch the mobile, then we have to type and save again these contacts on that device with the enormous keyboard of mobile phones. But that’s an old story now when you save a contact it’s automatically synced with the cloud and when you switch to new devices and login with the same account it imports all the contacts from the cloud.    

There are four main types of SIMs: Full size SIM, Mini-SIM, Micro-SIM, Nano-SIM, apart from all these four there is one more known as eSIM or Embedded SIM.

Now at some point, the industry is likely to move to the eSIM. This is where the SIM is embedded in the phone and activated remotely by the carrier.

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