How does a SIM card work?

How does a SIM card work, really? In the past, telephones had physical cables that connected the network so you could make calls to each other. The cables were used regardless of whether you made a call to the other side of the world (read our blog about the first transatlantic telegraph cable here) or if you had connected two tin cans with a piece of string to be able to talk to the neighbor on the opposite side of the road.

As soon as you had entered the telephone number you wanted to call, your telephone company could direct the number exactly where you wanted, through these cables.

But today, most of our phones have no cables. With all the wireless technology flourishing, the devices themselves must know where the signals should land. And this has to be right, every single time. This applies to both common signals for conversations and other exciting things like Instagram Stories and Snapchats, etc. that completely pour out of all the masts.

SIM-kort SIM Card

This is where SIM cards come into the picture and sort it all out. The abbreviation SIM stands for Subscriber Identity Module, an abbreviation that gives us a pretty good indication of what the SIM card actually does. It identifies which mobile subscriber is using a specific device.

  • SIM is an abbreviation for Subscriber Identity Module
  • It identifies which subscriber is using a particular device
  • Many SIM cards hold less than 1 MB of data
  • SIM card contains directory information
  • When phones have built-in SIM cards, this is called e-SIM


SIM cards have low capacity

Since these cards help to direct all traffic correctly, it is easy to imagine that they should be quite powerful. But that is not the case. SIM cards actually have quite low capacity, many of them contain well under 1 MB of data. But the little data they actually hold is of great importance. For example, when you want your mobile phone to be useful when you do not have Wi-Fi. The SIM card stores a 64-bit number that acts as a unique identifier. Although this only takes up 8 bytes of storage space, there are over 9 quintillions of unique numbers that can be stored in this way on a SIM card. In other words, this method of storing numbers should last a while longer, given that we are “alone” about 8 billion people on earth.

Hur fungerar ett SIM-kort How does a SIM-card work

When you start your phone and connect to the mobile network, your phone will send your SIM card ID number, together with an authentication key (which is also on your SIM card), to your operator. The operator then generates a random number and at the same time uses the key to create a response number. The random number is then sent back to the phone. The same calculation is made by the SIM card in the phone to obtain an answer number. If the two answer numbers then match each other, the operator will feel this and will connect your phone to the network. This ensures that it is actually you who is trying to connect, this to keep your data safe and to be able to see who you are.

The phonebook in the SIM card

In addition to identifying information, SIM cards can also store phonebook information. This may seem like an ancient and dated concept as today’s phones can store extremely much data themselves. But in the past, when the Nokia 3310 and the classic fold-in phones were super hot, this was an important feature because it helped to ensure that you did not lose all your contacts if you upgraded or replaced your phone.

The correct size of the SIM card

If you want to move a SIM card between different phones, make sure that the SIM card is the correct size for the different devices. Different devices have different SIM forms and some new phones do not even have room for a SIM card at all. It is mainly in the USA that some new phones now have their authentication information built directly into the memory instead. This is called an e-SIM.

Hur fungerar ett SIM-kort, how does a SIM card work

Although SIM cards can be useful when you want to change phone, it is not always possible to move a SIM card anyway. This is because the SIM card is locked. If you buy the phone from a major operator, you often get the SIM card locked to that particular operator. The SIM card only works with that particular operator. Operators do this so that they can subsidize the cost of the phone, which allows you to get new technology at a lower cost because the operator knows that they will receive money from you for a certain period due to your subscription.

SIM cards are small and can not hold much data, yet they are so important for our everyday lives. So the next time you hit a colleague or a friend, give an extra thought to what a great job your SIM card does.

Do you need to change SIM card to use 5G?

When the operators have switched between different networks, it has been required to change SIM cards to use the new networks. When it comes to 5G, it depends a bit on which operator you have. Some operators require you to switch to a new SIM card that is 5G ready. Some other operators have already started to change to 5G compatible SIM cards. It is best to check with your operator to see what applies.

Soluno is its own operator

We at Soluno are the operator and therefore have our own SIM cards that are delivered only to corporate customers. If your company needs to review its telephony, we are more than happy to help. Fill in the form and one of our knowledgeable advisers will contact you.

Latest update Nov 2020

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