What are SIM cards used for?

Almost every cell phone in existence today uses a SIM card. “SIM” stands for Subscriber identity module. Without it, you cannot make or receive phone calls. You also cannot use any internet data over a cellular connection.

You may already know this, but why Would your phone be useless without this little piece of metal and plastic? What are SIM cards really used for?

What does a SIM card look like?

SIM cards are pretty easy to identify. It’s a plastic card with a separate set of metal contacts. Inside the SIM card you will find an integrated circuit, which communicates with the phone through the aforementioned metal contacts.

SIM cards come in different shapes and sizes. In fact, there are a total of four sizes. The largest size is called “full size” and is actually obsolete. The full-size SIM cards measure 3.37 inches by 2.125 inches which is essentially the size of a smart access card.

The reason for this is quite interesting. In the early days of cell phone technology, many people would share a phone. The idea is that you will insert your giant SIM card into the phone when you want to use it. Since the card will spend more time in your wallet than it does in your phone, the large plastic body makes perfect sense.

These days, most SIM cards you’ll buy come in all three common sizes, with pieces of the plastic body that will break apart until you get to the size you want. Mini-SIM sizes are quite common these days and are mostly used in older devices or low-cost devices based on older designs.

Micro-SIM size is very common and most feature phones use this size. The Nano-SIM size eliminates virtually the entire plastic body around the SIM contacts and is essentially the norm for flagship and mid-range smartphones where internal space is extremely high-class. The SIM card is physically locked so that it cannot be inserted by mistake.

Here’s a tip: don’t throw away the Mini and Micro-SIM frames when you get your new SIM card. If you need to transfer SIMs to another phone, you may still need them!

What does a SIM card do?

The cellular network owned by your carrier is just that – one. They need to control who has access to that network for security reasons and to make the whole thing work!

So what is a SIM card? Your SIM card is an access card. It contains your login information. So in that sense it’s like a tag that stores your username and password. You “sign in” to the mobile network with a SIM card. Your phone number is your unique network identity.

No matter where you go, as long as you’re within range of your carrier’s cell tower, you can communicate and reach others.

What is stored on a SIM?

The SIM card has information that the network needs to authenticate your account. One of these is ICCID or Integrated circuit card identification code. This is a unique number for that particular physical SIM card.

The next important piece of data on the SIM is called authentication key. This is a cryptographic key used to sign data sent to the mobile network. Their key is run through a cryptographic function and then compared with another calculation performed with a copy of the key stored in the service provider’s database. If they match, you will be joined to the network.

The SIM also has storage on it, which you can use to save your contacts and text messages. However, this is rather limited, with most SIMs ranging in capacity from 64k to 256k. These days, most people are better off storing their contacts on the phone’s storage, along with a backup using something like icloud or their Google account.

Contract and Prepaid SIM Card

In general, there are two models that mobile service providers use to sell their services to customers. You can sign a fixed contract agreement or you can be a prepaid customer.

In either case, you will receive a SIM card. The cards themselves are no different. Rather, it’s the way the provider handles accounts that sets them apart. You can purchase a prepaid SIM card when it is available. Although in many countries you must register your purchase, using your government ID and proof of residency.

Contract SIM cards are allocated data, voice time and other resources according to your contract. The average amount is added to your bill and at the end of the billing cycle the money is deducted from your account. With a prepaid card, you’ll typically top up your SIM or buy prepaid talk time or mobile data.

Depending on where you live, certain contract handsets may only be locked to a SIM card from that carrier. Usually this is one way to get a phone at a substantial discount, but if you need to use a different SIM card, perhaps while traveling, you’ll find that it just doesn’t work! To avoid that, be sure to look for phones that are sold as “unlock”.

Switch SIM cards

Taking a SIM from one phone and transferring it to another is pretty easy. Given that both phones use the same SIM card size. If another phone uses a larger size, you will have to use an adapter to make it fit into the larger slot. If the SIM card you want to use is too large for the target phone, you have two options.

The first is to cut the size of the SIM. Some people are brave enough to do this using nothing more than a pair of scissors and some kind of template. If you accidentally cut into the circuit of the SIM card, the game will be over and you will need a new one. The safest way is to use a dedicated SIM cutter. Even then, you risk destroying the card, so go this route only at your own risk.

Another option is to switch SIM cards. Of course, if you just went out and bought a new SIM card, it will have another phone number associated with it! Each service provider may have a different process for transferring your phone number from one SIM to another.

In general, however, you just need to make one phone call and verify your identity. If your carrier has a physical store, you’ll even have the option to buy a new SIM, with your old SIM disabled.

These days, it is common to use text messages sent to specific SIM cards as a form of two-factor authentication. So now we have something called Fraud via SIM swap. Where criminals find ways to transfer your phone number to a SIM they control. SIM cloning is another method. It is definitely something to watch out for.

Life after SIM card

SIM cards, at least removable SIM cards, are no longer really needed. We have seen an increase in eSIM or Embedded SIM Card. This is a SIM card built directly into the phone. This card is programmable, which means you can easily change providers or use multiple providers at the same time.

In some countries, “dual SIM” phones are already very popular. With these, you can have two phone numbers, one for personal use, for example. You can also get a dedicated data SIM card, perhaps for a better price.

eSIM makes this redundant and turns access and authentication into something similar to using fixed line broadband. Where you just need to enter a username and password to access a certain service provider.

Very few current phones have an embedded SIM card at the time of writing, but that will change as the industry adopts this new standard. Your next phone most likely won’t have a place to put a SIM card, which would be the end of an era, but definitely a better move.

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