Simply about every cellular telephone in being today makes use of a SIM card. "SIM" is short for Subscriber Identity Module. Without it, you can't brand or receive phone calls. You also tin can't employ whatsoever cyberspace information over a cellular connectedness.

You probably already knew this, but why is your phone so useless without this pocket-sized slice of metal and plastic? Merely what is a SIM card actually used for?

What a SIM Card Looks Like

A SIM Card is pretty easy to identify. Information technology'south a plastic card with a distinct gear up of metal contacts. Inside the SIM card you'll find an integrated circuit, which communicates with the telephone through the aforementioned metal contacts.

SIM cards come in a diverseness of shapes and sizes. In fact, there are four sizes in total. The largest is known as "total-size" and is effectively obsolete. Measuring three.37 in past 2.125 in, full-size SIM cards were basically the size of smart access cards.

The reason for this is quite interesting. At the outset of mobile phone technology, multiple people would share a single telephone. The idea was that y'all would insert your enormous SIM card into the phone when you wanted to use it. Since the card would spend more time in your wallet than in a phone, the large plastic torso fabricated perfect sense.

These days most SIM cards you'll buy take all three of the common sizes, with pieces of the plastic body breaking away until you reach the size y'all desire. The Mini-SIM size is pretty uncommon these days and is mostly used in legacy devices or low-cost devices based on older designs.

The Micro-SIM size is very popular and most mainstream phones utilize this. The Nano-SIM size eliminates nearly all plastic body around the SIM contacts and is essentially the standard for flagship and mid-high-end smartphones, where internal space is at an farthermost premium. SIM cards are physically keyed to get in impossible to insert them the wrong style around.

Here's a tip: don't throw away the Mini- and Micro-SIM frames when you go a new SIM card. If you ever need to move the SIM to another telephone you may nonetheless demand them!

What Does a SIM Card Do?

The cellular network owned by your service provider is only that –  a network. They need to control who has access to that network both for security reasons and to brand the whole thing work!

Then what is a SIM card? Your SIM card is an access card. It contains your credentials. So in that sense it'due south like a card that stores your username and password. You lot "log in" to the cellular network with the SIM menu. Your phone number is your unique network identity.

No matter where yous go, every bit long equally you lot are within range of a cellular tower belonging to your service provider, you lot can exist reached and reach others.

What's Stored On a SIM?

The SIM carte has information on it that the network needs to authenticate your account. 1 of these is the ICCID or Integrated Circuit Carte Identifier. This is a number that'south unique to that specific physical SIM card.

The next of import chip of data on the SIM is known as the hallmark key. This is a cryptographic key used to sign data sent to the cellular network. They key is run through a cryptographic function and then compared to another calculation washed with a copy of the key stored in the carrier's database. If they friction match, yous're allow onto the network.

The SIM besides has retentiveness on information technology, that you can use to shop your contacts and text messages. This is pretty limited notwithstanding, with most SIMs sporting between 64k and 256k of infinite. These days most people are better off storing their contacts on the phone's own memory, forth with a fill-in using something like iCloud or their Google account.

Contract and Prepaid SIM Cards

There are generally 2 models cellular providers utilize to sell their services to customers. You lot can sign a fixed contract agreement, or you can be a prepaid customer.

In both cases, you'll become a SIM card. The cards themselves are no dissimilar from each other. Rather, information technology'southward how the provider handles the account that sets them apart. You tin can buy prepaid SIM cards off the shelf. Although in many countries you must register the buy, using regime ID and proof of residence.

Contract SIM cards are allocated data, talk time and other resources according to your contract. Overages are added to your bill and at the stop of the billing bicycle the money gets deducted from your business relationship. With prepaid cards, you lot'll ordinarily load a budgetary amount onto the SIM or otherwise purchase talk fourth dimension or mobile data upfront.

Depending on where yous alive, certain contract handsets may be locked to SIM cards only from that provider. Frequently this is a way to go a phone for a substantial discount, but if y'all ever need to use another SIM bill of fare, possibly when travelling, you lot'll find that it doesn't work! To avert that sort of situation, exist sure to await for phones sold as "unlocked".

Switching SIM Cards

Taking a SIM from one telephone and moving it to some other is pretty painless. Given that both phones employ the same SIM card size. If the other telephone uses a larger size, you'll take to employ an adapter to get in fit the larger slot. If the SIM card y'all want to use is too large for the target phone, you take ii choices.

The kickoff is to cut the SIM downwards to size. Some people are dauntless enough to do this using nada but a pair of pair of scissors and some sort of template. If you accidentally cut into the circuity of the SIM card, information technology'southward game over and yous'll need a new one. The safest way to do this is by using a defended SIM cutting tool. Fifty-fifty then, you run the gamble of destroying the carte du jour, so take this road just at your ain gamble.

The other option is to switch SIM cards.  Of course, if you lot only went out and bought a new SIM card, it would have a different phone number associated with information technology! Each service provider may have a different procedure to motility your phone number from one SIM to another.

In full general however, it should just take a phone telephone call and verification of your identity. If the provider has concrete stores, you lot should even have the option of going in for a new SIM, with your quondam one disabled.

These days it's popular to use text messages sent to specific SIM cards as a form of two-factor authentication. So now we have something known as SIM-swap fraud. Where criminals find means to take your phone number transferred to a SIM they control. SIM duplication is some other method. Information technology's certainly something to lookout out for.

Life After SIM Cards

SIM cards, at least discrete SIM cards, aren't really necessary anymore. We are already seeing the rise of the eSIM or embedded SIM card. This is a SIM carte built directly into the phone. This card is programmable, which means you can easily change providers or employ multiple providers at once.

In some countries, "dual-SIM" phones have been very popular. With these you could accept two phone numbers, one for personal use, for example. You could also take a dedicated information SIM card, perhaps with ameliorate rates.

eSIMs make this redundant and turns access and authentication into something like to stock-still-line broadband usage. Where you only enter a username and countersign to access a given service provider.

Few current phones have embedded SIM cards at the fourth dimension of writing, but that's prepare to alter equally the industry adopts this new standard. You adjacent phone might very well have no identify to put a SIM card at all, which would be the cease of an era, but certainly a movement for the ameliorate.