Just most every cellular telephone in existence today makes use of a SIM card. "SIM" is short for Subscriber Identity Module. Without information technology, yous can't make or receive phone calls. You also can't use any cyberspace data over a cellular connection.

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

What a SIM Carte du jour Looks Like

A SIM Card is pretty easy to identify. Information technology'south a plastic menu with a distinct set of metal contacts. Inside the SIM carte y'all'll find an integrated circuit, which communicates with the phone through the aforementioned metal contacts.

SIM cards come in a diversity of shapes and sizes. In fact, there are iv sizes in total. The largest is known as "full-size" and is effectively obsolete. Measuring 3.37 in by 2.125 in, total-size SIM cards were basically the size of smart access cards.

The reason for this is quite interesting. At the first of mobile phone technology, multiple people would share a single telephone. The idea was that you would insert your enormous SIM card into the phone when you wanted to use information technology. Since the card would spend more time in your wallet than in a phone, the big plastic trunk made perfect sense.

These days most SIM cards you'll buy have all iii of the common sizes, with pieces of the plastic body breaking away until you attain the size you lot want. The Mini-SIM size is pretty uncommon these days and is by and large used in legacy devices or low-cost devices based on older designs.

The Micro-SIM size is very popular and well-nigh mainstream phones apply this. The Nano-SIM size eliminates about all plastic body around the SIM contacts and is essentially the standard for flagship and mid-high-cease smartphones, where internal space is at an farthermost premium. SIM cards are physically keyed to make information technology impossible to insert them the wrong style effectually.

Here's a tip: don't throw away the Mini- and Micro-SIM frames when you become a new SIM card. If you lot ever need to move the SIM to some other phone you may nonetheless need them!

What Does a SIM Bill of fare Practice?

The cellular network endemic by your service provider is just that –  a network. They need to command who has access to that network both for security reasons and to brand the whole matter work!

So what is a SIM card? Your SIM card is an admission card. It contains your credentials. So in that sense information technology'south like a card that stores your username and countersign. You "log in" to the cellular network with the SIM carte du jour. Your telephone number is your unique network identity.

No matter where you lot become, as long as you are within range of a cellular tower belonging to your service provider, you lot can be reached and reach others.

What'southward Stored On a SIM?

The SIM menu has data on information technology that the network needs to authenticate your account. One of these is the ICCID or Integrated Circuit Carte du jour Identifier. This is a number that's unique to that specific physical SIM card.

The next important bit of data on the SIM is known as the authentication primal. This is a cryptographic key used to sign data sent to the cellular network. They key is run through a cryptographic office and then compared to another calculation done with a copy of the fundamental stored in the carrier's database. If they lucifer, you're let onto the network.

The SIM likewise has memory on it, that you can use to store your contacts and text messages. This is pretty limited however, with most SIMs sporting between 64k and 256k of space. These days near people are improve off storing their contacts on the phone'south own memory, along with a backup using something like iCloud or their Google account.

Contract and Prepaid SIM Cards

At that place are generally two models cellular providers use to sell their services to customers. You can sign a fixed contract agreement, or you lot can be a prepaid customer.

In both cases, y'all'll get a SIM bill of fare. The cards themselves are no different from each other. Rather, it'south how the provider handles the account that sets them autonomously. Y'all can buy prepaid SIM cards off the shelf. Although in many countries you must register the purchase, 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 beak and at the cease of the billing cycle the coin gets deducted from your business relationship. With prepaid cards, you'll usually load a budgetary amount onto the SIM or otherwise purchase talk time or mobile information upfront.

Depending on where y'all alive, certain contract handsets may be locked to SIM cards only from that provider. Often this is a style to get a phone for a substantial discount, merely if you e'er need to use another SIM card, perhaps when travelling, you'll find that it doesn't work! To avoid that sort of situation, exist certain to await for phones sold as "unlocked".

Switching SIM Cards

Taking a SIM from one telephone and moving it to another is pretty painless. Given that both phones use the same SIM card size. If the other telephone uses a larger size, you'll have to use an adapter to make it fit the larger slot. If the SIM card you want to utilize is too large for the target phone, you take two choices.

The beginning is to cut the SIM down to size. Some people are dauntless enough to do this using nil but a pair of scissors and some sort of template. If you accidentally cut into the circuity of the SIM carte, information technology's game over and you lot'll demand a new one. The safest way to do this is past using a dedicated SIM cut tool. Fifty-fifty then, you run the adventure of destroying the carte, so have this route only at your own chance.

The other pick is to switch SIM cards.  Of course, if you just went out and bought a new SIM card, it would have a different telephone number associated with it! Each service provider may have a unlike procedure to motility your phone number from ane SIM to some other.

In general however, it should only take a phone call and verification of your identity. If the provider has physical stores, you should even accept the pick of going in for a new SIM, with your old i disabled.

These days it'due south popular to use text messages sent to specific SIM cards as a class of 2-cistron authentication. So now we have something known as SIM-swap fraud. Where criminals find ways to have your telephone number transferred to a SIM they control. SIM duplication is another method. Information technology's certainly something to scout out for.

Life Afterward SIM Cards

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

In some countries, "dual-SIM" phones have been very popular. With these you could accept 2 telephone numbers, 1 for personal employ, for example. Y'all could as well accept a dedicated data SIM card, perchance with ameliorate rates.

eSIMs make this redundant and turns admission and authentication into something similar to fixed-line broadband usage. Where y'all just enter a username and password to access a given service provider.

Few current phones have embedded SIM cards at the time of writing, but that'southward prepare to change as the industry adopts this new standard. Y'all next phone might very well have no place to put a SIM card at all, which would be the end of an era, simply certainly a motility for the better.