Note: The information in this article should apply to both iPhones and Android phones (no matter who made your Android phone: Samsung, Google, Huawei, Xiaomi, etc).
SIM stands for subscriber identity module or subscriber identification module. It would then follow that a SIM card contains unique information that identifies it to a specific mobile network, which allows the subscriber (like you) to use the communication features of the device.
SIM cards are the small cards which contain a chip that you need to insert into a GSM phone before it will work. Without a SIM card, a GSM phone won’t be able to tap into any mobile network. The card holds all the critical information.
Some phones need a SIM card in order to identify the owner and communicate with the mobile network. So, if you have, say, an iPhone on Verizon’s network, it needs a SIM card so that Verizon knows that the phone belongs to you and that you’re paying for the subscription, but also so that certain features will work.
You may have been in a situation where you get a used phone that’s missing a SIM card and soon realize that it doesn’t work as anything but an expensive iPod. While you might be able to use the device on Wi-Fi and to take pictures, you can’t connect to any carrier’s mobile internet network, send text messages, or make phone calls.
What information does a SIM card hold? The most important bits of data include the IMSI (International Mobile Subscriber Identity) and the authentication key that validates the IMSI. The carrier provides this key.
If you’re interested in the nitty-gritty, SIM authentication goes like this:
On startup, the phone obtains the IMSI from the SIM card and relays it to the network. Think of this as the “request for access.”
The network takes the IMSI and looks in its internal database for that IMSI’s known authentication key.
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.
Long story short: this data not only determines which network to connect to but also acts as the “login credentials” which allow a phone to use said network.
Some SIM cards are mobile, which means if you put it in an upgraded phone you just purchased, the phone number and carrier plan details will now “magically” start working on that phone. On that note, if your phone runs out of battery and you desperately need to make a phone call, and you have a spare around, you can just put the SIM card into the other phone and immediately use it.
Note: In these instances with CDMA phones, the SIM card is most likely used because the LTE standard requires it, or because the SIM slot can be used with foreign GSM networks.
The first SIM cards were roughly the size of a credit card and were the same shape around all the edges. Now, both Mini and Micro SIM cards feature a cut-off corner to help prevent incorrect insertion into the phone or tablet.
Here are the dimensions of the different types of SIM cards.
Full SIM — 85 mm x 53 mm
Mini SIM — 25 mm x 15 mm
Micro SIM — 15 mm x 12 mm
Nano SIM — 12.3 mm x 8.8 mm
Embedded SIM — 6 mm x 5 mm