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Every month, you pay a bill for cell phone service. Somehow, that access needs to make it to your device. The SIM card, or "subscriber identity module," connects your phone with your phone bill.
While you might expect authentication to happen over a network with a simple username and password, cell phones predate the Internet, so SIM cards can seem a little old-fashioned. Still, the industry is trying to modernize itself while maintaining backward compatibility with older devices, and that has led to the rise of many different SIM card formats. The newest is iSIM, the latest advancement in a never-ending quest to save space in your smartphone.
Ye olde physical SIM card
Common physical SIM card sizes. (credit: Wikipedia)
Physical SIM cards have been around forever. They're the hunks of plastic you get from your phone carrier and slide into your cell phone. A small gold chip printed on the card—much like those on credit cards—makes electrical contact with the slot in your phone. Even these physical cards have their own standards and have shrunk over time; there's Mini SIM, Micro SIM, and Nano SIM, which all use the same chip with differing amounts of plastic around it.
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