Back in the good old days telephones used to have physical wires connecting one end to another. Whenever you dial a number your telephone company could row your call exactly where it needed to go using said cable, but in this brave new world of not only phone calls but vines, Facebook and other stuffs that the kids are doing that are being blasted out from cell towers everywhere. These signals need to know where exactly they are going. This is where SIM cards come in and it stands for “Subscriber Identity Module” which gives a pretty big hint of what it does. It identifies what the mobile phone subscriber is using a particular Iphone, Galaxy or a Windows device.
Although SIM cards are quite low capacity holding a megabyte of data, but the small amount of data they do hold extremely important piece of data, if you want your phone to be of any use when you are not using WiFi. SIM cards use a 64-Bit that use a unique identifier although that takes only 8 Bytes of storage it can store 9 quintillion unique numbers it can be stored in this way. So this should work no matter how many people signup for a single operator. When you turn on your phone on first connect to the cell network then the phone will pass your SIM card ID number along with an authentication key to your cell phone provider. The provider then generates a random number and uses key to spit out the response number at the same time and the random number is sent back to your phone and the same calculation is done with the authentication key to generate another response number. If the two number match then the service provider will recognize this and connect your phone to the network both ensuring that it is actually you that’s connected to keep your data safe and to see who you are so provider can block access if you don’t pay the bill or something like that.
Apart from all these SIM cards can also store phone book information’s, you can see this as a useless feature but it was very important earlier when people used to have phones like flip flop phones and you upgrade your phone( As we didn’t have feature like Google sync). So SIM cards are proved quite efficient when you switch your phone and don’t have to change your number.