A SIM - short for Subscriber Identity Module - is a piece of plastic that slots into your mobile phone.It acts as your unique ID and helps you connect to, make calls over a particular mobile phone network.Most SIM cards have a capacity between 32 to 128 KB.They store contacts and messages which can be transferred easily from one mobile to another. SIM also stores the ‘key’ needed to decrypt data that is encrypted when voice is trransfered from a phone to another.A unique ID called the IMSI (International Mobile Subscriber Identity) is used to connect someone calling your mobile number to your phone. GSM phones are the ones that utilize SIM cards .The most important bits of data include the IMSI (International Mobile Subscriber Identity) and the authentication key that validates the IMSI. This authentication key is provided by the carrier SIM authentication goes like this: On startup, the phone obtains the IMSI from the SIM card and relays it to the network,k...