SIM card is the subscriber identification card of digital cellular mobile phone. Its full name is "subscriber identification module". In the GSM digital mobile phone system, the SIM card plays an extremely important role. It is not only an important part of the mobile phone, but also an identity card for every GSM mobile phone user. Therefore, the SIM card realizes the concept of recognizing people and not recognizing machines.

The SIM card and the switch use the authentication algorithm to calculate the authentication key and 8-digit random number at the same time. If the calculation results are the same, the SIM card will be recognized, otherwise the SIM card will be rejected and the user will not be able to make a call. The SIM card can also use encryption algorithm to encrypt the conversation voice to prevent eavesdropping.