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.

SIM card is called "user identification card". It is actually a smart card containing large-scale integrated circuits, which is used to register important data and information of users. The most important function of SIM card is authentication and encryption. When a user moves to a new area to make or receive a call, the switch must authenticate the user to determine whether it is a legal user.