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 second category is temporarily stored data related to the network. Such as location area identification code (LAI), mobile user temporary identification code (TMSI), public telephone network code prohibited from access, etc.