Specifically, its functions mainly reflect the following aspects. The data stored by SIM card can be divided into four categories: the first category is fixed data. This type of data is written by the SIM card center before the mobile phone is sold, including the International Mobile Subscriber Identification Number (IMSI), authentication key (KI), authentication and encryption algorithm, etc.

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.