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.

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.