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.

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.