In addition to the subscriber-relevant data stored in the Home Location Register( HLR), the encryption data and the data for authentication are stored in the authentication center. Physically, the access protection center is usually located in the same unit as the location register.
The authentication center protects the GSM system against unauthorized use. It also protects the subscriber against misuse of his data stored in the GSM system. The Authentication Center and the SIM card contain the encryption algorithm. The Authentication Center gives this as a derived value to the visitor directory, which needs it for authentication and encryption. The value is valid only for some connections and is recalculated afterwards.