Terminology 

A3 Authentication Algorithm (SIM,AUC)
A5 Ciphering Algorithm (SIM,Mobile Switching Center)
A5/2 (export-insecure) A5/1 (somewhat secure)
A8 Ciphering Key Generation Algorithm (SIM,AUC)

AUC Authentification Center
BS Base Station
HLR Home Location Register
LAI Local Area Identification