According to GSM recommendations, authentication has to be done when the subscriber enters the network (IMSI Attach) and in Inter VLR Location update (“LOC UP NEW VIS). Authentication in other cases is necessary only for the changing of the ciphering key (Ciphering in used).
The usage of authentication increases the network load between the VLR and the HLR, and also increases call setup, SMS and LU times, meaning the fetching of the triplets from the AUC.
For security reasons ciphering key should be changed once in the while. That’s why to do Authentication in Mobile Originated and Terminated calls.
In this example(Nokia Printout) except LOC UP(Location Update) IMSI Attach and Periodic location update Auth is OFF even for MOC,MTC and…
LOC UP NEW VIS: 1 LOC UP: 1 PER UP: 1
IMSI ATTACH: 1 MO CALL: 0 MO SMS: 0
MT CALL: 0 MT SMS: 0
MT USSD: 0 SS OPER: 0
Hope it comes to use