So that user data passing over the radio path is protected. Anonymity Anonymity is provided by using temporary identifiers. When a user first switches on his radio set, the real identity is used, and a temporary identifier is then issued. From then on the temporary identifier is used. Only by tracking the user is it possible to determine the temporary identity being used.
Authentication is performed by a challenge and response mechanism.
The operator can check that, given the key of the mobile, the response to the challenge is correct. Eavesdropping the radio channel reveals no useful information, as the next time a new random challenge will be used. Authentication can be provided using this process. A random number is generated by the network and sent to the mobile. The network can check that the mobile really has the secret key by performing the same SRES process and comparing the responses with what it receives from the mobile.