Section: 9.4.9.3 Authentication Parameter AUTN
Content: This IE shall be present if and only if the authentication challenge is a UMTS authentication challenge.The presence or absence of this IE defines- in the case of its absence- a GSM authentication challenge or- in the case of its presence- a UMTS authentication challenge. The MS shall ignore the IE if a SIM is inserted in the MS. In UMTS, the MS shall reject the AUTHENTICATION & CIPHERING REQUEST message as specified in subclause 4.7.7.5.1 if this IE is not present and a USIM is inserted in the MS.