Question: In a scenario where an SGSN supports interworking between E-UTRAN and GERAN, what authentication method is used for a UE that supports it, when attached to GERAN?
Option 1: UMTS AKA (Authentication and Key Agreement)
Option 2: EPS AKA (Authentication and Key Agreement for Evolved Packet System)
Option 3: GSM AKA (Authentication and Key Agreement for Global System for Mobile Communications)
Option 4: LTE CA (LTE Conditional Access)
Explanation: According to the security architecture for interworking between E-UTRAN and GERAN, if a UE supports UMTS AKA, it is required to be authenticated using UMTS AKA when attaching to an SGSN that supports this interworking, even if the UE is currently attached to a GERAN network. This ensures seamless security context handling across different radio access technologies.