Question: In E-UTRAN, during Handover Preparation, what triggers the MME to send a HANDOVER PREPARATION FAILURE message to the source eNB when a CSG Id IE (Closed Subscriber Group Identifier) is received without a Cell Access Mode IE?
Option 1: Failure to authenticate the UE's (User Equipment) identity.
Option 2: Insufficient resources in the target cell for any of the bearers.
Option 3: Unsuccessful access control and no E-RAB (Evolved Radio Access Bearer) has a particular ARP (Allocation and Retention Priority) value.
Option 4: Handover request for a UE already in a call setup process.
Explanation: This is the correct answer because, according to the specifications, if the CSG Id IE is received without a Cell Access Mode IE and access control is unsuccessful, coupled with none of the E-RABs having a particular ARP value, the MME sends a HANDOVER PREPARATION FAILURE message. This scenario highlights the importance of access control and bearer prioritization in handover decisions within E-UTRAN, especially concerning closed subscriber groups.