Question: What type of security protection is applied to control plane signaling (including S-KeNB transfer) between Master eNodeB (MeNB) and Secondary eNodeB (SeNB) over the X2 reference point in a 3GPP System Architecture Evolution (SAE) network?
Option 1: X2-U security protection for both confidentiality and integrity
Option 2: X2-C security protection for control plane, with no protection specified for S-KeNB transfer
Option 3: No security protection is mandated for control plane signaling over X2
Option 4: IPsec (Internet Protocol Security) for all X2 reference point communications
Explanation: In a 3GPP SAE network, the control plane signaling between MeNB and SeNB over the X2 reference point requires robust security. X2-C security protection is specifically designed for this purpose, ensuring both confidentiality and integrity of control plane signals, including sensitive information like S-KeNB transfer.