Question: In Control Plane CIoT EPS optimization for a UE experiencing RLF, what is the primary purpose of the UE calculating and sending UL_NAS_MAC in the RRCConnectionReestablishmentRequest message?
Option 1: To authenticate the UE's identity to the target eNB and ensure it's communicating with a genuine network.
Option 2: To encrypt the NAS COUNT for secure transmission over the air interface.
Option 3: To synchronize the uplink NAS COUNT between the UE and the MME without an AS security context.
Option 4: To indicate to the network that the UE is capable of CIoT EPS optimization.
Explanation: The correct answer, Option 1, highlights the dual purpose of UL_NAS_MAC in authenticating the UE's request for re-establishment and verifying the network's genuineness. This is crucial since AS security is not available in Control Plane CIoT EPS optimization. The other options are incorrect because encrypting NAS COUNT (Option 2) is not the primary purpose here, synchronizing NAS COUNT (Option 3) is a secondary effect of the procedure, and indicating CIoT capability (Option 4) is not directly related to UL_NAS_MAC's role in this context.