Question: What action does a UE take when upper layers request establishment of an RRC connection while the UE is in RRC_IDLE and it has acquired essential system information?
Option 1: The UE initiates the procedure and ensures having valid and up to date essential system information before proceeding.
Option 2: The UE directly sends an RRCSetupRequest message without performing any checks or procedures.
Option 3: The UE enters a barred state and ends the procedure.
Option 4: The UE applies default L1 parameter values without considering SIB1.
Explanation: According to the specification, when upper layers request establishment of an RRC connection while the UE is in RRC_IDLE and it has acquired essential system information, the UE initiates the procedure. Before proceeding, the UE ensures having valid and up to date essential system information as specified in clause 5.2.2.2. This step is crucial for the UE to establish a successful RRC connection.