Question: In E-UTRAN, under what condition(s) does the network initiate the security mode command procedure to a UE in RRC_CONNECTED state, specifically in relation to the establishment of Signaling Radio Bearers (SRBs) and Data Radio Bearers (DRBs)?
Option 1: After both SRB1 and SRB2 are established, but before any DRB is set up.
Option 2: When only SRB1 (or for NB-IoT, SRB1 and SRB1bis) is established, prior to setting up SRB2 and/or any DRBs.
Option 3: Immediately after the UE transitions to RRC_CONNECTED, regardless of SRB/DRB establishment.
Option 4: Only when at least one DRB is established in addition to SRB1.
Explanation: This option correctly reflects the specified condition for initiating the security mode command procedure. It highlights the network's (E-UTRAN) action specifically when the connection is minimal, i.e., only the basic signaling bearer(s) [SRB1, and optionally SRB1bis for Narrowband Internet of Things (NB-IoT)] are established. This ensures security measures are put in place early on, before more significant data or signaling exchanges occur with the setup of additional bearers (SRB2 and/or DRBs).