When eNodeB wants to release existing radio connection for a particular UE. It sends RRC Connection release message to UE over DCCH channel. There might be various causes to get triggered this message from eNodeB to UE. This might be due to user inactivity, CS-fall back etc. When UE moves from RRC-IDLE to RRC-Connected, at that time only SRB1 is established. It means RRC connection setup refer SRB1 establishment but RRC Connection release refers releasing of SRB1 along with SRB2 and DRBs.
