Radio Resource Control (RRC)

The Radio Resource Control (RRC) protocol is used in UMTS and LTE on the Air interface. It is layer that exists between UE and eNB and exists at the IP level. This protocol is specified by 3GPP in TS 25.331[1] for UMTS and in TS 36.331 [2] for LTE. RRC messages are transported via the PDCP-Protocol.

The major functions of the RRC protocol include connection establishment and release functions, broadcast of system information, radio bearer establishment,reconfiguration and release, RRC connection mobility procedures, paging notification and release and outer loop power control.[3] By means of the signalling functions the RRC configures the user and control planes according to the network status and allows for Radio Resource Management strategies to be implemented.[4]

The operation of the RRC is guided by a state machine which defines certain specific states that a UE may be present in. The different states in this state machine have different amounts of radio resources associated with them and these are the resources that the UE may use when it is present in a given specific state.[4][5] Since different amounts of resources are available at different states the quality of the service that the user experiences and the energy consumption of the UE are influenced by this state machine.[5]