Different Tasks and States of RRC Protocol in LTE-EUTRAN
The RRC protocol for EUTRAN is responsible for the basic configuration of the radio protocol stack. But one should note, that some radio management functions (scheduling, physical resource assignment for physical channels) are handled by layer 1 and layer 2 autonomously. MAC and layer 1 signaling has usually delays that are within 10 ms, whereas RRC signaling usually takes something around 100 ms and more to complete an operation.
The RRC functional list is of course quite long.
System Information Broadcasting: The NAS and access stratum configuration of the network and the cell must be available to any UE camping on a cell. This information is coded as RRC message.
Paging: To locate an LTE_IDLE UE within a tracking area the RRC protocol
defines a paging signaling message and the associated UE behavior.
RRC Connection Management: The UE can have two major radio states:
RRC_CONNECTED or RRC_IDLE. To switch between the states an RRC connection establishment and release procedure is defined. With the state
RRC_CONNECTED the existence of signaling radio bearers and UE identifiers (CRNTI) is associated.
EUTRAN Security: Access layer security in EUTRAN consists of ciphering
(PDCP) and integrity protection for RRC messages.
Management of Point-to-Point Radio Bearers: Point-to-point radio bearers are signaling and user data radio bearers for SAE bearers. RRC is used to create, modify and delete such radio bearers including the associated lower layer configuration (logical channels, RLC mode, transport channels, multiplexing, …).
Mobility Functions: When a UE is in state LTE_ACTIVE, the mobility control is at the eNB. This includes handover from one EUTRAN cell to another or also inter system changes. To assist handover decisions in the eNB RRC defines procedures for measurement control and reporting. In LTE_IDLE mode the UE performs automatic cell re-selection, RRC takes control over this process within the UE.
MBMS (Multimedia Broadcast Multicast Service): RRC is used to inform UEs about available MBMS services in a cell and is also used to track UEs that registered for a certain multicast service. This allows the eNB to manage MBMS radio bearers which are usually point-to-multipoint.
QoS Control: The RRC protocol will be QoS aware, allowing implementation of radio bearers with different QoS within the UE.
Transfer of NAS Messages: NAS messages are sent and received through the EUTRAN protocol stack. RRC provides carrier services for such messages.
RRC will use one or two radio bearers exclusively used for signaling (Signaling
Radio Bearers). One will be for high, the other for low priority. The PDCP entities of these signaling radio bearers will be used for ciphering, but not for header compression.
The RRC protocol in EUTRAN defines two state for a UE: RRC_IDLE and RRC_CONNECTED. In the first state, the UE is not attached to a eNB and does free cell re-selection. In the second state the UE is connected to a eNB and the eNB handles all mobility related aspects of the UE via handovers. There is of course a close relationship between LTE-states and RRC states.