Radio Resource Control (RRC)
The Radio Resource Control (RRC) layer controls communications between a UE and an eNB at the radio interface and the mobility of a UE crossing cells. The RRC is the highest layer in the control plane of the Access Stratum (AS). The RRC also transfers messages of the Non-Access Stratum (NAS), which is located above the RRC layer. NAS messages are used to control communications between a UE and the Evolved Packet Core (EPC).
Signaling used to control transmissions of user data at the radio interface is provided by the RRC layer, the MAC layer, or the Physical layer in the AS. Compared to signaling provided by the MAC layer (i.e., MAC Control Elements, see Section 6.11.1) and signaling provided by the Physical layer (i.e., control information on the PDCCH), signaling provided the RRC layer (i.e., RRC messages) has the following characteristics:
- Higher extensibility: RRC messages can be extended easily to accommodate additional control information, for example, for enhancements in future releases of LTE.
- Secured transmission: Integrity protection and ciphering of an RRC message carried on the DCCH can be provided by the PDCP layer. Neither integrity protection nor ciphering is applicable for signaling provided by the MAC layer or the Physical layer.
- Reliable transmission: Missing parts of an RRC message carried on the DCCH can be retransmitted by ARQ operation provided in the RLC layer and HARQ operation provided in the MAC layer. The RRC layer ...