The RRC of NB-IoT processes the third layer information of the control plane between the UE and the eNB base station and performs system message broadcast, paging, RRC connection management, radio bearer control, radio link failure recovery, idle state mobility management, NAS layer Functions such as information exchange, access layer security, and provision of parameter configuration for underlying protocols. Módulo SPIThe RRC protocol has two states, namely RRC_Idle (idle state) and RRC_Connected (connected state). In NB-IoT, low-speed mobility of UE is supported, cell reselection in RRC_Idle idle state is supported, and cell handover in the RRC_Connected state is not supported.Generally speaking, the NB-IoT protocol stack is designed based on the LTE system, but according to the requirements of the Internet of Things business, some unnecessary functions are removed, and the overhead of the protocol stack processing process is reduced. Therefore, from the perspective of the protocol stack, NB-IoT is a brand-new air interface protocol.Taking SRB as an example, NB-IoT supports three SRBs: SRB0, SRB1, and SRB1bis. NB-IoT does not support SRB2 in the LTE system, the purpose is to reduce the encapsulation overhead of PDCP security functions, but at the same time, it introduces SRB1bis. DTU GPRSFor NB-IoT UE, SRB0 is used to transmit RRC messages (including connection establishment, connection recovery, and connection re-establishment), which are carried on the logical channel CCCH. In CP mode, SRB1bis is used to transmit RRC messages and NAS messages, which are carried on the logical channel DDCH. SRB1bis is a new dedicated SRB (compared to SRB1 but without PDCP layer configuration). In UP mode, only SRB1 is used to transmit RRC messages and NAS messages, which are carried on the logical channel DCCH.
