C subPDUs are concatenated one after another by LCP (see Section 7.6.6) in order to ease pipeline processing in both the receiver and the transmitter. Another difference compared to LTE is the asymmetric structure: in the downlink, the MAC CEs are multiplexed at the front of the PDU, while in the uplink they are appended at the end. This gives more time for the UE to process the MAC CEs it receives (in the downlink), and compute the MAC CEs it needs to send (in the uplink). Figure 7.14 summarizes the PDU structure in the downlink, while Figure 7.15 depicts the uplink. The RRC Protocol 7.7.1 Overview The main task of the RRC protocol is to configure the UE with the parameters needed by other protocol layers, and to maintain connectivity between the UE and the network. The RRC protocol configures UEs in all RRC states and is responsible for the connection 7.7 The RRC Protocol TA Command DRX Command SCell Activation/Deactivation Duplication Activation/Deactivation SP CSI-RS/CSI-IM Resource Set Activation/Deactivation Aperiodic CSI Trigger State Subselection TCI States Activation/Deactivation TCI State Indication SP CSI reporting on PUCCH Activation/Deactivation PUCCH Spatial Relation Activation/Deactivation SP ZP CS-RS Resource Set Activation/Deactivation Recommended Bit Rate MAC CE1 MAC CE padding subPDU- MAC PDU Figure 7.14 Downlink MAC PDU structure. Long BSR Short BSR Long Truncated BSR Short Truncated BSR Single Entry PHR Multiple Entry PHR Configured Grant Confirmation Recommended Bit Rate Query MAC CE MAC CE padding subPDU MAC PDU Figure 7.15 Uplink MAC PDU structure. control, broadcast of system information (SI), measurement configuration and report- ing, and other related functions (e.g. inter-RAT mobility, security activation, transfer of NAS protocol information). There are three RRC states in NR: IDLE, INACTIVE, and CONNECTED. All other state transitions except IDLE to INACTIVE are allowed states based on various triggers, as illustrated in Figure 7.16. The UE operations in each RRC state are different: 