 may only consist of a data field with no SDAP header. As shown in Fig. 2.21, the SDAP data PDU in the downlink or uplink consists of a header and data fields, where the headers for the downlink and uplink are different. For each downlink SDAP data PDU, in which reflective QoS indication (RQI) is set to one, the SDAP entity would inform the NAS layer of the RQI and QFI. The end-marker control PDU is used by the SDAP entity at UE to inform the gNB that the SDAP SDU QoS flow mapping to the DRB on which the end-marker PDU is transmitted, has stopped. The D/C bit specifies whether the SDAP PDU is an SDAP data PDU or an SDAP control PDU. The QFI field identifies the QFI to which the SDAP PDU belongs. The RQI bit indicates whether NAS should be informed of the updated SDF to QoS flow mapping rules and the reflective QoS flow to DRB mapping indication bit implies whether QoS flow to DRB mapping rule should be updated [3]. 236 Chapter 2 2.3 Layer 3 Functions and Services 2.3.1 Radio Resource Control Sublayer The RRC sublayer consists of control-plane set of protocols for connection control and setup, system configuration, mobility management, and security establishment. It is further responsible for broadcast of SI including NAS common control information and information applicable to UEs in RRC_IDLE and RRC_INACTIVE states, for example, cell selection/ reselection parameters, neighboring cell information, as well as information applicable to UEs in RRC_CONNECTED state, for example, common channel configuration information [14]. The RRC connection control functions include paging; establishment/modification/sus- pension/resumption/release of RRC connections including assignment/modification of UE identity; establishment/modification/suspension/resumption/release of SRBs except SRBO; access barring; initial security activation including initial configuration of AS integrity pro- tection (SRBs, DRBs), and AS ciphering (SRBs, DRBs); mobility management including intra-frequency and inter-frequency handover, associated secur