um access Control/configuration control (MAC) Transport channels| Physical layer (PHY) Control/configuration/ physical layer measurements Physical channels Figure 1.64 Mapping of lower NR protocol layers to OSI network reference model. varying the transmission format of the transport blocks, the MAC sublayer can realize dif- ferent data rates and reliability levels. The MAC sublayer receives the RLC SDUs mapped to various logical channels in the downlink, and generates the MAC PDUs that further become the transport blocks in the physical layer. The RLC sublayer provides RLC chan- nels to the PDCP sublayer and the latter provides radio bearers to the SDAP sublayer. As we mentioned earlier, radio bearers are classified into data radio bearers for user-plane data and SRBs for control-plane information. The SDAP sublayer is configured by RRC and maps QoS flows to DRBs where one or more QoS flows may be mapped into one DRB in the downlink; however, one QoS flow is mapped into only one DRB at a time in the uplink 5G Network Architecture User-plane Access and mobility function management (UPF) function (AMF) User-plane ((HII) protocol Control-plane stack protocol stack Control/ User data signaling Figure 1.65 NG-RAN protocol stack [16]. [16]. The introduction of a new sublayer in NR layer-2 was meant to support the improved flow-based QoS model in NR as opposed to bearer-based QoS model in LTE. MAC sublayer is responsible for mapping between logical channels and transport channels and multiplexing/de-multiplexing of MAC SDUs belonging to one or different logical chan- nels to transport blocks which are delivered to or received from the physical layer through the transport channels. The MAC sublayer further handles scheduling and UE measurements/ reporting procedures as well as error correction through HARQ (one HARQ entity per carrier in case of CA). It further manages user prioritization via dynamic scheduling, priority han- dling among logical channels of one UE through logical channel prioritization. A single MAC ins