alone deployments, the SDAP service/functionality is not used. As we mentioned earlier, the NG-RAN architecture supports disaggregated gNB where gNB functions are split into a central unit (gNB-CU) and one or more distributed units (gNB-DU) connected via F1 interface. In the case of a split gNB, the RRC, PDCP, and SDAP protocols, described in more detail below, reside in the gNB-CU and the remaining protocol entities (RLC, MAC, and PHY) will be located in the gNB-DU. The interface between the gNB (or the gNB-DU) and the device is denoted as the Uu interface. In the example shown in Fig. 2.4, the SDAP protocol maps the IP packets to different radio bearers, that is, IP packets n and n + 1 are mapped to radio bearer X and IP packet m is mapped to radio bearer y. The SDAP mapping function between a QoS flow and a DRB is due to the new QoS framework which is used in the new radio. The SDAP further marks the QFIs in the downlink due to the use of reflective QoS6 and in the uplink due to the use of new QoS framework. A single SDAP entity (as shown in Fig. 2.20) is configured for each individual PDU session, except for the dual connectivity scenario where two entities can be configured. Reflective QoS flow to DRB mapping is a QoS flow to DRB mapping scheme where a UE monitors the QoS flow to DRB mapping rule in the downlink and applies it to in the uplink [3]. 234 Chapter 2 PDU session PDU session QoS flows QoS flows SDAP-SAP SDAP-SAP SDAP entity SDAP entity SDAP sublayer Radio bearers SDAP-PDU PDCP-SAP PDCP-SDU PDCP-SAP entity entity PDCP sublayer entity entity UE/NG-RAN NG-RAN/UE (transmitting side) (receiving side) QoS flow QoS flow Transmitting Receiving SDAP entity SDAP entity SDAP entity SDAP entity Mapping of QoS flow to a DRB Removing SDAP header SDAP header is SDAP header is configured Reflective QoS flow to not configured DRB mapping Adding SDAP header SDAP header is SDAP header is configured not configured Radio interface (Uu) Figure 2.20 High-level SDAP sublayer functional architecture 3]. Fig. 2.20 illustra