tes one possible structure for the SDAP sublayer; however, the actual implementations may vary. The SDAP sublayer is configured by RRC. It maps the QoS flows to DRBs. One or more QoS flows may be mapped onto one DRB. However, in the uplink, one QoS flow is mapped to only one DRB at a time. The SDAP entities are located in the SDAP sublayer. Several SDAP entities may be defined for a UE. One SDAP entity is New Radio Access Layer 2/3 Aspects and System Operation 235 Octet 1 Octet 1 Octet 2 Octet N SDAP data PDU format without SDAP header Octet N Downlink SDAP data PDU format with SDAP header Octet 1 Octet 1 Octet 2 Uplink SDAP data PDU format with SDAP header Octet N Uplink SDAP data PDU format with SDAP header Figure 2.21 SDAP PDU formats [3]. configured for each individual PDU session. An SDAP entity receives/delivers SDAP SDUs from/to upper layers and transmits/receives SDAP data PDUs to/from its peer SDAP entity via lower layers. In the transmitting side, when an SDAP entity receives an SDAP SDU from upper layers, it constructs the corresponding SDAP data PDU and submits it to lower layers. In the receiving side, when an SDAP entity receives an SDAP data PDU from lower layers, it retrieves the corresponding SDAP SDU and delivers it to upper layers. Fig. 2.20 illustrates the functional block diagram of the SDAP entity for the SDAP sublayer [3]. Note that the reflective QoS flow to DRB mapping is performed at UE, if downlink SDAP header is configured. The SDAP sublayer transfers user-plane data and exclusively provides ser- vices to the user-plane upper layers. As shown in Fig. 2.20, the SDAP sublayer supports transfer of user-plane data, mapping between a QoS flow and a DRB for both downlink and uplink; marking QFI in both downlink and uplink packets; and reflective QoS flow to DRB mapping for the uplink SDAP data PDUs. The SDAP data PDU is used to convey SDAP header and user-plane data. An SDAP PDU or SDU is a bit string that is octet-aligned (Fig. 2.21). An SDAP SDU is included in an SDAP PDU. An SDAP data PDU