, which corre- sponds roughly to a logical channel). Such a split between the upper and lower layers of the DRB allows easier logical separation of the RLC bearer into a different logical node from the upper protocol layers. This split is applied for the CU-DU architecture as discussed in Section 4.2 and dual connectivity architectures as discussed in Section 4.3. Each RB can be configured with more than one of the RLC bearers that may reside in the same cell group for Carrier Aggregation (CA) or a different cell group in case of dual connectivity. If the ((Q)) ((Q)) Figure 3.4.1 NG-RAN architecture. 3 5G System Overview Application Relay Relay GTP-U UDP/IP UDP/IP UDP/IP UDP/IP NR-Uu Figure 3.4.2 User-plane protocol stack. RLC bearers are in different nodes as in dual connectivity, it can provide higher throughput for the RB. Alternatively, data packets can be duplicated and sent over the different RLC bearers for higher reliability, for example with CA duplication. The SDAP layer is common across all the RBs of a PDU session and its role is to split the data into the different RBs of the PDU session in accordance with the QoS information. The overall user-plane functionality can be summarized in Figure 3.4.3. The protocol layers are discussed in more detail in the following sections. Primary functionality of the SDAP (specified in 3GPP TS 37.324) is to map a packet to a DRB based on its QFI. The mapping table is configured either using RRC signaling (often called explicit) or in-band using a SDAP header (called AS reflective) and is discussed fur- ther below. The SDAP layer has an optional configurable header and carries different infor- mation in the uplink and downlink as shown in Figure 3.4.4. In the downlink, the SDAP header includes reflective QoS flow to DRB mapping Indica- tion (RDI) and RQI bits along with the QFI. The RDI bit is the AS reflective mapping bit. On receipt of a packet with this bit set, the UE updates the mapping table to map uplink packets of that QFI to the DRB the packet was received in