. The RQI bit is the NAS reflective mapping bit and is provided to the upper layers of the UE along with QFI of the data packet. In the uplink, the header carries only the QFI and is used by the network for handling the packet over the CN network nodes and interfaces. There is one SDAP entity in the UE per PDU session. On the network side, there may be up to two SDAP entities per PDU session - one in the MN and the other in the SN in case of dual connectivity (as described in Section 4.3). The PDCP layer (specified in 3GPP TS 38.323) provides the security functions of encryp- tion and integrity protection for the data packets. It also offers Robust Header Compression (RoHC) header compression for IP traffic. Both these functions are similar to LTE with the main exception that user data packets can also be configured with integrity protection for better security. PDCP data PDU consists of a header containing the Sequence Number, the data payload, and an optional integrity protection checksum. 3.4 NR Protocol Stack QoS Flows QoS flow QoS flow handling handling Radio Bearers Security Security Security Security RLC Channels Segm. Segm. Segm. Segm. Logical Channels Concatenation / Scheduling / Priority Handling Multiplexing UE1 Multiplexing UE, in Transport Channels Figure 3.4.3 Downlink Layer 2 Structure Protocol. (Source: Reproduced by permission of © 3GPP). Oct 1 Oct 1 Oct 2 Oct 2 Oct N Oct N Figure 3.4.4 Downlink (left) and uplink (right) SDAP Data PDU format with SDAP header. (Source: Reproduced by permission of © 3GPP). PDCP is also responsible for retransmission of packets based on PDCP status reports to ensure lossless delivery of packets for the cases where RLC cannot ensure lossless delivery. This could be when RLC bearers are released or moved to another network node, such as during handover, or with dual connectivity, or CA. PDCP can also be used to duplicate packets across multiple RLC bearers of the RB as a mechanism to improve reliability with lower latency compared with RLC-based retransmission. 3 5G S