layer is multiplexing data from different logical channels into a transport block and scheduling. Even though 5GC no longer uses the notion of bearers, the concept of radio bearers is kept in NR, which maps 5GC flows to DRBs based on QFI. RRC, which is the control-plane protocol of NR, is also similar to LTE. The protocol has a number of "transparent containers" defined, which can be used to convey e.g. UE configuration messages to be delivered to the UE by an intermediate network node without interpreting the content, e.g. in EN-DC operation (see Section 4.3). Other main differences are related to beam-based measurements and on-demand System Information Broadcast. Unlike LTE, NR RRC supports three different UE states - IDLE, CONNECTED, and INACTIVE. The INACTIVE state is similar to IDLE in terms of UE actions, with the main difference being that the NG-RAN maintains the UE context and the connection to 5GC. These are discussed in more detail in the following sections. 3.4 NR Protocol Stack 3.4.2 NG-RAN Architecture The RAN functions in NR reside in the logical node, gNB. The gNBs are connected by means of the NG interfaces to the 5GC, more specifically to the AMF by means of the NG-C inter- face and to the UPF by means of the NG-U interface. The gNBs are interconnected with each other by means of the Xn interface. The simplified NG-RAN architecture is shown in Figure 3.4.1. A gNB can be further split into different logical nodes as discussed in Chapter 4. 3.4.3 NR User Plane The primary role of the user plane is to transfer data across the radio interface efficiently and with the required QoS. Figure 3.4.2 shows the protocols across the different interfaces for the transfer of a user IP packet between the UPF and the UE. The radio protocols, also called AS, are shown in gray. The higher layers (also called NAS) indicate the QoS handling required for a packet. Each RB has four L2 protocol layers - the upper layers of SDAP (used only for DRBs) and PDCP, along with the lower layers of RLC and MAC (called RLC bearer