ntrol in connected state (i.e. handover) and in inactive state; UE measurements, including measurement configuration and processing of UE mea- surement reports; Routing of user-plane and control-plane packets toward user-plane function (UPF) and Access and Mobility Management Function (AMF), respectively; UE Quality of Service (QoS) flow management and mapping to radio bearers; Slicing; Tight interworking between NR and Long-Term Evolution (LTE), including multiple dual connectivity (between these technologies) variants; Radio access network sharing between multiple operators. In standards (e.g. 3GPP), a gNB is typically described in terms of the network and air inter- faces it supports, protocols it implements to run on these interfaces, and the functionality it provides. While the interfaces and protocols are typically specified in a sufficient level of detail to allow interoperability, the rest of the functionality (in particular, significant parts of the functionalities listed above) is intentionally described at the high level to allow different implementations and differentiation between vendors. As this book is primarily concerned with architectural aspects and because gNB implementation details are proprietary and dif- fer between different vendors, we therefore proceed with the same approach as the one taken by standards to describe the "monolithic" gNB architecture in terms of the interfaces and protocols it supports. Figure 4.1.1 illustrates the gNB architecture at the most abstract level. The gNB terminates the Xn (which is used to communicate with other gNBs) and NG (which is used to communicate with the 5GC) network interfaces and the Uu air interface (which is used to communicate with a UE). Detailed description of the Xn and NG network interfaces can be found in Section 3.3, while the physical layer and the protocol stack of the Uu interface are described in Sections 3.5 and 3.4, respectively. 4.1 Introduction Figure 4.1.1 Monolithic gNB architecture. Note that this gNB representation omits certai