ve an overview of key functions imple- mented in each of the functional layers from 3GPP as shown in Fig. 2-1 and delve deeper into those aspects that impact connectivity (relevant interface standards) and dimensioning. Next-Generation Radio Access Network overview and terminology Let's first discuss some 5G terminology. The whole 5G network, including base stations, core network components, and interfaces between them, is called the 5G system and is shown in Fig. 2-2. Note how the name for the base station itself evolved from BS (Base Station) in 1G to BTS (Base Transceiver Station) in 2G, NB (NodeB) in 3G, eNB (evolved NB) in 4G to gNB (next-generation NodeB) in 5G. The 5G air interface is called New Radio (NR or 5G/NR), and the radio access network is called the Next-Generation Radio Access Network or NG-RAN. Base Stations (also called NG-RAN Nodes) that build up the 5G NG-RAN can either be defined as gNB when they pro- vide a 5G/NR interface to the User Equipment (UE/client) as we discussed before, or ng-eNB if they provide an E-UTRA [4G Long-Term Evolution (LTE)] interface to the UE. The core network components are called 5G Core Network or 5GC. As if this is not confusing enough, we also have specific terminology for the interfaces between various components. The link between the NG-RAN and the 5GC is defined not only as the NG interface-but also as the N2 [control plane (CP)] or N3 [user plane (UP)] interface by the 3GPP 5G system architecture group documents. The combined N2 and N3 interface set is the equivalent to the S1 interface in 4G/LTE language. The link between gNB and NG-eNB is defined as the Xn interface, the equivalent of the X2 interface in 4G/LTE. These relationships are shown in Fig. 2-2, copied from 3GPP 38.300. Before we delve deeper into the communication links between the various components, we first look at the underlying protocol stacks (Fig. 2-3). Communication is done through a/ any Internet Protocol (IP)-based network: per 3GPP 38.410, UP traffic is carried through a Open Radio Acce