eNB ng-eNB Figure 3.3.1 Overall 5G System Architecture. (Source: Reproduced by permission of © 3GPP). 5 Only parts of 5GC and only monolithic (i.e. no-split) gNB are shown in the figure. 6 In some specifications, the NG interface is referred to as N2 and N3 (interfaces or reference points), corresponding to NG-C and NG-U, respectively. 7 Referred to as N2 and N3 in Chapter 3 and core network specifications. 3 5G System Overview handovers to LTE, but also tight NR-LTE interworking via dual connectivity (E-UTRA-NR Dual Connectivity [EN-DC]). Compared with Evolved Universal Terrestrial Radio Access Network (E-UTRAN), NG-RAN is somewhat more complex not only because it supports both NR and LTE access technologies, but also because it can be deployed in various split and non-split architecture variants, as explained in Chapter 4. As the NG-RAN and especially new architecture options introduced in 5G are the primary focus of this book, in the present section we only describe high-level NG-RAN functions and their differences compared with E-UTRAN. When discussing the NG-RAN interfaces, we make the (somewhat artificial) distinction between NG-RAN internal and external inter- faces, with the focus of the present section being the external ones. For the purpose of this discussion we currently assume that a gNB (or en-gNB) is a single monolithic network node, terminating network interfaces toward the core network and other NG-RAN nodes and implementing all of the required functionality: from network interfaces, to air interface protocol stack, to physical layer, RF and antennas. In 3GPP specifications, NG-RAN network nodes are defined in terms of radio and net- work interfaces they support, specifically: Functionality of NR (in the case of gNB) and LTE (in the case of ng-eNB) air interfaces toward a UE; Functionality of NG interface toward the core network (i.e. 5GC); Functionality of Xn interface toward other NG-RAN network nodes. The functionality required to support the radio and network interfaces mentioned above is som