scribed in this chapter should be viewed primarily as a model that implementations follow with varying degrees of rigor and precision. That being said, most implementations do follow these architectures. Furthermore, unlike 4G, in 5G we may see larger numbers of multi-vendor network deployments (which is already happening with at least one green field operator), in which case the standards-based network interfaces and architectures described in the present chapter will be of great importance. It is perhaps worth noting that the term "NG-RAN architecture" as used in this chapter (and often in the industry) is somewhat confusing, as sometimes it refers to a collection 5G Radio Access Network Architecture: The Dark Side of 5G, First Edition. Edited by Sasha Sirotkin. © 2021 John Wiley & Sons Ltd. Published 2021 by John Wiley & Sons Ltd. 4 NG-RAN Architecture of base stations and their interactions (as is the case of multiconnectivity described in Section 4.3) and sometimes it refers to an architecture of a "split" base station, in which a "monolithic" base station is further split into functional blocks with standards-based interfaces between them. With this in mind, we start by describing what we consider as a "monolithic" gNB archi- tecture, followed by high-level considerations about studies undertaken in 3GPP and other organizations on potential split of gNB functionalities into separate logical or physical network nodes. In the following sections of this chapter we finally describe various split architectures defined in relevant standards development organizations and industry fora. 4.1.1 Monolithic gNB Architecture In the "monolithic" architecture, a gNB is depicted as a single logical network node imple- menting all the required functionality, which include among other things: User equipment (UE) admission control over the radio interface; UE radio interface connection setup and release; Radio resource management, including UE radio bearer control, and uplink and down- link scheduling for a UE; UE mobility co