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. 1 Introduction Internet Core Network ((q)) ((p)) ((q)) ((q)) Figure 1.1 Radio Access Network (RAN). services in a single box (as shown in Figure 1.1), that is rarely the case. In practice, most base stations are split into multiple nodes in a variety of architectures, interconnected by sometimes standardized and sometimes proprietary network interfaces in a variety of architectures. Network interfaces themselves, illustrated as straight lines, in practice are anything but straight. What is often overlooked is that these interfaces run on a transport network, which often consists of various technologies - multiple transport network nodes inter- connected in various network topologies. This book is dedicated to the topic of RAN architectures and technologies. It is structured as follows: In Chapter 2 ("Market Drivers") we describe the technological, regulatory, and business driving forces behind 5G in general and how these diverse requirements, challenges, and marketing considerations affect the RAN. Before we dive into the details of RAN architectures, in Chapter 3 ("5G System Overview") we provide a high-level overview of all the components of a 5G system: the core network, the air interface protocol stack, and the air interface physical layer. These help put the RAN architectures discussed afterward into a proper context. Chapter 4 ("NG-RAN Architectures") is perhaps the main part of the book, where we describe in detail all the 5G RAN architectures defined in the 3rd Generation Partner- ship Project (3GPP), O-RAN Alliance, and Small Cell Forum, specifically: the high-level Introduction gNB CU/DU (central unit-distributed unit) split, the multi-connectivity architectures, the gNB architecture with control/user separation, the low-level gNB intra-PHY split, and the small cell architectures. Chapter 5 ("NG-RAN Evolution") is dedicated to NG-RAN e