Before diving into the details of NG-Radio Access Network (RAN) architecture, it is important to have at least a high-level understanding of the whole 5G System (5GS). To provide such a high-level picture, this chapter describes the functionalities of the physical layer, the protocol stack, the NG-RAN, and the 5G Core (5GC) network. It provides an overview of all the components of 5GS, with emphasis on what is new compared with 4G. 5GS architecture is functionally split into 5GC and RAN (NR) functionalities similar to older cellular technologies. 5GC functionality is responsible for the overall user registrations and session management functions, while NR provides the radio access-related functionalities. The chapter also describes the subset of the most commonly used NG-Application Protocol procedures and the New Radio physical layer, focusing on differences compared with Long-Term Evolution.