nd others are defin- ing standardization of functional splits of various units that make up the RAN, as well as the interfaces between these units. This introduces the concepts of the central unit, distributed unit, and radio unit as well as the Integrated Small Cell. Between them, these units make up the bulk of the O-RAN-based RAN deployments. Chapter 1 Open radio access network overview 41 It is best to understand the various aspects involved in system design by going through an example. We pick one of an indoor, enterprise deployment that provides sufficient cover- age for small- and medium businesses and use this example to define the set of questions that the system architect needs answers to in order to define the "sandbox" she or he gets to play in. This starts defining which boxes need to be build, connectivity between them, per- formance targets, and SO on. We will use this (simple!) example where required throughout the remainder of this book as a use case where relevant to show dimensioning aspects. References [1] O-RAN Alliance. Available at https://www.o-ran.org. [2] Study on new radio access technology: Radio access architecture and interfaces. TR, 38 (801), 2016. [3] Small Cell Forum. Available at https://www.smallcellforum.org/ Further reading "Study on new radio access technology: Radio access architecture and interfaces, V14.0.0 (2017-03)," 3GPP, Sophia Antipolis, France, Rep. TR 38.801, 2017. 802.1CM Time-Sensitive Networking for Fronthaul (802.1CM). Available at https://1.ieee802.org/tsn/802- 1cm/. This page intentionally left blank System components, requirements, and interfaces This chapter outlines the key functionality that encompasses the O-RAN system: the central unit (CU), distributed unit (DU), radio unit (RU), and integrated small cell (ISC). We also dis- cuss the key interfaces between these components and include relevant standard references. The objective of this chapter is not to provide a deep-dive training on 3GPP-this is done very well in many books and online tutorials. We gi