dio access network overview 21 Active Ethernet Backplane Back Haul 100GbE To Core Base Station Line Card PDCP/Transport Base Station Line Card PDCP/ Low Physical High Physical MAC/RLC Transport Layer Layer Front Haul To Radio 10/25/50/ Heads 100 GE/CPRI FIGURE 1-17 Centralized RAN architecture. RAN, radio access network. correct interface between these units, and where are the protocols/interfaces defined in an open and interoperable manner? Given high product development cost and timelines, the challenge is now to define appropriate "split" point in the 3GPP processing stack as well as interoperable physical and logical interfaces that allow a minimum set of physical products to target as broad as possi- ble an O-RAN market. This chapter discusses these topics, introducing the O-RAN concepts of central unit, dis- tributed unit, and radio unit. Distributed versus centralized processing Splitting up processing chains into different physical components/locations is a well-studied topic with extended coverage in literature. In various previous C-RAN-related publications, functional splits are presented by different names and focusing on different directions. 3GPP standards started to define these functional splits in 38.801, 2 to formalize potential protocol split options, without dictating an implementation. Functional splits are intended to allow functions to run in a central location (e.g., the IT room in an enterprise deployment) versus distributed in the radio heads (the antenna units that are connected to the baseband unit). 3GPP functional split options are shown in Fig. 1-18. The picture shows that 3GPP allows for any functional split. Practical implementations zoom in on specific use cases and involve trade-offs that center around the following four issues: Fronthaul interface throughput and latency requirements. As more functionality is pushed toward a central location (lower level split), fronthaul throughput increases and latency requirements get tighter. 22 Open Radio Access Network (O-RAN) Systems Arch