processing requirements of different applications. As shown in Fig. 1.25, a CU typically hosts non-real-time RAN protocols and functions offloaded from the core network as well as MEC services. Accordingly, a DU is primarily responsible for physical layer processing and real-time processing of layer-2 functions. In order to relax the 5G Network Architecture transport requirements on the fronthaul link between the DU and the RRUs, some physical layer functions can be relocated from the DU to the RRU(s). From the equipment point of view, the CU equipment can be developed based on a general-purpose platform, which sup- ports RAN functions, functions offloaded from core network, and the MEC services, whereas the DU equipment must be (typically) developed using customized platforms, which can support intensive real-time computations. Network function virtualization infra- structure allows system resources, including those in the CU and the DU, to be flexibly orchestrated via MANO, SDN controller, and traditional network operation and maintenance center, supporting fast service rollout. In order to address the transport challenges between CU, DU, and RRU(s), the NGFI standard has been developed where an NGFI switch net- work is used to connect the C-RAN entities. Using the NGFI standard, the C-RAN entities can be flexibly configured and deployed in various scenarios. In case of ideal fronthaul, the deployment of DU can also be centralized, which could subsequently support cooperative physical layers. In case of non-ideal fronthaul, the DU can be deployed in a fully or par- tially distributed manner. Therefore C-RAN architecture in conjunction with the NGFI Evolved packet core (EPC) 5G core (5GC) Separation Backhaul Backhaul Central unit plants L2/L3 Baseband processing unit (BBU) CU-DU interface (F1) Distributed unit (DU) Fronthaul Remote radio Remote radio Radio- head (RRH) head (RRH) Figure 1.25 BBU architecture and evolution to 5G [47]. 62 Chapter 1 standard can support CU and DU deployments [47]. Latency, cost, and