ace, SO CPRI carries the necessary phase/time information. This means that the timing signal needs only to be delivered to the BBU, and CPRI looks after the distribution of that signal to the RU. So, it is important for the network engineer to understand that the phase/time signal needs to be de- livered only to the BBU/eNB, wherever it is located. From that point onward, CPRI looks after the synchronization between the BBU/eNB and the RU. The different timing points for phase alignment are shown in Figure 10-24. Mobile core site +1.1 us PRTC to eNB hotel +1.1 us PRTC to eNB 400 ns over CPRI links to RU 400ns ((cp)) ((c)) ((c)) Cell site Distributed RAN Centralized RAN Figure 10-24 Time Synchronization Architecture with CPRI-Based front-haul As shown in Figure 10-24, the network phase requirement of +1.1 us is to be delivered to the BBU/eNB. The other 400 ns of budget to meet the overall radio requirement of +1.5 us is consumed in the radio equipment and the CPRI network. By centralizing the processing, the CRAN architecture can better utilize advanced techniques like eICIC and CoMP to improve the efficiencies and quality of the available radio resources. This re- duction of the latency between the BBUs/eNBs allows closer radio coordination, supports improved spectrum utilization, and reduces cost by optimizing the amount of equipment and improving opera- tional efficiency. CPRI over the Packet Network CPRI can be thought of as digitized radio signals because it represents the raw output after all the baseband radio processing has taken place (the same in the receive direction, CPRI is carrying raw digitized radio signals before radio processing in the BBU). There are three important characteristics of CPRI traffic that need to be explained because they im- pact directly on 5G: It is a TDM-like transmission, meaning that it is always communicating and that it must trans- mit at specific time intervals. It can be thought of as being like an E1/T1 circuit, and like E1/T1, CPRI circuits need frequency synchronizatio