d hence adversely affecting the user experience. To support heterogenous deployments and minimize issues with densification, the standard distrib- uted eNB-based architecture evolved to support a centralized eNB architecture. The distributed ar- chitecture is known as the distributed RAN (DRAN), while the centralized version is called the centralized RAN (CRAN). Figure 10-23 shows the transition from the (traditional) DRAN to the CRAN architecture, with a comparison between the two architectures. Mobile core site Backhaul hotel Backhaul Fronthaul Fronthaul ((ip)) Cell site Distributed RAN Centralized RAN Figure 10-23 Distributed RAN Versus Centralized RAN Architecture in LTE In conventional 4G LTE, the Radio Unit (RU) connects to the Baseband Unit (BBU) or eNB for baseband processing using the Common Public Radio Interface (CPRI) or the Open Base Station Architecture Initiative (OBSAI) interface. In this RAN architecture, all the components are situated out at the remote cell site with the baseband processing, radio functions, and antennas all co-located there. In the DRAN architecture, the CPRI network is local to the cell site and the BBU/eNB connects dir- ectly to the mobile core over the back-haul network, as illustrated in Figure 10-23. However, using dark fiber or passive optical networks, CPRI can be carried over an extended dis- tance (tens of km), which allows the baseband functions of a group of cell sites to be pooled into what is known as a baseband hotel (BBH) while the radio functions (RF) remain at the remote cell site. In this CRAN architecture, the CPRI interface is extended up to the centralized BBH location, and this extended CPRI network is referred to as the front-haul network. CPRI, being a synchronous protocol, requires a frequency synchronization signal. But the RAN also has phase/time synchronization requirements to support the advanced radio techniques covered in the preceding sections. The advantage to using CPRI is that it natively supports transfer of syn- chronization over its interf