o achieve its full potential. In the next section of this chapter, we describe three current LTE RAN configurations, and show how each configuration could use NFV/SDN technology. 11.5.1 LTE RAN CONFIGURATIONS The basic structure of the LTE RAN for conventional early LTE deployments followed the distrib- uted RAN (or dRAN) architecture. This architecture is similar to the architecture of earlier genera- tions of RAN technologies, and is shown in Figure 11.13. In this architecture, all of the components of the RAN node are located at the cell site, including the radio frequency (RF) components (such as the antenna and the radio amplifier) and the baseband unit (BBU), which does the digital process- ing of the radio signals. The RF and antenna are typically located at the top of the cell tower, and the BBU is typically located at the base of the cell tower. The BBU and RF are interconnected by a short fiber interface that typically uses a protocol such as Common Public Radio Interface (CPRI) designed to operate for relatively short distances over a dark fiber [8]. The BBU then has a backhaul connection to the core network, typically using a high-speed Ethernet link. Applying NFV and SDN techniques to the dRAN architecture is very challenging, because cell sites located at cell towers are distributed geographically. New deployments of cell sites are also shifting to small cells, which are even more distributed. In typical configurations, small cells may be self-contained units mounted on utility poles, light poles, or on the ceilings inside buildings. In situations where the density of traffic and cell sites are very high, and where dark fiber trans- port is available to cell sites, network operators have been deploying a centralized RAN (cRAN) architecture, shown in Figure 11.14. In this architecture, only the RF equipment is located at the cell site. A collection of BBUs for a number of cell sites within a small distance can be located together at a central site, referred to as baseband pooling. The pool of BBUs at