erface between CU and DU. 144 Open Radio Access Network (O-RAN) Systems Architecture and Design Server Server CU-CP CU-CP PDCP-CP PDCP-CP CU-UP CU-UP IPSec PDCP-UP IPSec PDCP-UP Ethernet Ethernet Ethernet Ethernet FIGURE 4-3 iNIC solutions for disaggregated CU. Combined, the F1 interfaces separate transport network layer and radio layers. Depending on required balance between performance, power and scalability, this CU unit can be implemented on a single device or split across multiple devices and even Instruction Set Architectures, balancing capacity/performance, cost, and power. Let's look at a few options from high levels of integration with relatively low performance to low integra- tion but highly scalable solutions. At the highest level of integration, a single Intelligent Network Interface Card (iNIC) card can support combined CP and UP functions or be plugged in to a server where UP is imple- mented in the iNIC and CP in the server environment. These options are shown below (left and right) in Fig. 4-3: An obvious next level of scale-out can be supported by utilizing multiple iNICs, separat- ing UP functions (as shown below in Fig. 4-5): An active Ethernet backplane is provided by a TOR switch becomes the method to scale- out as performance requirements increase (Fig. 4-6): Chapter 4 Hardware architecture choices CU-CP CU-UP PDCP-CP PDCP-UP PHY-High eCPRI/O-RAN PHY-Low FIGURE 4-4 Key CU interfaces as defined by 3GPP. In scale-out option, multiple accelerators devices are placed on a processing blade that supports increments of 100 Gbps or more of packet processing per function with an Ethernet switch to provide connectivity between devices. Multiple blades are placed into a 100GbE backplane to provide close to infinite scaling. To support this deployment model, the S1 interface can no longer be implemented as a single (IPSec) tunnel but needs to be split into multiple tunnels that can each be routed to a unique target accelerator. We assume that the statistical traffic properties are such that issues arou