coming traffic based on header fields such as IP protocol (IPSec) or UDP port (2152 for GTP). After classification, packets can be directed by hardware to use a specific buffer memory (Linux kernel: sk_buff or a proprietary buffer for optimized stacks) and a separate queue that interfaces to either the optimized implementation or to the Linux kernel for "all other traffic." This gives the system integrator the best of both worlds: Features and ease-of-use associated with Linux with hardware opti- mized performance (Fig. 5-11). IPSec Fwd Performance (normalized) Stack Linux FIGURE 5-10 IPSec performance comparing Linux to more optimized implementations. Chapter 5 System software Server Network Interface Card GPP Complex Ethernet MAC(s) Bman/QMan GPP Cores Buffer Managent Application A Ethernet L2/L3/L4 (eg U-Plane in Interface(s) Classification Queue DPDK) Management and QoS Application B (eg C-Plane in kernel) FIGURE 5-11 Hardware traffic bifurcation. L2/L3 Application Software RF/DFE/ Control Config Config L1 Platform Software FIGURE 5-12 Functional application platform interface internal APIs. Copied from small cell forum. Functional application platform interface The Functional Application Platform Interface (FAPI) specification suite is an initiative within the Small Cell Forum to create interoperability and innovation for providers of hardware/ software platforms and application software by providing a common API. With a common API, those two parts can be "interchangeable." The FAPI suite includes three specifications: SCF222, '5 G FAPI: PHY API' - main data path (P7) and PHY mode control (P5) interface SCF223, '5 G FAPI: RF and Digital Front End Control API' - (P19) for Frontend Unit control SCF224, 'Network Monitor Mode API' - (P4) for 2 G/3 G/4 G/5 G The FAPI interface (Fig. 5-12) defines internal interfaces to (in O-RAN language) the DU and is agnostic to O-RAN or other RAN architectures. 202 Open Radio Access Network (O-RAN) Systems Architecture and Design PHY API SCF222 defines the interfaces to L1 for