erface between DU and RU: the fronthaul. 3GPP standards are more important in the mid- and backhaul portion of the network that connects the CU and DU components to the 4G and 5G core network as shown in Fig. 1-29. These standards include: the Higher Level Split between DU and CU, or the F1 interface as defined by 3GPP and the El interface between the CU User Plane (CU-UP) and CU Control Plane (CU-CP). Relevant standards documents for both are defined in 3GPP 38.47x series. We will discuss these standards in more detail elsewhere in this document. nFAPI We have already mentioned that the functional split 6 connects the base station and the radio head over a Ethernet network interface at the MAC/PHY boundary. Although neither defined by 3GPP or by O-RAN, there is a standard worth mentioning in this regard: nFAPI or "Networked Femto Application Programming Interface." nFAPI builds on the femto API that has been the industry-standard interface for connecting L2 (MAC/RLC/PDCP) stacks and L1 (physical layer) stacks from different vendors, typically in an ISC/femtocell environment. This standard has been available from the Small Cell Forum³ since the late 2000s when fem- tocells started entering the market. It has enabled relatively pain-free interoperability 5G Core (LTE core) 5G gNB-CU eNB CU X2u/Xnu gNB CU UP gNB CU CP (LTE) (5G/NR) (5G/NR) X2c/Xnc eNB DU (LTE) gNB DU (5G/NR) O-RAN O-RAN FIGURE 1-29 3GPP-defined mid- and backhaul standards. Chapter 1 Open radio access network overview 33 between software vendors and proven an enabler of ecosystem approaches in base station implementations. As the name says, the nFAPI standard extends this "vendor-independent API" concept to a networked version to enable O-RAN and similar implementations with a centralized/virtua- lized MAC layer and a network connected physical layer. The standard is defined for both 4G (4G-nFAPI) and 5G (5G-nFAPI). Other relevant standards Recent contributions to IEEE standards show increasing focus on Time-Sensitive Networking (TSN). TSN is part o