n important aspects of the physical connec- tions implementing the logical network interfaces shown. For the network interfaces, these details are explained in Section 6.6. The Uu interface is described in Chapter 3. The descrip- tion of the network interfaces provided in these sections omits the details of the transport network (3GPP specifications generally assume IP transport and do not define it in further detail). To fill this gap, we describe transport network aspects in Section 6.6. With regard to the Uu interface, while the specifications do provide a rigorous description of all the functionality, one aspect is sometimes overlooked - the connection to the physical antenna (3GPP specifications operate at the logical abstraction of an antenna port). To fill this gap, we describe the antenna interface (which is used to configure the antenna) and the Common Public Radio Interface (CPRI) interface (which is commonly used to transfer data between a gNB and a Remote Radio Head [RRH]) in the following subsections. 4.1.2 Common Public Radio Interface (CPRI) CPRI is the interface between a Radio Equipment (RE) (sometimes referred to as an RRH, or remote unit [RU]), and the rest of the base station (i.e. gNB), referred to as Radio Equipment Control (REC). CPRI is primarily used to transfer user data (along with relevant control information) between REC and RE. Formally, CPRI specification is not a standard as it is produced by an industrial cooper- ation by a few network equipment vendors. Nevertheless, CPRI has historically had large market adoption, and some portions of CPRI and its evolutions have offered a minimal level of openness. That is, in the CPRI architecture the "monolithic" gNB shown in Figure 4.1.1 is split into RE and REC, as shown in Figure 4.1.2. The CPRI physical layer can support an electrical interface (primarily for internal connec- tions of an integrated base station) and optical interface (primarily for remote installations). CPRI was originally defined for 3G, later extended to support 4G, an