lit into two (high-level and low-level) logical nodes, with a standardized F1 interface between them. This is just one deployment option - as mentioned above, a gNB can also be implemented as a single physical network box. Moreover, multiple gNB split architectures have been defined, as we explain further in the book. The current section only describes the RAN aspects of these procedures supported by the F1 interface, which assumes some knowledge of the air interface and core network functionality. For an overview of these, please refer to Chapter 3. 4.2.1 Key Ideas LTE eNB is defined as a single logical network node, implementing RRC, PDCP, RLC, MAC, and PHY layers. Many implementations follow 3GPP LTE logical architecture; however, some vendors chose to split an eNB into two (or even more) physical nodes. The interface between these nodes is proprietary and therefore not interoperable between multiple vendors. In 5G, 3GPP have defined both a single-node "monolithic" NG-RAN architecture and a split architecture. In the latter, a gNB is split into two logical nodes: a centralized gNB central unit (gNB-CU) node and one or many distributed gNB distributed unit (gNB-DU) nodes, connected via the standardized F1 interface. 4.2 High-Level gNB-CU/DU Split The underlying assumption is that a single gNB-CU controls a large number of cells (much larger than a typical gNB would), thus allowing centralized RRM across many cells for improved performance, resource pooling across many gNBs (at least for PDCP functionality) for lower cost and energy, and virtualization support. In the split architecture, a gNB-CU hosts PDCP, Service Data Adaptation Protocol (SDAP), and RRC layers, and a gNB-DU node hosts RLC, MAC, and PHY layers. This functional split does not allow the realization of all the benefits of centralization (discussed above), as most of the functionality resides in a gNB-DU and there is relatively little to be centralized in a gNB-CU. 3GPP has decided to define a high-level split as a compromise to support at least d