s Group (2018). Tower mounted low noise amplifier (ST-TMA). Available at: http://aisg.org.uk(accessed May 29, 2020). 4 NG-RAN Architecture 4.2 High-Level gNB-CU/DU Split Sasha Sirotkin Intel Corporation, Israel In this section we discuss the gNB-CU/DU split architecture, which is equivalent to option 2 from the 5G study, which we mentioned in Section 4.1. The RAN architecture has been evolving over the years and 3GPP Releases in somewhat circular fashion. In UMTS, the RAN consists of two network nodes: Node B and RNC. The LTE RAN architecture, on the other hand, is flat - there is only one eNB node, which combines the roles previously assigned to Node B and the RNC. In 5G, 3GPP enabled the hierarchical architecture again. However, even though the high-level architecture may look similar to UMTS, there are important differences, as we will show. Generally, 3GPP defines a logical network architecture, meaning that logical network nodes defined in the specifications can be mapped in different ways into physical network nodes in actual implementations. Multiple logical nodes can be combined into a single physical node, in which case network interfaces "collapse" into internal Application Pro- gramming Interfaces (APIs), which may or may not follow the standard. The same does not necessarily apply to the case of "splitting" a single logical node into multiple physical ones - this can, of course, be done in a proprietary manner, but if multi-vendor interoper- ability of the split nodes is desired, such architecture must be standardized. On the other hand, having both split and non-split architecture in the specification is not required. The fact that the NR standard, contrary to the UMTS specifications, supports both architectures (single-node gNB and a gNB split into two logical nodes) illustrates the internal 3GPP strug- gles to define open and multi-vendor interoperable network interfaces and the reluctance of some parties to standardize these. In this section we describe the NG-RAN architecture in which a gNB is sp