le network node hosting all gNB functionalities. One important exception is the MR-DC architecture, which splits the functionality across two base sta- tions (which can be 5G gNBs or LTE eNBs) and which can also function independently (e.g. for UEs that do not support MR-DC). These split architectures offer numerous benefits in terms of deployment flexibility, inde- pendent upgrades, and scaling of network components (only where scaling is needed). Furthermore, split deployments are typically easier to virtualize. An important non-technical benefit of split architectures is that (if defined, implemented, and tested properly) they may allow network operators to source different network nodes from different vendors, thus increasing competition and potentially driving cost reduc- tions. One must remember though that mixing and matching equipment from different vendors shifts the burden of interoperability testing and troubleshooting from a vendor to an operator, or an integrator employed by an operator for that task. Another important point to keep in mind is that, while most network vendors do follow architecture standards to some extent, many chose to add proprietary features and some even develop non-standardized architectures. One such example (not covered in this book, as we focus on standardized architectures) is the Distributed Antenna System (DAS). Another interesting trend we are observing is that some NG-RAN architectures that have been initially defined for 5G (e.g. gNB-CU/DU split and gNB control-user plane separation) are being "backported" to LTE. This chapter is dedicated to NG-RAN architectures defined by 3GPP in Release-15 and by O-RAN and Small Cell Forum around the same time frame. In the next chapter we discuss how NG-RAN evolves in Release-16 and how it is likely to evolve further in Release-17 and beyond. NG-RAN Evolution 5.1 Introduction In the previous chapter we discussed various NG-RAN architectures, defined in 3GPP, other standards bodies, and industry fora. The architectures described SO 