ation was not complete at the time, SO some NSA 5G networks started to emerge. The period of 2019-2020 was really the time in which the first Phase-1 deployments of standardized 5G based on 3GPP Release 15 commenced. However, most of the networks that were deployed in 2019 centered around NSA operation, which is composed of 5G RAN with Evolved Packet Core (EPC). Phase-1 deployments are only centered on eMBB services. It is Phase 2 where we see those additional two pillars of 5G, namely URLLC and mMTC. Phase-2 deployments are based on a combination of Release 15 and Release 16 features. We see full SA operations take place with the various features relating to URLLC and mMTC as well as eMBB. In this section, we looked at the key drivers, performance synopsis, and standardization of 5G, which will help us understand the forces driving the technology we will be studying in this book. We will now look at 5G NR and NG-RAN. 5G NR and NG-RAN Figure 1.4 shows the end-to-end architecture for the 5G system. 5G System 5G Core NG-RAN Network Network NR Uu Figure 1.4 - 5G system high-level architecture 5G systems largely comprise the 5G NR, NG-RAN, and, finally, 5GC. NG-RAN architecture Figure 1.5 shows key elements within the architecture of the NG-RAN. The User Equipment (UE) can be in the form of a mobile device, but it could also be in all manner of different forms, as can be appreciated with the advent of IoT. In order to provide RAN coverage, there are gNBs, which stands for New Radio Node B. A radio interface is needed to create connectivity between the UE and the gNB. That radio interface is called 5G Uu. Notice also that there is connectivity between the gNBs. So, the Xn reference points allow these gNBs to communicate with each other. Finally, connectivity between gNBs and the core network is needed, which is also achieved by the N reference points. In the diagram, the N2 reference point is providing the control plane flow, whereas the N3 reference point is used for the user plane, which carries user traffic and use