The first phase of new radio (NR) standard, which was finalized in December 2017, is called a non-standalone (NSA) NR as it is based on dual connectivity between an LTE radio access technology (RAT) and the NR RAT, known also as E-UTRA – NR Dual Connectivity(EN-DC). This chapter presents an overview of 5G core (5GC) network and functionalities by presenting the specified logical network nodes which are the standardized building blocks for the 5GC and by giving an example of a signaling flow between the nodes for connection establishment. It discusses the different dual connectivity options and related terminology. Two different protocol stacks exist in NR system: control plane protocol stack, carrying control information between user equipment (UE) and gNB or Core Network, and user plane protocol stack, carrying data between different applications in the UE and in the network.