Tutorial on communication between access networks and the 5G core

Fifth-generation (5G) networks enable a variety of use cases, e.g., Ultra-Reliable and Low-Latency Communications, enhanced Mobile Broadband, and massive Machine Type Communication. To explore the full potential of these use cases, it is mandatory to understand the communication between User Equipment (UE), Radio Access Network (RAN), and 5G Core (5GC), which support new network concepts and paradigms. For example, network slicing plays a crucial role in the communication system to address the challenges expected by the 5G networks. 3rd Generation Partnership Project has recently published Release 16, including the protocols used to communicate between RANs and 5GC, i.e., Non-Access Stratum (NAS) and NG Application Protocol (NGAP). The main goal of this article is to present a comprehensive tutorial about NAS and NGAP specifications using a didactic and practical approach. The tutorial describes the protocol stacks and aspects of the functionality of these protocols in 5G networks, such as authentication and identification procedures, data session establishment, and allocation of resources. Moreover, we review message flows related to these protocols in UE and Next Generation Node B (gNodeB) registration. To illustrate the concepts presented in the tutorial, we introduce a 5GC tester that implements NAS and NGAP for availing of three open-source 5GC projects on a black-box testing methodology.