## 2 Mobile Networking Background  
This paper focuses on the most recent 5G New Radio protocol as standardized and specified by the Third Generation Partnership Project (3 GPP) [14; 15; 16]. 3 GPP standardizes and specifies the technologies for the radio access, backend core network, and service capabilities for mobile telecommunications, thus guiding mobile networking research and development and enabling interoperability between the different cellular service provider services. While 6G's protocol design and standardization are currently ongoing, 5G will provide a building base for 6G as 6G will inherit most of the existing technologies, including those described in this section. Section 2.1 provides a high-level overview of the current mobile networking focusing on 5G, including the different protocol steps for the wireless/RF communication channel setup vs. the digital setup and the critical messages and identifiers/credentials used for the channel setup (which can become the targets for security protection in future security research). Building on the 5G architecture, Section 2.2 describes the incorporation of the UAV flying base station to the 5G architecture. Existing Telecommunications Networking Protocol: 3 GPP Standardization Protocol and User vs. Base Station vs. Core Network  
The connectivity provider infrastructure to provide connectivity to the mobile user includes the base station, the core network, and the intermediate routers/switches (which forward the networking packets across the physical distances after the base station). The lower row in Figure 1 depicts these entities from the user (left) to the cloud (right), where those between the base station and the core network are within the cellular service provider infrastructure. Beyond the core network is outside of the cellular service provider infrastructure and relies on the collaborations and agreements with other service provider entities. The base station connects to the mobile user wirelessly and serves as the bridge gateway to transition to the wireless communications for the mobile users in telecommunications networking. Because the mobile user uses a wireless communication link, while the networking from the base station to the internet and cloud are in the form of wired communications, the base station serves as the gateway between wireless vs. wired networking. The user and the base station communicate via radio-frequency (RF) wireless communications, while communication from the base station to the core network and then to the cloud is via wired communications involving other nodes such as routers and switches. Therefore, the base station is the first and the last hop to the mobile users communicating in RF. Beyond the base station, the core network is responsible for much of the digital processing to set up the connectivity services to the mobile user, including user registration, security setup, and service as well as access control. The communications  
Figure 1: Telecommunications networking architecture and entities, including the UAV flying base station in the upper row. and packets from the user are processed by the core network before it leaves, e.g., to the public internet. Figure 2 describes the logical interactions for the control communications between the user, base station, and core network. The brief description abstracts from the telecommunications implementations and details from [14; 15; 16] and focuses on the information needed to understand this review paper. The user and the base station require radio resource control (RRC) and a wireless channel for setting up the RF wireless communications, where the communication resources are shared with multiple other users coexisting in the nearby air medium. The RRC begins with the broadcasting messages by the base stations, including the master information block (MIB) and system information block (SIB).