## 2 Architectures of 5G and 6G Networks  
The evolution from fourth-generation (4G) Long-Term Evolution (LTE) systems to fifth-generation (5G) and eventually to the forthcoming 6G represents a paradigm shift in network design, operational flexibility, and service delivery. Whereas 4G focused primarily on increasing data rates and improving spectral efficiency, 5G introduces a broader transformation that leverages softwarization, virtualization, and distributed computing capabilities to support diverse use cases. Finally, 6G builds upon the advanced capabilities of 5G by further enhancing them, while also integrating emerging technologies such as AI-driven communication, advanced integrated sensing, and ubiquitous connectivity. The use cases driving the evolution from 5G to 6G [7], along with newly imposed characteristics, are depicted in Figure 1.  
Three use cases of 5G include enhanced mobile broadband (eMBB), ultra-reliable and low-latency communication (uRLLC), and massive machine-type communication (mMTC) [8]. Looking ahead, the envisioned 6G is expected to build upon and extend the previous three categories--now referred to as Immersive Communication, Massive Communication, and Hyper-Reliable Low-Latency Communication (indicated by * in Figure 1)--while integrating new technologies such as AI-driven communication, integrated sensing and communication, and ubiquitous connectivity. Notably, 5G and future 6G systems integrate three key principles--Software-Defined Networking (SDN), Network Function Virtualization (NFV), and edge computing--to deliver high-bandwidth, low-latency, and flexible network slicing for various vertical industries (e.g., Industry 4.0, autonomous vehicles, and telemedicine). While these technologies enable unprecedented levels of performance and tailoring of services, they also bring new security issues that need  
Figure 1: Use cases highlighting the evolution from 5G to 6G networks and their fundamental key features. strong and flexible defense mechanisms. The subsections below detail each component of the architecture and give real case studies, leading to a discussion on securing future wireless networks. * **Software-Defined Networking (SDN):** Software-Defined Networking decouples the control plane from the data plane, allowing centralized management of network elements and policies. This contrasts with conventional, tightly coupled hardware-based solutions in which configuration and operation reside within proprietary devices. By abstracting network intelligence into a logically centralized controller, SDN increases agility; therefore, network operators can dynamically configure traffic flow, enforce Quality of Service (QoS) policies, and deploy security rules in real time via programmable interfaces [9]. However, SDN also introduces potential single points of failure; if an attacker compromises the SDN controller, they can gain sweeping control over the network, redirect traffic, or inject malicious flow rules. Consequently, rigorous security-by-design principles--such as controller redundancy, secure communication channels between the controller and switches, and continuous monitoring of controller activities--become essential [10]. A 2020 proof-of-concept study demonstrated how a compromised SDN controller in a simulated 5G testbed could manipulate routing tables, leading to partial or complete Denial of Service (DoS) [11]. The researchers implemented multi-controller architectures and controller-level intrusion detection modules to detect anomalous requests in real time, mitigating the issue. The findings highlight the need to integrate security features within the SDN orchestration layers more so as the 5G networks diverge and converge with the IoT devices that require near-zero latency. The authors in [12] highlight key security challenges and examine how integrating SDN and NFV (discussed further in the next paragraph) can strengthen the security and adaptability of 6G networks.