## 2 Exploring SDN, NFV, Policies, ML, Network Slicing and Telecom Security
### Network Slicing  
A network slice is a virtual network architecture built over a physical network, giving the impression to the slice tenant that they are operating their exclusive physical network, and it is a pivotal technology within 5G and future cellular networks [41]. Network slicing as a service and the unification of the 5G end-to-end service platform can be enabled by network _softwarization_ and virtualization using SDN, NFV, and cloud computing. This needs new designs and implementations across various 5G network segments like RAN, transport, core, mobile-edge networks, and network clouds to meet business demands and drive innovation [20]. Some of the network characteristics needed to enable and implement this can include a high-capacity backhaul based on high-speed optical fiber and free-space optical systems (FSO) that must enhance the backhaul connectivity. This is a challenge for 5G and beyond networks as it will not be possible to have always optical fiber connectivity as a backhaul due to geographical constraints and complexities. FSO associated with RF can be seen as a possibility to overcome limitations presented by the atmosphere and overcome some of the limitations associated with the infrastructure that enables 5G and beyond networks [42]. The fundamental principles that underlie network slicing and its operations within software-based 5G networks include the following [20]: automation of network operations; high reliability, scalability and isolation; programmability; hierarchical abstraction; slice customization and network resource elasticity.  
Network slicing addresses the inflexibility of traditional \"one-size-fits-all\" network architectures, which struggle to accommodate the diverse performance requirements of emerging applications. By leveraging SDN and NFV, network slicing allows operators to partition a physical network into multiple, isolated virtual networks (slices), each optimised for specific use cases [43]. Network slicing introduces several compelling benefits for operators and industries:  
* **Flexibility and Customization**: Slices can be tailor-made for applications requiring high bandwidth (e.g., video streaming), ultra-low latency (e.g., remote surgery, industrial automation), or support for massive device connections (e.g., smart cities, Internet of Things) [44].
* **Improved Resource Efficiency**: Network slicing facilitates the dynamic allocation of resources based on real-time slice demands, maximising efficiency and reducing costs [45].
* **New Revenue Streams**: Operators can offer custom slices to enterprise customers or other service providers, unlocking new market opportunities [43].
* **Management Complexity**: Orchestration of multiple slices with distinct configurations demands sophisticated management and automation tools [45].
* **Security Concerns**: Meticulous security measures are needed to ensure slice isolation and prevent interference or attacks. This is critical as slices share a common infrastructure [46].
* **Standardisation**: Ongoing efforts by bodies like 3 GPP focus on defining interoperability standards, which are crucial for multi-vendor compatibility [46].  
The landscape of mobile communication networks is undergoing a significant transformation driven by the emergence of many diverse use cases with vastly different requirements. The challenges posed by these diverse use cases and how traditional, one-size-fits-all network architectures fall short in addressing their unique needs. Those use cases are mainly associated with the three prominent categories in network slicing seen previously: eMBB, URLLC, and mMTC, often referred to as an important feature of the Internet of Things (IoT) [41].