## 3 What Is SDN?
### How SDN Will Help Cellular Network 5G and Beyond  
SDN is anticipated to be a key player in the evolution and operation of 5G networks and beyond. SDN, in conjunction with 5G, will revolutionize network capabilities, offering significant opportunities for network operation, which allows for dynamic allocation of network resources, thereby improving network efficiency and flexibility [73]. Integrating SDN with 5G and beyond networks can enhance AI and automation capabilities, leading to more efficient network management and operation. Furthermore, SDN can reduce associated costs by minimizing the need for physical infrastructure and allowing for the more efficient use of network resources [74]. SDN can also enhance the delivery of network-based services, IT systems, and applications, improving user experience and enabling new services. Despite the limitations related to performance and scalability of a centralized controller approach in SDN for extensive 5G and beyond networks, advancements in SDN technology are addressing these challenges, making it a promising solution for managing the complex and large-scale networks of 5G and beyond [75]. SDN can also be integrated with other technologies like NFV to enhance the capabilities of cellular networks further. In essence, SDN is seen as an enabling technology that can help realize the huge promises of a 5G network by providing a flexible, programmable, and centrally managed network infrastructure [76].  
[PERSON] et al. [77] presented an SDN network in which the OpenFlow data plane was controlled. Moreover, functions were executed on OF commodity switches within a logically centralized data center. Additionally, an SDN controller (SDNC) was employed. A virtualized Serving Gateway (vS-GW) was deployed on top of the Northbound SDN as an application. However, the Serving Gateway User Plane (S-GW UP) was carried by the Regional Router (RR). The UP referred to the data path, which was inherently managed by the data plane. It functioned as the network component responsible for anchoring user sessions, managing mobility, and providing access to external networks. The architecture was found to be similar to the one proposed by [PERSON] et al. [78]. Furthermore, the approach closely resembled the work of [PERSON] et al. [79], in which an SDN control plane was used to manage OpenFlow devices, primarily in the core network rather than in the access network. [PERSON] et al. [45] proposed the management of OpenFlow switches by the SDN control plane; however, their approach was implemented within the core network.  
Figure 7: Architecture of SDN [72].  
[PERSON] et al. [80] proposed a method to connect trains and data centers using SDN and cellular technology. A general controller was placed on the train (local), while an external controller (data center) was suggested for additional functionality via REST API, as shown in Figure 8. [PERSON] et al. [81] also investigated seamless handover in vehicular networks. Vehicular Ad Hoc Networks (VANETs) frequently experience interruptions, and SDN integration was expected to mitigate this issue. Two different SDN architectures were proposed, and it was observed that the one with lower complexity provided better performance in terms of delay reduction, packet loss mitigation, and minimized network overhead. Additionally, for P4 (Figure 4c), a local controller was suggested for handling reactive forwarding, while the remaining functionalities (such as telemetry) were managed at the data center.