## 5 A Framework for SDN-SDR-Based Maritime Communications
### The SDN Approach  
#### 5.2.1 SDN Background  
There has been an increase in the use of software-defined networks (SDNs) in recent years, which promise to solve the problem of bundling between the control plane and the data plane. In an SDN, the control plane decides how network traffic should be handled, and the data plane forwards the traffic by the control plane's decisions. Logically centralized controllers simplify policy enforcement, network (re)configuration, evolution, and scalability by implementing control logic [60]. Figure 3 depicts mapping SDN and SDR to an OSI model. Some of the key advantages of SDNs are as follows:  
* Host Multiple Connections: The ability to host multiple connections means that combinations of Wi-Fi, satellite, and mobile communication networks can be utilized as a single connection to provide a more streamlined and less complex maritime network. This also provides a certain level of redundancy for other connections in that, should one connection falter or go down, multiple others could take any redirected traffic onward to its destination. * Intelligent and Application-Based Routing: One of the lesser celebrated but essential benefits of SDN solutions are their application-based routing [61] and intelligence capabilities [62]. This allows operators to build intelligence into their networks in order to understand the applications they run and their particular bandwidth requirements. Using the multiple wide area network (WAN) links available, maritime software-defined WAN administrators are able to benefit from dynamic application-level routing as well as implement application-based intelligence to overlay traditional packet-based routing. This enables the network to intelligently allocate the best possible connection for each individual bit of traffic. * Remote Management and Updates: Leveraging SDN technologies can also save enterprises money, space, and resources by allowing SDN networks at their customers' sites to be managed by service providers from a centralized location. This also allows for networks to be updated remotely through software, instead of needing to house an IT specialist for the duration of every trip. In summary, SDN provides centralized control and management, allowing administrators to dynamically allocate and prioritize network resources based on real-time needs. This flexibility enables efficient traffic management, ensuring smooth and reliable communication between maritime assets. Additionally, SDN's programmability enables the implementation of advanced security measures [63]. It allows for fine-grained access control, enabling administrators to create and enforce security policies across the network. \begin{table}
\begin{tabular}{c p{284.5 pt}} \hline \hline
**Reference** & **Summary** \\ \hline
[55] & Joint sleep scheduling and opportunistic transmission scheme in maritime wireless communication networks using SDN to balance energy consumption and delay. \\ \hline
[56] & Use of multiple controllers for reducing response delay. \\ \hline
[57] & Integration of SDR-based AIS receiver into AAUSAT3 satellite, noting versatility and adaptability of new detection algorithms. \\ \hline
[58] & CrossFlow approach for application development in SDR networks, combining SDR and SDN principles for flexibility and modular architecture in wireless radio networks. \\ \hline
[59] & Discussion of SATCOM bandwidths on ships, highlighting the cost-effectiveness of indirect communication. \\ \hline
[52] & Presentation of a prototype for AIS and VDES using a low-cost SDR platform, with additional filtering and amplification for improved performance. \\ \hline
[53] & Proposal for a low-cost AIS transmitter using SDR technology and open-source software, confirming feasibility through tests. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Cont. Moreover, SDN's separation of control and data planes enhances security by reducing the attack surface and preventing unauthorized access, especially when used in critical infrastructure such as maritime communication networks.