## 5 A Framework for SDN-SDR-Based Maritime Communications
### A lse Case
Several other works, including [71, 72, 73, 74, 75, 76], have evaluated the integration of SDR in numerous satellite-based applications. With an SDN-SDR unified communications framework, the ship's crew can prepare for changing RF requirements based on the geographical region and regulations ahead of time by keeping the SDR infrastructure ready with the requisite physical-layer standards with simple software reconfiguration tasks. This will alleviate the burden of carrying several hardware components to meet the communications standards of different regions. In [58], the authors present CrossFlow, a principled approach for application development in SDR networks. CrossFlow provides a flexible and modular cross-layer architecture using the principles of SDR and a mechanism for centralized control using the principles of SDN. Through the convergence of SDN and SDR, CrossFlow works towards providing a target-independent framework for application development in wireless radio networks. Each vessel in the CSG has a SATCOM link for communication with other vessels and offshore sites and, more importantly, for accessing the services on the GIG. It is important to note that small- and medium-sized ships within the CSG have limited SATCOM bandwidth, typically 256-512 Kbps, which prevents them from accessing large volumes of data. However, they can use the excess bandwidth of the 4-8 Mbps available to the larger ships within the strike group [59]. Figure 7 depicts our proposed SDN-SDR-driven communications between vessels for a representative CSG. Each vessel in the CSG--surface and underwater--is fitted with multiple SDRs and managed by an SDN controller, denoted as SDN-C. Note that the Carrier can have multiple controllers, i.e., SDN-Cs, along with one SDN master controller, denoted as SDN-MC to manage the SDN-Cs. Each vessel in the CSG establishes and maintains an SDN-SDR-driven communications link with the Carrier,i.e., V2C communication as previously noted. To maintain the V2C link, each vessel's SDN-C communicates with the Carrier's SDN-C using the SDN-SDR unified cross-layer network architecture. With multiple SDRs, each vessel has the flexibility to dedicate specific SDRs for specific functions, such as AIS, secure COMMS, etc. While the hardware remains the same, the embedded software is adapted for each application. The Carrier frequency, bandwidth, modulation, and other signal characteristics are specified to meet each communication system's requirements. The use of the same hardware is an advantage in case of an SDR failure. Ready spares can be brought online, i.e., deployed, with the needed software functionality and replaced at sea. In a worst-case scenario, a working SDR for a lower priority system can replace a broken SDR in a critical system. Each Vessel in the CSG has its own SDN-C, which is responsible for managing a specific domain or segment of the network, ensuring optimal performance and handling local network events. However, this teamwork does not stop within one ship, rather it extends to the Carrier itself and reaches out to other vessels in the CSG. The Carrier has its group of SDN-Cs, working together to make sure everything runs smoothly. In order to make sure everyone is in synchronization, the Carrier has a special, global controller called the SDN master controller (SDN-MC). The SDN-MC oversees the entire CSG network infrastructure and acts as a central point of coordination for all vessels in the CSG. The SDN-MC's primary role includes global policy enforcement, network-wide optimization, and traffic engineering. It uses the insights provided by the SDN-C to make strategic decisions that promote efficient resource utilization, load balancing, and overall network resilience. Cooperation between the SDN-C and the SDN-MC is essential for ensuring seamless network operation. The SDN-C continuously provides real-time updates and status reports to the SDN-MC, enabling it to make informed decisions.