## 5 A Framework for SDN-SDR-Based Maritime Communications
### A lse Case  
Merchants and cruise ships travel far and wide across the vast oceans between places. In the case of Navy vessels, they remain afloat for months at a time and may be required to anchor far offshore. In either case, the communications environment has physical and environmental challenges. More importantly, once away from the shore, the ships have no access to land-based high-bandwidth communications infrastructure. Furthermore, when ships want to pull up to ports in different parts of the world, they are often faced with differing communication standards. For example, countries may choose to operate wireless communications equipment across different spectrum allocations [47]. With dedicated equipment operating on a specific carrier for a given bandwidth, there is no guarantee of compatibility with another system's specifications. While SATCOM is a viable solution in such scenarios, the cost can be prohibitive, and the latency and signal interference may not be acceptable for time-critical missions. Furthermore, the nature of the satellite constellation can introduce spatial or temporal communication gaps. For instance, a geosynchronous satellite in orbit approximately 35,000 km above the earth has a period of approximately 24 h. This results in the perception  
Figure 6: SDN-based framework for attack mitigation. to a user on the earth that the satellite stays relatively fixed at a given point in the sky. As a maritime vessel traverses east or west across the oceans, this point in the sky eventually will appear below the horizon, requiring the vessel to obtain coverage from an additional satellite to communicate. Additionally, due to the curvature of the earth, geosynchronous satellite coverage above 70 degrees north latitude and below 70 degrees south latitude is greatly diminished [70]. While these issues can be overcome with different solutions such as using a polar or highly elliptical orbit, they bring their own complications, such as tracking and pointing requirements. In particular, Navy operations require connectivity among a diverse set of platforms, including submarines, surface ships, aircraft, and shore sites (see Figure 2). The links among these platforms support a wide range of applications supporting strategic and tactical C4 ISR functions. While high bandwidth communications are available on large ships, small ships do not have the ability to leverage this bandwidth by dynamically selecting the most capable link available [59]. Consequently, ships do not efficiently utilize the available bandwidth within the strike group, limiting the ability for smaller ships to effectively gain access to services on the global information grid (GIG) [59]. SDN-SDR-based unified communications infrastructure can be very useful for deployment with the Navy's carrier strike groups (CSGs), which is composed of ships, submarines, aircraft, and personnel to support an extensive range of operations, all of which primarily rely on SATCOMs. A more robust, low-cost, high-bandwidth, and low-interference communication infrastructure should be considered to address the afore-explained situations, especially for vessel-to-vessel (V2V) and vessel-to-aircraft-carrier (V2C) communications. With regards to a CSG, one possible solution is to fit the vessels in the CSG with an SDN-SDR-based unified communications framework integrated with the existing SATCOM infrastructure. It is important to note that today's satellites indeed leverage SDRs for more flexible and varied applications. In [57], the authors discuss how the SDR-based AIS receiver has been integrated into the AAUSAT3 satellite. The authors note that, due to its versatility, new detection algorithms can be easily deployed on an SDR-based AIS receiver, and they easily adapt to the new AIS transmission channels. As previously mentioned, SDN and SDR provide great flexibility to the network and physical layer functionalities of the communications stack.