Software-Defined Networks and Software-Defined Radios in Maritime Communications

Version	Summary	Created by	Modification	Content Size	Created at	Operation
1		Nadia Niknami	--	5022	2024-01-17 15:44:01	\|
2	Reference format revised.	Lindsay Dong	+ 4 word(s)	5026	2024-01-19 03:14:16	\| \|
3	Main text revised.	Lindsay Dong	-1 word(s)	5025	2024-01-22 02:52:51	\|

This entry is adapted from the peer-reviewed paper 10.3390/network3040025

Effective maritime communication is vital for ensuring the safety of crew members, vessels, and cargo. The maritime industry is responsible for the transportation of a significant portion of global trade, and as such, the efficient and secure transfer of information is essential to maintain the flow of goods and services. With the increasing complexity of maritime operations, technological advancements such as unmanned aerial vehicles (UAVs), autonomous underwater vehicles (AUVs), and the Internet of Ships (IoS) have been introduced to enhance communication and operational efficiency.

autonomous underwater vehicles IoT maritime communication software-defined networking software-defined radio

1. Introduction

An Internet of Things (IoT) system involves the interconnection of devices using the Internet as a communication medium. A gamut of application domains have and continue to benefit from the emergence of IoT, including traffic management, critical infrastructure, and transportation sectors, such as the shipping industry. In the domain of ship technology, the IoT offers new opportunities to make use of interconnections between the physical and cyber layers, as well as the integration of all the ship’s components with the cyber systems, including global navigation satellite systems (GNSS), automatic identification systems (AIS), and electronic chart display systems (ECDIS). Maritime communications are also crucial for IoT applications such as monitoring the environment and controlling climate change. While it is true that 4G and 5G can be installed in maritime environments, their application to onshore environments is restricted since base stations cannot be located offshore.

Internet of Ships (IoS) and maritime IoT are becoming increasingly popular ^[1], and their numerous applications are presented in Figure 1. A more crucial aspect of the maritime industry and scientific research is the ability to connect devices in order to facilitate navigation and data collection. The maritime environment differs significantly from conventional land propagation environments since there are no physical barriers such as buildings or hills to obstruct the wireless signals. However, when considering wireless communications over inland waterways, it is important to account for potential obstacles such as buildings, bridges, and vessels passing beneath. These obstructions primarily impact large vehicles and buildings located near the waterways. It is also important to consider equipment located in offshore areas and vessels located in deep-sea regions when considering wireless communications over the sea.

Figure 1. Key emerging applications of IoS ^[1].

It has become increasingly important to maintain maritime communication as oceanic activities have increased dramatically, including naval shipping and logistics, offshore oil exploration, wind farms, fishing, and tourism, among others ^[2]. There are three broad types of maritime communication systems: 1. radio communication, 2. satellite communication (SATCOM), and 3. shore-based mobile communication. Below are the three most significant communications challenges currently faced by the maritime industry:

Range: A Wi-Fi signal can travel approximately 330 feet (100 m) from an on-land base station, but this may be affected by obstructions between the base station and the receiver, which limits their reliability. Even though Wi-Fi appears to be an economical and fast networking solution, its use is limited by its range of restrictions.
Latency: There has been considerable effort toward solving the latency problem. While it is not possible to completely eliminate latency, significant improvements have been made to minimize the latency ^[3]^[4]. Using satellites requires sending data as radio waves from ground equipment (transmitter) to a satellite (in space), which is then re-transmitted by the satellite to a receiving ground equipment (receiver).
Service Demands: With advances in networking, IoT, and smart technologies, the demand for data services is at an all-time high ^[5]. The provision of network connectivity on a cruise ship requires robust and flexible technology, not only for safety, communication, and navigational purposes but also for recreational and business purposes. As a result of this increase in service demands, reliable, flexible, robust, and agile maritime networks have become an essential component of maritime infrastructure ^[3].

1.1. Maritime Communication

The right communication protocol must be used for a vessel network to achieve a clear signal. Ensuring communication and navigational safety in maritime activities is paramount, and this is where maritime communications, spanning both inland waterway and sea communications, play a pivotal role as a critical technology ^[6]. In addition to waterway and sea communications, there are also aerial, space, and terrestrial communications. Developing a robust communication solution for IoT applications in the maritime environment is crucial to minimize external interference and ensure reliable data transmission from senders, such as smart containers, to receivers, such as cloud services. This is vital to maintain the integrity and efficiency of maritime IoT systems, enabling seamless connectivity and effective data exchange in the face of potential disruptions or signal disturbances. Communications technologies can be broadly categorized into the following categories:

Near Range—The near range technologies are for short-range communications within a limited area or proximity, such as ship-to-ship or ship-to-shore communication over short distances.
Wide Area—The wide area technologies are for long-range communication over larger geographic areas, often facilitated by SATCOM systems providing global coverage.
Narrowband—Narrowband technologies are for communication systems operating within a narrow frequency range, typically used for transmitting small amounts of data, such as text messages or basic voice communication.
Broadband—Broadband technologies are for high-speed data transmission allowing for transferring large amounts of data, enabling multimedia applications, video streaming, and high-quality voice communication. Examples include cellular networks, very-small-aperture-terminal (VSAT) systems, and dedicated maritime broadband services.

Due to the small amount of data typically transmitted by near-range communication devices, such as those utilizing NFC, the required battery size is small, enabling extended battery life. It is possible to send larger amounts of data using other near-range communication protocols. The Wi-Fi system, for example, can deliver a large amount of data, but the Wi-Fi signal range is generally limited to 100 m from a single access point ^[7]. To cover larger areas, such as an ocean vessel, multiple access points or repeaters are often necessary for Wi-Fi deployment. Bluetooth technology, on the other hand, has the capability to establish communication over extended distances, particularly with the advancements in Bluetooth 5. However, it typically operates at slower data rates compared to Wi-Fi. It is worth noting that Bluetooth can be a suitable choice for certain applications, even though it may have slower data transfer speeds when compared to Wi-Fi. Mobile cellular communication is a potential wide-area technology for the transmission of large amounts of data over large geographical areas, which maritime communications can benefit from. Mobile cellular services generally have the capability to send and receive larger amounts of data compared to Wi-Fi. The exact data capacity and speed of mobile cellular networks depend on various factors, such as the specific generation of cellular technology (e.g., 3G, 4G, 5G), network coverage, and available bandwidth. It is a licensed technology that requires mobile operators to obtain permission to operate a network within a specific geographical region. However, due to the higher power consumption of cellular technology, it can significantly impact the battery life of low-power devices.

While there are other communication protocols that are capable of transmitting data over a wide area, they do suffer from low data rates and can typically only transmit kilobytes of information instead of megabytes. Among these protocols is the long range (LoRa) protocol. LoRa is an unlicensed technology that employs spread spectrum modulation and enables the transmission of small amounts of data over long distances while consuming less power ^[8]. A typical application of the LoRa protocol would be monitoring water or gas meters over a large geographical area. Due to the small amounts of data transmitted, the battery life of the device running protocols such as LoRa is relatively long.

1.2. Unmanned Aerial Vehicles (UAVs)

An aerial network consists of multiple unmanned aerial vehicles (UAVs). Since UAVs are capable of flying, it is very attractive to deploy them as aerial base stations and relays ^[9]. The airborne UAVs can communicate directly with other airborne UAVs or with ground stations ^[10]. However, the management of UAV airborne networks poses significant challenges. These networks involve the exchange of vast amounts of information, including flight and control information, protocol stack information, sensing information, and data obtained from ground terminals. Furthermore, due to the constant motion of UAVs, wireless links and network topologies are continuously changing. As a result, managing and utilizing information, handling intermittent links, and maintaining a fluid topology have become highly challenging tasks ^[11]^[12]. In maritime environments, UAV communication also faces challenges to the channel characteristics, including temperature of troposphere above the ocean, pressure, waveguide impact due to humidity, signal attenuation due to climate change, and uneven scatterer effect on UAV-to-ship communication due to irregular sea waves ^[13].

Due to the difficulty of deploying communications infrastructure on the ocean, the performance of existing maritime communication networks (MCNs) is far behind 5G ^[14]. However, the ubiquitous deployment of UAVs for wireless communication purposes can offer a potential solution to bridge the 5G gap between networks in the air and those on ground. With agile UAVs, there is an opportunity to provide maritime coverage in fixed sea lanes, without relying on costly satellite systems or on-shore stationary base stations. By employing a hybrid architecture, UAVs can establish connections with terrestrial base stations along the coast while utilizing satellites for backhaul over long distances. To better optimize the UAV deployment in response to the sporadic presence of ships in shipping lanes, it can be tailored to address the specific needs and requirements of users ^[14]. The use of tethered UAVs and helicopters is an alternative option for large ships and cruise ships. Tethering UAVs simplifies the provision of energy by allowing continuous power supply through the tether. This eliminates the challenge of limited battery life and enables the UAVs to operate for an extended duration ^[15]^[16].

1.3. Autonomous Underwater Vehicles (AUVs)

The autonomous underwater vehicle (AUV) is a robot that travels underwater without the assistance of an operator. Underwater projects and mapping missions using AUVs can help commercial and recreational vessels detect and map submerged wrecks, rocks, and obstructions that may present navigational hazards ^[17]. Deep-sea data collection is a unique application of delay-tolerant networks (DTNs). There are several reasons for this, including the need for timely data reporting and the inevitable delay in acoustic transmissions in the ocean ^[18]. A AUV surfaces frequently to transmit the data obtained from sensors—in two-dimensional or three-dimensional search space—to surface stations. Whenever an AUV ascends to the surface to establish communication with the surface stations, an additional delay occurs in the data transmission process.

1.4. Unmanned Surface Vessels (USV)

Unmanned surface vessels (USVs) and unmanned surface vehicles are autonomous vehicles capable of performing tasks in diverse and complex environments without human intervention. In maritime communications, the limitations of UAVs have prompted industry and academia to explore the potential of USVs as a competitive solution to achieving the vision of 6G networks with seamless coverage and autonomous capabilities ^[19]. USVs’ robust, waterproof technology allows for long-term operation, even in challenging weather conditions. They possess the ability to autonomously sense and gather information from various maritime terminals, promptly identifying risks and responding accordingly. These USVs are dispatched to periodically collect maritime data from buoys and sensor nodes across vast areas. Equipped with high-gain antennas and computation units, USVs are well-prepared for future mobile communications in computing and caching-enabled networks. They can perform local computations or offload tasks to satellites or base stations as needed ^[5]. USVs offer numerous advantages for future maritime wireless communication systems.

2. IT, IoT, IIoT, IoUT, and OT

An IoT system is a gateway for the maritime industry. The gateway is used to communicate between the hardware components, such as sensors and machinery, and the cloud. Additionally, gateways can be configured to provide only selective information to each other and complete information only to the cloud. Information technology (IT) in maritime communication involves the use of computer systems, software applications, and networks to manage and process information related to maritime operations. This includes data storage; transmission; analysis; and communication between vessels, shore-based facilities, and other stakeholders involved in maritime activities. Operational technology (OT) refers to the specialized systems and technologies used to control and monitor devices, events, and processes in enterprise or industrial operations. With maritime operations, OT includes equipment; software; and networks that enable real-time communication, navigation, vessel control, safety systems, and other operational functions specific to the maritime industry.

The Industrial Internet of Things (IIoT) refers to the extension and use of IoT in industrial applications and sectors. In marine and underwater environments, the Internet of Underwater Things (IoUT) is an emerging communication ecosystem that connects underwater objects ^[20]. IoUT technology plays a crucial role in various aspects of the maritime industry, including smart boats, ships, shores, and oceans. It enables automatic marine transport, precise positioning and navigation, underwater exploration, and disaster prediction, and prevention, as well as intelligent monitoring and security ^[21]. Although IoUT devices generally do not generate new data at high frequencies, it is essential to establish connectivity through methods such as acoustic, magnetic induction, and onboard wireless communication. Maritime IoT and IoUT devices encounter the challenge of ensuring continuous power or managing battery replacements.

A further sub-category of IoT applications in maritime networks can be identified as follows:

Cargo: In the maritime industry, one of the key applications of IoT technology is the monitoring and tracking of large items, such as shipping containers, as they move along shipping routes. Additionally, IoT devices can be utilized for fleet management, which can automate and improve the efficiency of logistics, supply, maintenance, and operations associated with the fleet ^[22].
Cruise/ferry: The utilization of satellite technology is prevalent in various domains such as fishing, cruise, ferry, and leisure markets. As vessels incorporate specialized equipment for IoT sensor data and IoT connectivity, the use of satellite technology is projected to increase even further.
Fishing: There has been a significant improvement in the efficiency of monitoring, controlling, and supervising fishing vessels as a result of vessel monitoring systems (VMS). The use of VMS has become mandatory in several countries in recent years to ensure that fishing vessels report their catches to fishery management agencies.

The maritime network is characterized by the involvement of nodes such as ships and buoys in the development of an IoT setup, which led to the idea of Internet of Ships (IoS). Through high-level virtualization of the core network, the IoS enables the coordination of node computation to achieve forecasting analysis by using machine learning and artificial intelligence methods. UAVs can also be used to collect information in addition to IoT maritime sensor nodes. The battery life of the UAVs may limit this method, particularly if the head node is mobile and gathers data from the sensor nodes. There are a number of industrial and scientific applications that require the connectivity of underwater objects, including oil exploration, environmental monitoring, disaster prevention, and disaster recovery ^[1]. Ships, buoys, and autonomous surface vehicles can serve as data collection stations, or sinks, by gathering data from underwater sensor networks and transmitting them to a control center via radio waves. This approach allows for efficient and cost-effective data collection and monitoring in marine environments.

3. Current Security Challenges in Maritime Networks

There is no standardized cybersecurity strategy for maritime transportation, which is a safety-critical activity. Cybersecurity attacks on shipping lines may have severe consequences, such as maritime accidents and supply chain disruptions. Cyber-attacks can have the most severe consequences as autonomous vessels become more prevalent ^[23]. The threat of maritime cyber-attacks adds a level of complexity to the traditional maritime threats of piracy, illegal activities, maritime terrorism, and accidents at sea. Digitalization, automation, and connectivity are increasingly prevalent in the global maritime sector. In recent years, cyber threats have increased significantly. In the past few years, cyber-attacks on shore-based maritime-related systems have increased nine-fold, while GPS and AIS spoofing have frequently been observed ^[24]. Infiltrating and controlling a commercial vessel in order to capsize, collide, or cause environmental damage is now well within the realm of possibility ^[24]. A variety of complex automated systems are installed on modern and autonomous ships, including navigation systems, radio detection and ranging (radar), automatic identification systems (AISs), communication systems, and control systems to control a wide range of electro-mechanical systems, including the main engine, generator, and converter drive. In modern ships, the extensive use of automation and IT systems presents new opportunities for hackers and malicious actors to implement cyber-attacks that could have catastrophic consequences and cause major safety losses ^[25]. The main goal of the attackers is to gain remote access to ships and vessels, extract sensitive and valuable information for future attacks, or disrupt the ship’s operations by tampering with crucial components and rendering automated systems non-functional ^[26]. Below are some potential vulnerabilities, the consequences of an attack, and actual incidents that have been reported:

There is no authentication or integrity check on the AIS transponders, which makes them vulnerable to hacking, where they could be used to spread fake messages. Attackers use software-defined radios to transmit false man-in-the-water signals, enabling them to remain undetected and transmit false weather reports ^[25]^[26].
Navigation and GPS technologies are actively used in the maritime sector, which is a target for a number of cyber-attacks that aim to exploit design flaws in order to destabilize services that rely on them ^[27]. By spoofing GPS signals, attackers are able to reroute a vessel without triggering an alarm or alert.
Autonomous vessels depend on enhanced SATCOMs to transmit operational commands and sensor data, making them susceptible to cyber-attacks such as denial-of-service attacks and man-in-the-middle attacks ^[28].
The maritime very-small-aperture-terminal (VSAT) is an essential component for high-speed data transmission during naval operations. However, it lacks authentication, encryption, security, or personal information verification, making all devices vulnerable to attacks at the implementation level. Attackers could send false signals or malicious codes to disable or compromise the system, potentially endangering the safe navigation of the vessel ^[29].
It is common for the system to be run on old computers without security updates. It is possible to compromise the system when updating the maps by downloading them from the Internet or manually uploading them via USB. The use of this updated medium can expose the system to many security risks ^[30].
Despite the fact that radar signals are harder to interrupt than satellite signals, they are still susceptible to interference and DDoS attacks. Radar can provide inaccurate information about nearby objects in the event of a cyber-attack due to false echoes caused by external radar waves. Inaccurate information can lead to ship collisions.
In the maritime industry, several network types are used for the transmission of data collected and processed by networked information systems. These technologies include SHIPNET, SAFENET, C3I system, RICE 10, SHIP system 2000, Smart Ship, and TSCE. Several security vulnerabilities exist in these technologies, as the design and configuration of communication links between IT networks neglect to consider authentication and encryption methods, leading to potentially vulnerable and outdated systems being available on the Internet ^[31].

4. A Framework for SDN–SDR-Based Maritime Communications

SATCOM offers numerous advantages over traditional point-to-point terrestrial communications, with a wide geographical coverage being the most significant advantage. However, SATCOM’s potential for providing extensive coverage across global regions, the high cost of implementation, and extended propagation delays pose significant hurdles to its deployment within the maritime sector ^[6]. The maritime industry largely relies on SATCOM systems, which are expensive and have a low data rate ^[32]. Frequent maritime activities, on the other hand, require high-speed and reliable data transmission in order to ensure smooth communication between vessels and the control center. Current maritime wireless communication systems, however, do not meet this demand. SDN and SDR have tremendous potential to revolutionize maritime communications. With the proper design, integration, and deployment of SDN–SDR-based wireless communications infrastructure with the existing SATCOM infrastructure, both network and physical layer complexities can be alleviated. Maritime communications can be better managed with the planned deployment of software at these two layers to meet the demands and constraints of constantly changing on- and off-shore maritime communication environmental conditions and jurisdictional regulations.

4.1. The SDR Approach

The International Convention for the Safety of Life at Sea (SOLAS) requires maritime operators to comply with a host of requirements that specify certain constraints on shipboard radio equipment ^[33]. Likewise, the International Telecommunication Union (ITU) has specified equipment to comply with the SOLAS Convention requirements ^[34]. Among these requirements are two-way VHF voice communication and, depending on the vessel, could also include AIS, satellite equipment, and emergency-position-indicating radio beacons. The ITU also identified trends that include E-navigation, VHF data exchange systems, and VHF voice digitalization.

4.1.1. SDR Background

An SDR provides traditional hardware-based signal processing in a reconfigurable software environment ^[35]. Where each new iteration of a communication standard requires legacy radios to upgrade to the most recent chipset and transceiver components to take advantage of new features, an SDR can be adapted to incorporate the changes by reprogramming the software that runs on programmable hardware ^[36]. Adopting the use of SDRs can allow for switching from one communication standard to another, or even using the same communication standard in countries that have differences in the physical layer protocols. Using the 5G spectrum as an example, countries in the Americas, Europe, and Asia are licensing different frequencies to operate equipment ^[37]. Instead of carrying on board and maintaining a multitude of radios, SDRs can be reprogrammed and use infrastructure common among varying implementations. While the use of a purpose-built radio for a globally agreed standard need not be discarded in favor of an SDR, there may be times or operating conditions when the SDR is superior. Marine VHF radios use analog modulation for voice, while AIS is also a VHF band system that uses a digital modulation ^[38].

A single SDR can take the place of both of these radios and implement further functionality, such as the translation of voice-to-text information or even communication similar to SMS messages. Additionally, the SDR allows for agility in transmitting or receiving signals and can be adapted for the wireless environment. Atmospheric ducting has a significant impact on electromagnetic wave propagation. Signals may travel well beyond expected distances and cause unintended interference.

4.1.2. SDR Adoption

SDR solutions exist and may be implemented in maritime communication systems and enhanced to keep up with the increasing demand for novel functionality. The US Navy has been developing SDRs for implementation on surface ships and submarines, saving space, reducing maintenance requirements, and adding functionality ^[39]. There are over 900 radios that have been initially delivered, and these systems are being improved with new features that also include National Security Agency-certified encryption for both voice and data ^[40]. The maritime industry can make ready use of the lessons learned from the US Navy’s investment and enjoy the benefits of capable SDRs. While still meeting SOLAS Convention and ITU requirements, SDRs offer an opportunity to improve current data rate capabilities and remain flexible for future changes in communication protocols.

4.2. The SDN Approach

4.2.1. 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 ^[41]. Figure 2 depicts mapping SDN and SDR to an OSI model. Some of the key advantages of SDNs are as follows:

Figure 2. Mapping SDN and SDR to OSI model.

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 ^[42] and intelligence capabilities ^[43]. 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.

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.

4.2.2. SDN Adoption

In maritime networks, establishing stable communication links is a significant challenge due to the constantly changing sea surface, which increases the risk of link fragility caused by sea waves. Therefore, finding a stable route is crucial to ensure network stability and minimize delays ^[44]^[45]. Recently, SDN has emerged as a solution to reduce the complexity of network management tasks ^[46]^[47]^[48]. By utilizing SDN, networks can be deployed and managed with greater flexibility, which reduces cost and increases availability. Additionally, the SDN controller integrates and learns from the information contained in the network itself to make intelligent decisions continuously.

In ^[49], the authors present a joint sleep scheduling and opportunistic transmission scheme in delay-tolerant maritime wireless communication networks based on SDN to find a viable trade-off between energy consumption and delay. Another solution to reducing the delay of response is through the use of multiple controllers, as presented in ^[50]. Figure 3 presents an SDN with a maritime communications network.

Figure 3. Maritime communications platforms with SDN.

4.3. Maritime Communications Security with SDN

An SDN-based framework can be used to mitigate attacks in an automated manner for improved resilience in the ship’s communication network ^[51]. There are sensors and actuators attached to the different components of the ship that are responsible for controlling the bridge, the engine, and the propulsion. The sensors transmit the data related to these physical devices to the controller for analysis. The controller, known as the Monitoring Controller, plays a crucial role in overseeing the operation of the ship’s components. It continuously monitors the data received from the sensors and analyzes them to ensure the proper functioning of the various ship components. Depending on the information obtained from the bridge devices, the Monitoring Controller issues commands to start or stop the propulsion control system and can even reroute the ship on a different route if necessary.

With the help of the Detection Engine, the Monitoring Controller can quickly identify any anomalies or deviations from normal behavior. The Detection Engine examines the network traffic within the ship’s communication network and detects suspicious or malicious activities by employing various techniques and underlying algorithms. It analyzes the network traffic patterns, protocols, and payload contents to identify potential threats or attacks. Once a suspicious activity is detected, the Detection Engine raises an alert and informs the Monitoring Controller about the potential security breach.

Figure 4 shows these components and the relations between them. Note that each of the three controllers within the controller layer, depicted in Figure 5, can have the Detection Engine and the Mitigation Engine, for more robust overall security.

Figure 4. SDN-based framework for attack mitigation.

Figure 5. SDN-based maritime network.

4.4. A Use 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.

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.

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 ^[52]. 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. The links among these platforms support a wide range of applications supporting strategic and tactical C4ISR functions.

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.

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 ^[53]. Figure 6 depicts a 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.

Figure 6. Logical, i.e., network layer (SDN), COMMS with the proposed SDN–SDR cross-layer unified framework.

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