## 2 Background
### Interoperability in IoUT
#### 2.5.3 Semantic Interoperability  
Semantic interoperability concerns the ability of computer systems and applications to exchange data unambiguously, i.e., under a common and shared understanding of the context and the domain of discourse [40]. A global shared and commonly agreed ontology followed by the common interpretation of semantic information, would be the ideal case [41]. However, the lack of standardization, and the extension of ontology when reusing it, makes the issue of data exchanging with shared meaning constantly bigger. Semantic interoperability provides the ability to enable machine-computable logic, and (therefore) the skill of exchanging meaning with data (i.e., data semantics), in combination with the packaging of data (data syntax). It is a recommended solution to interconnect all devices in an IoUT environment, but also to integrate the human factor. For example, considering that in a swarm of UUVs, various types of vehicles exist (e.g., attacking, surveillance and cyber-surveillance), thus different capabilities, this results in the need to integrate a variety of their heterogeneous sensors and systems. Consequently, in an underwater network, a torpedo firing system, a surveillance radar, and a radar of a UAV that coordinates a target, should all be able to exchange information between each other in a common language. NATO aims to provide solutions to a wide range of underlying problems in interoperability of heterogeneous platforms [10], in processing and interpretation of increased volumes of sensor data, including military command and control (C2) systems, in combination with low-power and long-endurance operations, by experimentation and standardization of protocols (STANAGs). More specifically, the CME Data Knowledge and Operational Effectiveness (DKOE) team developed the Fusion and Reasoning under Uncertainty Modelling (FORUM) research platform, developing an ontology for describing experimental evaluations for the recognition of different classes of ships [10].  
Although UUVs constantly gain popularity, the problem of interoperability between heterogeneous platforms for a multi-vehicle, multi-domain missions are still open [39]. It is crucial to work on a cross-domain communication and interoperable protocol, in order to connect platforms of different manufacturers, architectures and interfaces, similar to the work conducted on JANUS in CME. The road towards overcoming issues of interoperability of UUVs is challenging. It is essential to exchange information such as geographical position or trajectory elements between surface and/or air platforms, as well as to ensure the integrity of the information and to avoid packet loss, considering the restriction of low energy consumption. Furthermore, the robustness of an underwater network is important to guarantee the availability of data. Motivated by this necessity, a routing protocol is presented, named Efficient Void Aware (EVA), to detect the void nodes and suppress them from the eligibility to forward data packet [42]. The general idea is similar to the \"ping\" command used to check connectivity between two end devices over a network. According to the proposed protocol, before every transmission of information from a UUV to a heterogeneous platform and vice versa, a \"test\" packet is being sent to evaluate the availability of other devices. Although the proposed idea is ostensibly effective, many security and bit-rate issues are emerging due to the increase level of the packet transmissions, which results in network congestion. Nevertheless, there is a long way until a framework can connect all these heterogeneous sensors and systems, being able to manage heterogeneous data in such a way that all the platforms communicate under a commonly understandable language.