## 2 Background
### Interoperability in IoUT
#### 2.5.1 Technical Interoperability  
The principal obstacle in achieving a robust interoperability is the seamless interaction between devices and people with devices [33]. Technical interoperability is probably the most demanding among the three levels, mainly due to the lack of standardization in communication protocols [34]. It is divided into three types: interoperability of devices, networks, and platform as follows.  
* Device Interoperability  
This concerns the need to add a new sensor or device in an existing network (USWN), with its own communication protocol and power consumption architecture, interoperating with existing devices of the network [31]. The key issue at this level is the heterogeneity of platforms of institutions or the NATO standards. NATO identified the need to further investigate the key issue of interoperability, the standardization, and the lack of common communication protocols. A representative approach for tackling this issue is the initiative to define a common language to achieve initial contact and data exchange between nodes, namely, the JANUS at NATO STO Centre for Maritime Research and Experimentation (CMRE) [9; 10; 35; 36; 37]. This is the first underwater digital communications protocol which was promulgated as a NATO Standardization Agreement (STNAAG), enabling interoperability between heterogeneous military and civilian devices.  
* Network Interoperability  
This concerns the communication protocols used to meet the requirements of a device to interoperate within a network. The key issues at this level are the quality of service (QoS), scalability, fault tolerance, and security of the network. To ensure robust underwater communication networks, an initial discovery protocol is required and, as mentioned above, JANUS is such a protocol. Essentially, JANUS provides all the requirements in order for all assets of the underwater domain to be able to communicate each other in a standard and common language [38].  
* Platform Interoperability  
Different data sharing policy and operating systems in each platform, such as Microsoft Azure Cloud or Apple Home Kit, result in heterogeneous systems that increase the problem of interoperability [31]. Referring to IoUT, UUVs should be capable of exchanging information efficiently with other underwater platforms, but also with surface and air platforms. Some of the most important data or messages needing to be exchanged, among others, are related to the a) coordination of friendly units or enemy targets, b) their trajectory information, (speed, course, depth), c) their classification in recognized types, d) fuel and battery residues, and e) ammunition residues. An effort to achieve effective exchange of such data between heterogeneous platforms has been proposed by researchers from the Applied Research Laboratory at the University of Hawaii (ARL at UH), the RIP Laboratory at the University of Hawaii at Manoa, the Naval Undersea Warfare Center Division Newport (NUWCDIVNPT), and the Underwater Systems and Technology Laboratory at the University of Porto (LSTS) [39]. Numerous unmanned platforms, including USVs, and UUVs, utilized different open-source software stacks in order to interoperate and \"speak\" the same language. Moreover, an underwater node had the task to continuously order new higher-priority plan-paths to the UUV. Several challenges were encountered, with the most significant being the integration of different communication protocols of platforms, concluding that integrating the new JANUS standard is probably a solution for most of the challenges in interoperability in the underwater domain.