## 2 State of the Art
### Multi-Level Interoperability  
In the rapidly evolving landscape of the IoT, heterogeneous wireless communication technologies have emerged to facilitate seamless connectivity among diverse devices and networks [40]. Prominent among these technologies are those that enable wireless personal area networks (WPANs) like Bluetooth low energy (BLE), those based on the IEEE 802.15.4 standard [41] (e.g., Zigbee, Thread, and 6 LoWPAN), those based on the IEEE 802.11 standard [42] (e.g., WiFi and WiFi HaLow), those encompassed as low-power wide area networks (LPWANs) (e.g., LoRa and SIGFOX), and cellular-based solutions (e.g., NB-IoT and the upcoming 5G NR releases).  
Each of the aforementioned IoT wireless communication technologies has its specific use cases and advantages. However, their coexistence highlights the complex issue of interoperability. This challenge primarily derives from the utilization of distinct physical (PHY) and medium access control (MAC) layers, which impedes the seamless integration of such heterogeneous technologies into a unified ecosystem.  
As IoT continues to expand its reach, achieving robust interoperability among diverse communication protocols remains a vital goal. Due to the inherent differences in IoT communication technologies at the PHY/MAC, interoperability efforts are focused on upper layers. Industry alliances, standards organizations, and consortia are working toward creating common standards and frameworks that enable devices using these protocols to communicate seamlessly with one another. For instance, the Open Connectivity Foundation (OCF) [43] is striving to establish cross-protocol compatibility.  
In addition to the communication technologies described above, which allow the link between the IoT devices and the edge (or even the fog/cloud) layer, multiple communication protocols for the application layer are currently available in the IoT-edge-cloud continuum, ranging from the traditional IoT protocols (i.e., CoAP, MQTT, HTTP, AMQP, DDS, and XMPP) [44] to those expected to play an important role in the future, like gRPC [45].  
Indeed, the most common way to push updates from IoT devices is through MQTT, which is a low-footprint topic-based broadcast messaging protocol. Clients connect to a broker that forwards messages to the subscribers of their respective topics. However, the semantics of the topics and the content of messages are not standardized. Most implementations use string-formatted content such as headeless CSV or JSON, but some prefer sending binary data, for example, using Protobuf serialization. MQTT can be used directly over TCP or via WebSockets. Other solutions opt for using HTTP REST to push updates directly, bypassing the need for an intermediate broker. However, HTTP REST is generally less efficient for high-frequency updates than MQTT.  
The interoperability challenge extends beyond the IoT-edge-cloud continuum, encompassing the broader spectrum of data exchange and collaboration across diverse ecosystems. Standardization efforts, such as the development of common ontologies like the Smart Appliances REFerence (SAREF) [46] or the utilization of modeling tools like the Eclipse Modeling Framework (EMF) [47], play a crucial role in addressing this challenge. COG-NIFOG encourages the adoption of well-known standards with the goal of facilitating the communication among heterogeneous components. An example of such an effort can be observed in the Kentyou DataHub module, where EMF is used to generate its data model in a standardized way, promoting interoperability within the ecosystem.