2.2. Interoperability Using the OGC SensorThings API
Interoperability is a major challenge for the IoT. The real potential of IoT lies in the “systems of IoT systems” rather than with disparate IoT silos [29,30]. An interoperable IoT system of systems provides a uniform way for sharing, finding, and accessing IoT sensing and tasking capabilities, the Internet, and different applications [31]. Interoperability requires layers of standards in order to address the heterogeneity issues amongst sensors, data, and networks [32]. Data and sensor interoperability refer to the ability to exchange and understand data formats, protocols, and sensor models. Network interoperability has no value if the bits and bytes are delivered but cannot be interpreted, i.e., if the data being exchanged over the network cannot be understood by machines a priori. Further, various levels of interoperability include synthetic, semantic, and cross-domain interoperability which mean the standardization of conceptual models, practices, and policies from disparate systems.
The OGC SensorThings API (OGC STA) [22,25] is an OGC and United Nation’s International Telecommunication Union Telecommunication (ITU-T) standard that defines a data model and an API for IoT sensing and tasking interoperability. The OGC STA is part of the well-established OGC Sensor Web Enablement (SWE) suite of open international standards [23]. SWE standards are in use by many large-scale organizations such as the Department of Homeland Security (DHS) [33], National Aeronautics and Space Administration (NASA), National Oceanic and Atmospheric Administration (NOAA), United States Geological Survey (USGS) [34], Natural Resource Canada (NRC), the World Meteorological Organization (WMO), and many others, including private sector companies [29,35,36]. The previous generation of SWE standards, such as the Sensor Observation Service (SOS), are heavyweight when it comes to running applications in edge devices with limited resources [37]. OGC STA represents the new generation of SWE standards that was specifically designed for IoT applications and is thus efficient and lightweight, e.g., it uses the REpresentational State Transfer pattern (RESTful) and the efficient JSON encoding. The OGC SensorThings API follows the ODATA (Open Data Protocol) for managing the sensing resources. As a result, it has a REST-like API and supports the Hypertext Transfer Protocol (HTTP) create, read, update, and delete operations (i.e., GET, POST, PATCH, DELETE) and ODATA query options (select, expand, filter, orderby, top, skip) for data retrieval [38]. In addition to supporting HTTP, the OGC SensorThings API has an extension for supporting Message Queuing Telemetry Transport (MQTT) for the creation and real-time retrieval of sensor Observations [39].
The OGC STA enables interoperability for two layers: (1) Service interface, and (2) Data model [40]. With regards to the service interface layer, the STA defined a RESTful pattern, based on the OASIS OData standard, that allowed different STA services to exchange and filter entities defined by the STA data model. As for the data model aspect, the STA data model was based on the International Organization for Standardization (ISO) and OGC Observation and Measurement standard model [41]. As a result, the data model can interoperate and is backward compatible with the OGC Sensor Observation Service (SOS) Web service. The following UML diagram describes the entities of the STA data model. In the OGC STA, every Thing can have zero or more locations in space or time ((Figure 2). Furthermore, each Thing can have zero or more “Datastreams”. A Datastream is a collection of “Observation” entities grouped by the same “ObservedProperty”. An Observation is an event performed by a “Sensor”, that is a process producing a result with a value that estimates the ObservedProperty of a “FeatureofInterest”.
The OGC STA provided an interoperable framework with which to build the proposed IoCT. STA’s O&M-based data model and query functions have been shown to work for a very wide range of IoT systems from simple weather stations to complex drone systems. By using the OGC STA, we were able to develop an IoCT that interconnects heterogeneous IoT devices, data, and applications over the Web. In order to deal with the pandemic’s fast-changing requirements, IoT developers need an established working architecture that will work not only for today, but also for future, COVID-19 applications. In addition, healthcare applications are often near real-time and need to be scalable and performant, i.e., able to accommodate a very large number of devices that are sending high frequency data simultaneously without sacrificing performance. The goal for the IoCT is to build an interoperable foundation for future expansion and integration using various existing and new COVID-19 IoT applications.