With some 25 billion devices expected to be connected to the Internet by 2015 and 50 billion by 2020, providing interoperability among the things on the IoT is one of the most fundamental requirements to support object addressing, tracking, and discovery as well as information representation, storage, and exchange.

The lack of explicit and formal representation of the IoT knowledge could cause ambiguity in terminology, hinder interoperability and mostly semantic interoperability of entities in the IoT world. Furthermore, lack of shared and agreed semantics for this domain (and for any domain) may easily result to semantic heterogeneity – i.e. to the need to align and merge a vast number of different modelling efforts to semantically describe IoT entities, efforts conducted by many different ontology engineers and IoT vendors (domain experts). Although there are tools nowadays to overcome such a problem, it is not a fully automated and precise process, and it would be much easier to do so if there is at least a partial agreement between the related stakeholders – i.e. a commonly agreed IoT ontology.

In these circumstances, an ontology can be used as a semantic registry for the facilitation of the automated deployment of generic and legacy IoT solutions in environments where heterogeneous devices also have been deployed. Such a service can be delivered by IoT solution providers, supporting the interoperability problems of their clients/buyers remotely when buying third-party devices or applications. Practically, this will require the existence of a central point – e.g. a web service/portal for both end users (buyers of the devices) and the IoT solution providers (sellers of the applications) to register their resources, i.e. both the devices and the IoT solutions, in an ontology-based registry.

The Sensor Web Enablement (SWE) standards enable developers to make all types of sensors, transducers and sensor data repositories discoverable, accessible and usable via the Web. Sensor technology, computer technology and network technology are advancing together while demand grows for ways to connect information systems with the real world. Linking diverse technologies in this fertile market environment, integrators are offering new solutions for plant security, industrial controls, meteorology, geophysical survey, flood monitoring, risk assessment, tracking, environmental monitoring, defence, logistics and many other applications. The SWE effort develops the global framework of standards and best practices that make linking of diverse sensor-related technologies fast and practical. Standards make it possible to put the pieces together in an efficient way that protects earlier investments, prevents lock-in to specific products and approaches, and allows for future expansion. Standards also influence the design of new component products. Business needs to drive the process. Technology providers and solutions providers need to stay abreast of these evolving standards if they are to stay competitive.

Semantic Web technologies have been proposed as a means to enable interoperability for sensors and sensing systems in the context of SWE. Semantic Web technologies could be used in isolation or in augmenting SWE standards in the form of the Semantic Sensor Web (SSW). Semantic technologies can assist in managing, querying, and combining sensors and observation data, thus allowing users to operate at abstraction levels above the technical details of format and integration, instead of working with domain concepts and restrictions on quality. Machine-interpretable semantics allows autonomous or semi-autonomous agents to assist in collecting, processing, reasoning about, and acting on sensors and their observations. Linked Sensor Data may serve as a means to interlink sensor data with external sources on the Web.

One of the primary outcomes of the SSW research is the Semantic Sensor Network (SSN) ontology (by W3C Semantic Sensor Network Incubator Group). This IoT ontology provides all the necessary semantics for the specification of IoT devices as well as the specifications of the IoT solution (input, output, control logic) that is deployed using these devices. These semantics include terminology related to sensors and observations, reusing the one already provided by the SSN ontology, and extended to capture also the semantics of devices beyond sensors – i.e. actuators, identity devices (tags), embedded devices, and of course the semantics of the devices and things that are observed by sensors, that change their status by actuators, that are attached to identity tags, etc. Furthermore, the ontology includes semantics for the description of the registered IoT solutions - i.e. input, output, control logic – regarding aligning and matching their requirements with the specifications and services of the registered devices.