# Semantic Distributed Data for Vehicular Networks Using the Inter-Planetary File System
## 2 Semantic Data and Ontologies  
The semantic representation of the data published by vehicles and RSUs makes possible these agent-based machine-to-machine interactions. For instance, a vehicle circulating in Whitechapel could easily ask the network if the Tower Bridge is now open for traffic. There are endless ways to portray transportation data using semantics, and for this reason, agents and data producers need ontologies to provide coherence and unambiguity to the data. Quoting [PERSON], one of the originators of the Semantic Web, an ontology is defined as a \"set of knowledge terms, including the vocabulary, the semantic interconnections, and some simple rules of inference and logic for some particular topic\" [3]. This abstract concept is easier to understand with an example. Figure 2 shows a basic usage of semantic data describing a person using the Friend of a Friend (FOAF) ontology. Semantic data are represented by triples (subject-predicate-object). In this example, the subject is the depicted person, the predicate is indicated with an arrow, and the object is the pointed value. For example, the person's name is [PERSON] and this person is the friend of [PERSON]. This generic structure can be used to describe highly complex concepts and their relationships. In regard to vehicular networks, the ontology or ontologies needed to represent and distribute the data should include numerous details about the different states of the vehicles, RSUs, and their surroundings. For instance, basic details could be the description of how to share the information captured by a traffic camera, or the possible readings of weather stations. Additionally, they should also include semantic definitions about static elements of the vehicular networks, such as city buildings, roads, and traffic rules. These definitions, although not directly used while sharing data, would be needed to establish a common communication framework. A comprehensive ontology would also include specific details, such as the difference between a rotary and a roundabout, how many lanes have the different kinds of roads, and the traffic rule to apply when two vehicles arrive at the same time to an uncontrolled intersection. The task of defining a single large and consistent ontology for every possible concept and situation in every different city and every particular vehicular network would be a tremendous and nearly impossible task. It is more realistic for researchers and developers to give support to multiple interlinked ontologies, promoting a system with the ability to adapt to current and future changes in ontology definitions. The first published ontology for transportation networks was published by [PERSON] et al. in 2005 [27], but numerous alternative ontologies have been proposed since then. The recent survey by [PERSON] and [PERSON] in [28] summarized and analyzed 11 different  
Figure 2: This example shows how to use semantic data and the Friend of a Friend (FOAF) ontology to describe people and their relationships. Image of [PERSON] from the Library of Congress, copyright by [PERSON] (circa 1922). transportation ontologies. The most prevalent and complete ontology is the Km4 City ontology [29]. It provides classes for streets, points of interest (POIs), public transport, and more. Besides, vehicular networks do not need only transportation ontologies. Additional ontologies, such as the Car Accident Ontology for VANETs (CAOVA) [30], can be used to describe car accidents. Others, such as the Semantic Sensor Network (SSN) W3C recommendation, can be used to create detailed reports of the available sensors. Figure 3 provides an example of the CAOVA ontology. It is important to note that the task of adding support for existing or future ontologies should not require changes in the network protocols or infrastructure. Ideally, a system change should only comprise a small software update thanks to its compliance with the Resource Description Framework (RDF) and the Web Ontology Language (OWL) W3C recommendations.