Semantic web: Concepts, Technologies and Applications.

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Computer communication BThe Semantic Web


BibliographyThe Semantic Web, Scientific American, May 2001, Tim Berners-Lee, James Hendler and Ora Lassila. Breitman, K.K., Casanova, M.A., & Truszkowski, W. (2007) Semantic web: Concepts, Technologies and Applications. Springer Verlag, London Antoniou, G., Van Harmelen, F. `(2004) “A Semantic web Primer”(see library or Pdf copy)


The semantic web: some definitionsThe Semantic Web is not a separate Web but an extension of the current one, in which information is given well-defined meaning, better enabling computers and people to work in cooperation (Berners-Lee, Hendlers, J. & Lassila, O., 2001) “The Semantic Web is a vision: the idea of having data on the Web defined and linked in a way that it can be used by machines not just for display purposes, but for automation, integration and reuse of data across various applications (W3C, 2003) “Soon it will be possible to access the Web resources by content rather than just by keywords (Anutariya et al, 2001)


The semantic web (SW): definitions“The Semantic Web is a vision: the idea of having data on the Web defined and linked in a way that it can be used by machines not just for display purposes, but for automation, integration and reuse of data across various applications (W3C, 2003) “Soon it will be possible to access the Web resources by content rather than just by keywords (Anutariya et al, 2001)


Semantic Web: IntroductionThe content of the present Word Wide Web is nowadays only accessible and can be elaborated only by people The Semantic Web is an enlargement of the WWW with semantic information that can be used by computers With the help of semantic information the content of pages could be processed automatically and computers could make inferences about a search


The semantic web: characteristicsThe semantic web is not different from the www, is actually a developing part of it. The infrastructures and characteristics should be common Use URI (Uniform resource Identifiers) addressing Use protocols that a have a small and universally understood set of commands (like HTTP: Hypertext Transfer Protocol) Be decentralized (like the www) Function on a large scale


The semantic web: The layer cake


The semantic webTwo characteristics for the construction of the semantic web Downward compatibility Agents fully aware of a layer should also be able to interpret and use information written at lower levels. For example, agents aware of the semantics of OWL can take full advantage of information written in RDF and RDF Schema. Upward partial understanding On the other hand, agents fully aware of a layer should take at least partial advantage of information at higher levels. For example, an agent aware only of the RDF and RDF Schema semantics can interpret knowledge written in OWL partly, by disregarding those elements that go beyond RDF and RDF Schema.


XML: Extensible Markup Language 1It is a general purpose markup Language for creating specific purpose mark-up languages Follows the SGML-standards (Standard Generlised Markup Language) With XML the single users can create their own tags (which is not possible with HTML) Differences between HTML and XML HTML (Hypertext Markup Language) Has a fixed set of tags It is most frequently used to define the lay-out Does not focus on the logical content or on the structure XML It is possible to personally define the tags Tags reflect a content The layout is defined in a separate document (stylesheet)




Semantic web: XML


XML: Extensible Markup Language 2A XML document consists of plain text and markup, in the form of tags. A XML document is interpreted by application programs A XML document can be represented in a form of a “tree”


XML: Extensible Markup Language 3A XML document consists of Elements formed by A start-tag A content A matching end-tag Elements can be nested in a tree form Every element is named after the start-tag


XML: Extensible Markup Language 4A start-tag can have zero or more attributes Name (followed by the equal sign) Value (between double quotes) Every XML-document has to follow a specific syntax: Every start-tag needs a matching end-tag (see previous slide) Elements need to be nested in other elements An XML-document can contain a XML-schema (defines additional constraints on the document structure) valueName


RESOURCE, URIs and NAMESPACESA resource is anything that has an identity Digital (i.e an electronic document) Physical (i.e. a book) A URI (Uniform Resource Identifier) is a character string that identifies a resource on the Web URIs can follow different schemes FTP (File Transfer Protocol) HTTP (Hypertext Transfer Protocol


NamespacesNamespaces are contexts, the domain of specific elements Namespaces are identified by a URI URIref: It is a URI with an optional fragment identifier attached to it, preceded by #


The semantic web: The layer cake


RDF: Resource Description Framework 1RDF is a general-purpose language for representing information in the web Useful to represent metadata about Web resources RDF describes resources (Both abstract or concrete subjects) identifiable via an URI The syntax of RDF is based on XML RDF-documents are written as XML-documents with the tag rdf:RDF


RDF Statements A RDF-statement is described by a triple (S, P, O) S= Subject of the statement (It’s a URIref) P= Property (Predicate) of the statement (URIref) The value of a property can be a simple value (ordinary number), or can be a literal (string of characters) O= Object


RDF-SchemaA RDF-schema: Offers the bases to model hierarchies and classes of properties.


The semantic web: The layer cake


Ontology: definitionsOntology comes from: Ontos (greek)= Being + Logos=Word Gruber (1993): “An ontology is a formal explicit specification of a shared conceptualization” WC3-consortium “Ontology is a term borrowed from philosophy that refers to the science of describing the kinds of entities in the world and how they are related “ Should be machine readableA abstract model


OntologyOntology categorizes concepts (which are defined by a set of common properties) into classes based on common characteristics Ontology is the representation of the knowledge of a domain where a set of objects and their relationships is described by a vocabulary. Ontologies should provide descriptions for Classes (things) in the various domains Relationships among things Properties of these things


Ontology Ontologies should satisfy certain demands: Expressivity: domains should be described Consistency: it should not give contradictory information It should support reasoning processes Ontologies are useful in sharing and exchanging information between software agents Ontologies do not necessarily reflect the human way of thinking of how knowledge is classified Ontologies should therefore not be seen as a reflection of human intelligence


Ontology vs TaxonomyTaxonomy Is a classification of terms in form of a hierarchy using typically a father-son relationship (i.e. Type of) Example The taxonomy of the leaving beings Kingdom: Animalia Filo: Chordata Subfilo: Vertebrata ……


Web Ontology LanguagesThey are designed to define ontologies They are based on RDF and RDF-schema SHOE Oil (Ontology inference Layer) OWL (Web Ontology Language) It is an ontology description language It is a standard language for the modeling of ontologies Facilitates the interpretability of the Web content (more than XML or RDF) Less complex than RDF-schema Has additional vocabulary based on description logic

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Last Updated: 8th March 2018

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