computer communication b
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Computer communication B. The Semantic Web. Bibliography. The 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 - PowerPoint PPT PresentationTRANSCRIPT
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Computer communication BThe Semantic Web
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Bibliography The 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
http://www.w3.org/ Antoniou, G., Van Harmelen, F. `(2004) “A
Semantic web Primer”(see library or Pdf copy)
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The semantic web: some definitions The 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)
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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)
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Semantic Web: Introduction The 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
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The semantic web: characteristics
The 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
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The semantic web: The layer cake
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The semantic web Two characteristics for the construction of the semantic web
1. 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.
2. 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.
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XML: Extensible Markup Language 1 It 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)
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WWW: HTML
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Semantic web: XML
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XML: Extensible Markup Language 2
A 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”
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XML: Extensible Markup Language 3
A 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
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XML: Extensible Markup Language 4
A 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)
value
Name
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RESOURCE, URIs and NAMESPACES A 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 WebURIs can follow different schemes
FTP (File Transfer Protocol) HTTP (Hypertext Transfer Protocolhttp://www.mysite.com/food.html
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Namespaces
Namespaces 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 #
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The semantic web: The layer cake
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RDF: Resource Description Framework 1
RDF is a general-purpose language for representing information in the webUseful 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
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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
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RDF-Schema
A RDF-schema:Offers the bases to model hierarchies and
classes of properties.
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The semantic web: The layer cake
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Ontology: definitions
Ontology 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 readable
A abstract model
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Ontology Ontology 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
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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
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Ontology vs Taxonomy
Taxonomy 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 ……
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Web Ontology Languages They are designed to define ontologies They are based on RDF and RDF-schema
SHOE Oil (Ontology inference Layer)
OWL (Web Ontology Language)http://www.w3.org/TR/owl-features/ 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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OWL 1 Describes classes, properties and relations to
facilitate machine interpretability of the Web content
Owl is defined as a vocabulary (like RDF) but is semantically richer
In OWL classes of entities can be specified in different ways, for example: Which individuals belong to a specific class Which qualities should have the elements that belong
to that class If individuals belong to a particular underclass
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OWL 2
It is possible to limit a membership in a subclass
With RDF-schema it is possible to produce subclasses
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OWL: example 1 Definition of “name”
Properties, classes and things are distinguished Things can be summarized in classes
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OWL: example 2 Definition of “marine mammal”
Precise definition of classes: How many subclasses are there?
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OWL: example 2 What is dolphin?
A class is defined like a cross-section of two classes
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The semantic web: The layer cake
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Logic 1: Logic rules With logic it is possible to go further than the explicit
information It is possible to control if till now the collected information is
consistent or if it has to be revised With the rule of logic it is possible to derive novel information
Two inference rules Modus Ponens
If A → BATherefore BIf: “if x is a person than this person has a mother”“Jan is a person” → “Jan has a mother”
Modus Tollens (from “tollere” take away from) If A → B⌐BTherefore ⌐AIf: “if x is a person than this person has a mother”“Jan does not have a mother” → “Jan is not a person”
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Logic 2 Logic is the study of the principles of valid inferences
and demonstrations (traditionally considered a part of philosophy) Logic rules are used to create reasonings Their formulation is close to the formulation used in natural
language Logic is about the rules themselves, and not about the
way they have to be applied. Classical logic is unfortunately too limited to model all
types of human reasoning Default-reasoning:
“if x is a bird then x can fly” “if x a penguin is then x can not fly”
More complicated forms of logic are more difficult to process
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The semantic web: The layer cake
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Proof There is a difference between the rule
formulation and their application to solve a problemThere must be an intelligent choice of rules
and facts (a good order for the rules)There should be good strategies for efficient
argumentations Proof-systems are important topics in
informatics But not much has been done in the
domain of the Semantic Web
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Trust Trust can be perceived at different levels
Is the processed information plausible? How can be trust be verified?
On human judgmentsBased on social trust (trust people that are
trusted by other people)