By Galina Vitkova
The Semantic Web brings the further development of the World Wide Web aimed at interpreting the content of the web pages as machine-readable information.
In the classical Web based on HTML web pages the information is comprised in the text or documents which are read and composed into visible or audible for humans web pages by a browser. The Semantic Web is supposed to store information as a semantic network through the use of ontologies. The semantic network is usually a directed or undirected graph consisting of vertices, which represent concepts, and edges, which represent relations among the concepts. An ontology is simply a vocabulary that describes objects and how they relate to one another. So a program-agent is able to mine facts immediately from the Semantic Web and draw logical conclusions based on them. The Semantic Web functions together with the existing Web and uses the protocol HTTP and resource identificators URIs.
The term Semantic Web was coined by sir Tim Berners-Lee, the inventor of the World Wide Web and director of the World Wide Web Consortium (W3C) in May 2001 in the journal «Scientific American». Tim Berners-Lee considers the Semantic Web the next step in the developing of the World Wide Web. W3C has adopted and promoted this concept.
The Semantic Web is simply a hyper-structure above the existing Web. It extends the network of hyperlinked human-readable web pages by inserting machine-readable metadata about pages and how they are related to each other. It is proposed to help computers “read” and use the Web in a more sophisticated way. Metadata can allow more complex, focused Web searches with more accurate results. To paraphrase Tim Berners-Lee the extension will let the Web – currently similar to a giant book – become a giant database. Machine processing of the information in the Semantic Web is enabled by two the most important features of it.
First – The all-around application of uniform resource identifiers (URIs), which are known as addresses. Traditionally in the Internet these identifiers are used for pointing hyperlinks to an addressed object (web pages, or e-mail addresses, etc.). In the Semantic Web the URIs are used also for specifying resources, i.e. URI identifies exactly an object. Moreover, in the Semantic Web not only web pages or their parts have URI, but objects of the real world may have URI too (e.g. humans, towns, novel titles, etc.). Furthermore, the abstract resource attribute (e.g. name, position, colour) have their own URI. As the URIs are globally unique they enable to identify the same objects in different places in the Web. Concurrently, URIs of the HTTP protocol (i.e. addresses beginning with
http://) can be used as addresses of documents that contain a machine-readable description of these objects.
The architecture of the Semantic Web can be represented by the Semantic Web Stack also known as Semantic Web Cake or Semantic Web Layer Cake. The Semantic Web Stack is an illustration of the hierarchy of languages, where each layer exploits and uses capabilities of the layers below. It shows how technologies, which are standardized for the Semantic Web, are organized to make the Semantic Web possible. It also shows how Semantic Web is an extension (not replacement) of the classical hypertext Web. The illustration was created by Tim Berners-Lee. The stack is still evolving as the layers are concretized.
As shown in the Semantic Web Stack, the following languages or technologies are used to create the Semantic Web. The technologies from the bottom of the stack up to OWL (Web Ontology Langure) are currently standardized and accepted to build Semantic Web applications. It is still not clear how the top of the stack is going to be implemented. All layers of the stack need to be implemented to achieve full visions of the Semantic Web.
As mentioned, top layers contain technologies that are not yet standardized or comprise just ideas. May be, the layers Cryptography and Trust are the most uncommon of them. Thus Cryptography ensures and verifies the origin of web statements from a trusted source by a digital signature of RDF statements. Trust to derived statements means that the premises come from the trusted source and that formal logic during deriving new information is reliable.
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