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The Semantic Web
will be composed of a series of
standards. These standards have to be organised into a certain structure that
is an expression of their interrelationships. A suitable structure is a
hierarchical, layered structure.
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The slide
illustrates the different parts of the Semantic Web in the vision of Tim
Berners-Lee. The notions are explained in an elementary manner here.
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Layer 1. Unicode
and URI
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At the bottom
there is Unicode and URI. Unicode is the Universal code. Unicode codes the
characters of all the major languages in use today.
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Layer 2. XML,
namespaces and XML Schema
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Layer 3. RDF en
RDF Schema
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The first two
layers consist of basic internet technologies. With layer 3 the Semantic Web
begins.
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RDF Schema
(rdfs) has as a purpose the introduction of some basic ontological notions.
An example is the definition of the notion “Class” and “subClassOf”.
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Layer 4. The
ontology layer
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The definitions
of rdfs are not sufficient. A more extensive ontological vocabulary is
needed. This is the task of the Web Ontology workgroup of the W3C who has
defined already OWL (Ontology Web Language) and OWL Lite (a subset of OWL).
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Layer 5. The
logic layer
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In the case
study the use of rules was mentioned. For expressing rules a logic layer is
needed. An experimental logic layer exists.
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Layer 6. The
proof layer
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In the vision of
Tim Berners-Lee the production of proofs is not part of the Semantic Web. The
reason is that the production of proofs is still a very active area of
research and it is by no means possible to make a standardisation of this. A Semantic Web engine should only need to
verify proofs. Someone sends to site A a proof that he is authorised to use
the site. Then site A must be able to verify that proof. This is done by a
suitable inference engine. Three inference engines that use the rules that
can be defined with this layer are: CWM, Euler and RDFEngine developed as
part of this thesis.
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Layer 7. The
trust layer
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Without trust
the Semantic Web is unthinkable. If company B sends information to company A
but there is no way that A can be sure that this information really comes
from B or that B can be trusted then there remains nothing else to do but
throw away that information. The same is valid for exchange between citizens.
The trust has to be provided by a web of trust that is based on cryptographic
principles. The cryptography is necessary so that everybody can be sure that
his communication partners are who they claim to be and what they send really
originates from them.
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The trust policy
is laid down in a “facts and rules” database. This database is used by an
inference engine like RDFEngine. A user defines his policy using a GUI that
produces a policy database. A policy rule might be e.g. if the
virus checker says ‘OK’ and the format is .exe and it is
signed by ‘TrustWorthy’ then accept this input.
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The impression
might be created by the slide that this whole layered building has as purpose
to implement trust on the internet. Indeed it is necessary for implementing
trust but, once the pyramid of the slide comes into existence, on top of it
all kind of applications can be built.
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Layer 8. The
applications
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This layer is
not in the figure; it is the application layer that makes use of the
technologies of the underlying 7 layers. An example might be two companies A
and B exchanging information where A is placing an order with B.
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