**3.2 The Semantic Web stack**

The Semantic Web stack also called the Semantic Web cake is basically a hierarchy of the technologies composed of different layers. Each layer takes advantages of the capabilities concerning all the sub-layers. The following figure 5. illustrates the Semantic Web cake.

Fig. 5. The Semantic Web Stack.

Spatialization of the Semantic Web 173

engines. This layer has not been formally defined so subjected to certain degrees of

The top two layers in the stack are not yet fully conceptualized in terms of their applicability. These layers contain technologies which are not standardized yet but still they point toward maintaining the authenticity in the knowledge generated. The layer describing proof is therefore presumed responsible for providing evidence for the accuracy. At present no technology recommended to support this layer exists but there is an attempt for developing a proof language called Proof Mark-up Language (PML (da Silva et al., 2004; Al-Feel et al., 2009)) by knowledge systems laboratory at Stanford University. The top most layer Trust is to certify the knowledge reliability and there is a degrees of confidence in the knowledge generated within the layers under it. Again, at present there is no technology to

The figure 5 can hence be updated with the three categories defined in this section and is

Semantic Web technologies are built up through the Web technologies that could hold up contents. The emergence of the eXtensible Markup Language (XML) marked the beginning of content based information in the Web environment. The language can encode information in machine readable format. The XML syntax is recommended in various data models and this syntactical approach laid a foundation for data models for defining metadata as Resource Description Framework (RDF). Resources are conventionally described through their metadata. The W3C recommended RDF as a standard to define the resources on Web.

manipulation.

support the layer.

illustrated in figure 6.

Fig. 6. The layers of the Semantic Web stack.

**3.2.1 The syntactic layer**

There is a degrees of uncertainty in which the Semantic Web cake is defined. There are four versions of this cake till date, and none of them have been published in the literatures. All the four versions are presented by Berners-Lee in his presentations (Gerber et al., 2008). The components and their relationships are hence been defined profoundly. It is thus necessary to isolate each component and discuss their role in terms of the Web. The definitions of some layers within the semantic cake are illusive and could be interpreted in many ways. However, some layers especially the lower layers have clear definitions. Here, these hierarchical layers are discussed in terms of the knowledge representation approach. The layers in the Semantic Web stack can generally be divided into three categories: syntactic layers, knowledge layers and certifying layers composing of different technologies to support the technology.

The bottom layers are information holding layers and are either presented in uniform language or the through XML based information. The components within this layer hold the technologies that are direct descendent technologies from the hypertext Web. Though they are carryovers from basic technologies, they provide strong base to the Semantic Web. The technologies within these layers present syntactical representation of the information and thus be grouped into one common category of syntactic layers. They are capable to hold huge amount of information in each of the individual technologies within the level. These technologies include basic technologies as URI or content based technologies as XML and RDF. Despite rich with contents they lack interpretations as they do not possess semantics within.

The middle section contains layers which represents knowledge. These layers generally represent the technologies standardize by W3C for processing knowledge and can be grouped together to knowledge layer. The technologies here utilize the syntactically rich technologies in layers beneath. The knowledge is generated through attaching semantics to the information. RDFS provides vocabulary to RDF thus providing semantics to the structured statements representing the information as triplets. Through RDFS technology it is possible to derive hierarchical representations of objects and relate the objects to each other. The technology bridges the gaps between syntactically rich contents and tools to interpret knowledge from these contents. RDFS can define ontologies. Ontologies play important roles in order to provide semantics to the information or to the contents by providing suitable vocabulary to the contents and uplift contents to resources which could be related to real world objects. As a result of the work of the W3C Web Ontology Working Group, the "Ontology" layer has now been instantiated with the Web Ontology Language (OWL (Smith et al., 2004)) (Horrocks et al., 2005) due to its extended constructs to describe the semantics of the RDF statement. The semantic within the ontologies and expressed through OWL can be used within the ontologies and the knowledge bases themselves for the inferences. However, in order to express the rules independent to the languages two standards are emerging in the form of RIF (Boley & Kifer, 2010) and Semantic Web Rule Language (SWRL) (Horrocks et al., 2004). The rules are supported through inference engines. Simple Protocol and RDF Query Language (SPARQL) (Prud'hommeaux & Seaborne, 2008) is SQL equivalent language for querying data stored as RDF resources. As OWL is basically written in RDF pattern so the query could be applied to it as well. The topmost layer within knowledge layer is the unifying logic layer. This layer provides the logic behind knowledge manipulation through the reasoning capabilities of reasoning 172 Semantics – Advances in Theories and Mathematical Models

There is a degrees of uncertainty in which the Semantic Web cake is defined. There are four versions of this cake till date, and none of them have been published in the literatures. All the four versions are presented by Berners-Lee in his presentations (Gerber et al., 2008). The components and their relationships are hence been defined profoundly. It is thus necessary to isolate each component and discuss their role in terms of the Web. The definitions of some layers within the semantic cake are illusive and could be interpreted in many ways. However, some layers especially the lower layers have clear definitions. Here, these hierarchical layers are discussed in terms of the knowledge representation approach. The layers in the Semantic Web stack can generally be divided into three categories: syntactic layers, knowledge layers and certifying layers composing of different technologies to

The bottom layers are information holding layers and are either presented in uniform language or the through XML based information. The components within this layer hold the technologies that are direct descendent technologies from the hypertext Web. Though they are carryovers from basic technologies, they provide strong base to the Semantic Web. The technologies within these layers present syntactical representation of the information and thus be grouped into one common category of syntactic layers. They are capable to hold huge amount of information in each of the individual technologies within the level. These technologies include basic technologies as URI or content based technologies as XML and RDF. Despite rich with contents they lack interpretations as they do not possess semantics

The middle section contains layers which represents knowledge. These layers generally represent the technologies standardize by W3C for processing knowledge and can be grouped together to knowledge layer. The technologies here utilize the syntactically rich technologies in layers beneath. The knowledge is generated through attaching semantics to the information. RDFS provides vocabulary to RDF thus providing semantics to the structured statements representing the information as triplets. Through RDFS technology it is possible to derive hierarchical representations of objects and relate the objects to each other. The technology bridges the gaps between syntactically rich contents and tools to interpret knowledge from these contents. RDFS can define ontologies. Ontologies play important roles in order to provide semantics to the information or to the contents by providing suitable vocabulary to the contents and uplift contents to resources which could be related to real world objects. As a result of the work of the W3C Web Ontology Working Group, the "Ontology" layer has now been instantiated with the Web Ontology Language (OWL (Smith et al., 2004)) (Horrocks et al., 2005) due to its extended constructs to describe the semantics of the RDF statement. The semantic within the ontologies and expressed through OWL can be used within the ontologies and the knowledge bases themselves for the inferences. However, in order to express the rules independent to the languages two standards are emerging in the form of RIF (Boley & Kifer, 2010) and Semantic Web Rule Language (SWRL) (Horrocks et al., 2004). The rules are supported through inference engines. Simple Protocol and RDF Query Language (SPARQL) (Prud'hommeaux & Seaborne, 2008) is SQL equivalent language for querying data stored as RDF resources. As OWL is basically written in RDF pattern so the query could be applied to it as well. The topmost layer within knowledge layer is the unifying logic layer. This layer provides the logic behind knowledge manipulation through the reasoning capabilities of reasoning

support the technology.

within.

engines. This layer has not been formally defined so subjected to certain degrees of manipulation.

The top two layers in the stack are not yet fully conceptualized in terms of their applicability. These layers contain technologies which are not standardized yet but still they point toward maintaining the authenticity in the knowledge generated. The layer describing proof is therefore presumed responsible for providing evidence for the accuracy. At present no technology recommended to support this layer exists but there is an attempt for developing a proof language called Proof Mark-up Language (PML (da Silva et al., 2004; Al-Feel et al., 2009)) by knowledge systems laboratory at Stanford University. The top most layer Trust is to certify the knowledge reliability and there is a degrees of confidence in the knowledge generated within the layers under it. Again, at present there is no technology to support the layer.

The figure 5 can hence be updated with the three categories defined in this section and is illustrated in figure 6.

Fig. 6. The layers of the Semantic Web stack.

#### **3.2.1 The syntactic layer**

Semantic Web technologies are built up through the Web technologies that could hold up contents. The emergence of the eXtensible Markup Language (XML) marked the beginning of content based information in the Web environment. The language can encode information in machine readable format. The XML syntax is recommended in various data models and this syntactical approach laid a foundation for data models for defining metadata as Resource Description Framework (RDF). Resources are conventionally described through their metadata. The W3C recommended RDF as a standard to define the resources on Web.

Spatialization of the Semantic Web 175

RDFS or the RDF Schema is the semantic extension of RDF. The applications using RDF uses it to describe its resources and those descriptions can be modeled as relationships among Web resources. These models constitute of interrelationships among the resources. They are carried out through the named properties and values. It however lacks the mechanism of defining the relationships between properties and other resources. Furthermore RDF data models do not declare these properties. They are hence information without any semantics. RDFS is designed to address these shortcomings. RDFS provides mechanisms for describing

OWL or the Web Ontology Language is a family of knowledge representation language to create and manage ontologies. It is in general term an extension of RDFS with addition to richer expressiveness that RDFS lacks through its missing features (Antoniou & Harmelen, 2003). The OWL Working Group has approved two versions of OWL: OWL 1 and OWL 2. This research work uses OWL 1 for the applications of ontology as this version was the most used version at the time of research. The later version of OWL 1 was just evolving during

The expressiveness of OWL depends upon the level of serialization. The expressiveness of OWL comes at the cost of computational efficiency and reasoning effectiveness. This tradeoff between expressiveness and reasoning support was addressed through classifying

OWL Full contains the maximum expressiveness but may lack in computational processing capability. It may also have restricted reasoning efficiency. OWL Full is completely compatible with RDF/RDFS both syntactically and semantically. OWLDL is compatible to the components of description logics and provides the functionalities of DLs. It provides the complete computational efficiency and reasoning capabilities. It is sub language of OWL Full with all OWL language constructs which could be used only through certain restrictions (McGuinness & Harmelen, 2004). This restriction is even more in OWL Lite – the third sublanguage of OWL. The advantage of this language is its easiness to understand and implement but the drawback is it is just a simple and fast migration from thesauri and other

It has been stated before that RDF statements store data in the form of informative contents. In this manner, it could be easily argued RDF documents are datasets complimenting the data storage capability of its conventional counterparts as database systems. As database systems provide efficient retrieval of the data through its query language in form of

statements should be rigorously audited for the model to be reliable enough. The semantic within the model should not be ambiguous. The meaning of each

statement within the model should be clear and unambiguous.

groups of related resources and the relationships between these resources.

the period. This research work discusses its activities in terms of OWL 1.

OWL into three sub languages by the W3C Web Ontology working group.

**The Web Ontology Language (OWL)** 

**RDFS** 

taxonomies.

**The SPARQL language** 

knowledge model because they decide the reliability of the model. If any model contains statements which contradict with each other, the model cannot be considered reliable. For an example A cannot be a father and son of B at the same time. Such

W3C has defined five major reasons for developing the standard (Klyne et al., 2004). They focus on automatization of the information processing through serialization. That means the contents inside the documents are machine processable. In order for the documents to be machine processable they need to be machine readable and since the syntax of RDF is based on XML, it provides a mechanism to represent the information in machine readable manner.

The RDF (Resource Description Framework) is a graph data model. It is basically a framework to represent information on the Web. It has also been assigned as the standard model for data interchange on the Web by W3C because it can merge different sets of data irrespective to the underlying schema. RDF is conceptualized through graph data model which demonstrates the underlying structure of its expression. The nodes in the graph model are resources which can represent Uniform Resource Identifiers (URI reference or simply URIRef) or literals or even blank. The link in the graph representing properties are generally URI references. The literals within RDF expressions are generally assigned values of certain data types. RDF syntax is primarily based on its predecessor XML and is defined by RDF abstract syntax. This abstract syntax is the syntax over which the formal semantic are defined. It is a set of triples known as RDF graph (Klyne et al., 2004). It consists three parts which are normally called RDF triplet and represent a statement of relationship between the objects.
