Computer Standards & Interfaces 31 (2009) 846–855

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Computer Standards & Interfaces

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OWL-L: An OWL-based language for Web resources links

I-Ching Hsu a,⁎, Yuan Kwei Tzeng b, Der-Cheng Huang b

a Department of Computer Science and Information Engineering, National Formosa University, 64, Wenhua Rd., Huwei Township, Yunlin County 632, Taiwanb Department of Computer Science and Engineering, National Chung-Hsing University, 250 Kuo-Kuang Rd, Taichung, Taiwan

⁎ Corresponding author.E-mail address: husic@nfu.edu.tw (I.-C. Hsu).

0920-5489/$ – see front matter © 2008 Published by Edoi:10.1016/j.csi.2008.09.029

a b s t r a c t

a r t i c l e i n f o

Article history:

Although providing metad Received 22 December 2007Received in revised form 25 July 2008Accepted 28 September 2008Available online 17 November 2008

Keywords:OWLOWL-LXLinkOntologySemantic Web

ata association, XLink lacks computer-interpretability to support knowledgerepresentation for intelligent applications. This study proposes an OWL-based language, called OWL-L, tomake Web resources links computer-interpretable. Two aspects of OWL-L, link profile and link model aredescribed. The link profile provides the information required for an agent to discover a link, while the linkmodel provides information that enables an agent to exploit a link. Finally, this study describes the feasibilityof using role/arcRole properties of links to represent OWL-based ontologies, which can thus seamlesslyinteroperate and integrate with OWL-L to enhance knowledge representation.

© 2008 Published by Elsevier B.V.

1. Introduction

The XML Linking Language (XLink) [1] provides a surface syntaxfor generating both basic unidirectional links and complex linkingstructures. It allows an XML document to associate metadata with alink, and asserts linking relationships among more than two Webresources. Clearly, XLink is a powerful complement to conventionalHTML hyperlinks. The main limitation of XLink is that it is an XML-based development, and thus focuses on syntax and format ratherthan semantics and knowledge. Therefore, XLink has the advantage ofmetadata association, but it lacks computer-interpretability to supportknowledge representation for intelligent applications.

To improve this problem, a mapping from links of XLink tostatements in an RDF (Resource Description Framework) model hasbeendefined [2]. Such amapping allows XLink elements to beharvestedas a resource of RDF statements. Additionally, RDF and relatedspecifications are designed to make statements about the resource onthe Web (that is, anything that has a URI), without the requirement tochange the resource itself. This feature enables document authors toannotate and encode the semantic relationships among resources on theWeb. However, RDF alone does not provide basic common structuresthat help describe the resource classes and represent the types ofrelationships between resources. A specification with more facilitiesthan those found in RDF to express semantics flexibly is needed. TheSemantic Web [3,4] can help solve this problem.

The Semantic Web is rapidly becoming a reality through thedevelopment of Semantic Web markup languages such as RDF, RDF

lsevier B.V.

Schema [5], DAML+OIL [6], and OWL (Ontology Web Language) [7].According to Fig. 1, these markup languages enable the creation ofarbitrary domain ontologies that support the unambiguous descrip-tion of Web content. XLink provides some extra built-in linkfunctionality over RDF, but does not encode semantic relationshipsas effectively. XLink is a viable alternative for adopting the full powerof the Semantic Web technology for encoding extended and externallinks. OWL, defined by W3C, is more able to express semantics thanthese languages, and it is superior to others in marking itself asmachine-readable. Hence, this study proposes an OWL-based ontol-ogy for linking Web resources, called OWL-L, to make Web resourceslinks computer-interpretable.

The emerging web ontology language OWL can create an ontologymultiple-layered architecture [8] for OWL-L to support the followingfunctions:

1) Supporting expression of semantics to implement domain ontolo-gies [9]. For instance, OWL adds more vocabulary for describingproperties and classes: relations between classes (e.g. disjointness),cardinality (e.g. exactly one), equality, richer typing of properties,characteristics of properties (e.g. symmetry), and enumeratedclasses. OWL has the advantages of efficient reasoning support,sufficient expressive power, and convenient expression.

2) Providing the high level taxonomy from a specific domain torepresent classification of concepts and instances [10,11]. Thetaxonomy ontology can be used to inherit knowledge and facilitatesearching. OWL-L is developed to provide the infrastructure forinteroperability of terms in the domain of Web resources links.

3) Retrieving the appropriate information from objects by providing astructure to annotate the metadata of a object with semanticinformation [12,13]. For instance, the link provider adopts OWL-L

2

Fig. 1. Layered approach to ontology language development.

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to annotate Web resources links metadata, which help softwareagents to locate, select, and compose links automatically.

In XLink, both the role and arcRole attributes provide semanticinformation. The role attribute indicates the property of a Webresource, while the arcRole attribute describes the objects of an arc'sending Web resource with respect to its starting resource. The valuesof both the role and arcRole attributes are URI references that identifythe Web resources of the intended property. The format of this Webresource is not standardized in XLink, and hence is open forproprietary semantic extensions [14]. OWL-L defines the range ofthe role/arcRole properties as an owl:Thing, allowing a Web resourceto refer to additional semantics through the role/arcRole properties toassign a specific ontology class.

This paper is organized as follows. Section 2 presents an upperontology for OWL-L. Section 3 introduces the link profile ontology.Section 4 gives the details of the link model ontology and itscomponents used in OWL-L. In Section 5, an applicable demonstrationis presented to illustrate how the OWL-L can support more power ofsemantic description capabilities than XLink. Section 6 presents somerelated works. Finally, summary and concluding remarks are included.

2. An upper ontology for OWL-L

The current version of OWL-L builds on the Ontology WebLanguage (OWL) by following the layered approach to SemanticWeb development [15]. The main contribution of OWL-L is to allowsoftware agents to locate and compose links automatically. OWL-L andOWL-S [16,17] are located on the same layer, as shown in Fig. 1. Thus,OWL-L and OWL-S have similar functions and frameworks, describingWeb resources to improve the capabilities of search agents. OWL-Sand OWL-L differ mainly in that former OWL-S describes thecharacteristics and capabilities of Web Services, while the latterannotates Web resources links. The Web Services are described withOWL-S, and are referred to as target resources of the links described byOWL-L. Web Services are Web resources that can be described byOWL-L and OWL-S for different purposes.

In OWL, abstract concepts of entities or events are defined in termsof classes and properties. OWL-L is based on XLink to define a set ofclasses and properties, specific to the description of Web resourceslinks, within OWL. The OntoLink, LinkProfile, LinkModel and rdfs:Resource classes are defined in the top level ontology1 of the OWL-L,as shown in Fig. 2.

The class OntoLink provides an organizational point of referencefor declaring Web resources links. The properties presents,describedBy and providedBy belong to OntoLink. The classesLinkProfile, LinkModel and rdfs:Resource are the respective rangesof those properties. The link profile offers the information required

1 The top level ontology of OWL-L can be found at http://sparc.nfu.edu.tw/~hsuic/sw/OWL-L/1.0/OntoLink.owl.

by an agent to determine whether the link meets its roughrequirements. Meanwhile, the link model describes what happenswhen the link is followed. The link model enables an agent to do thefollowing: (1) analyze in more detail whether the link complieswith its requirements; (2) compose link descriptions from multipleextended links to perform a specific application; (3) decompose alinkbase to extract the required extended links, and (4) trace andrecord the navigational behavior of users.

Each instance of the OntoLink class is an annotation documentencoded in OWL-L. This annotation document contains a link profileand model for describing metadata of Web resources links. Thefollowing sections discuss the link profile and model in greater detail.

3. Link profile

While describing what a link does, the link profile ontology2

characterizes the link for purposes of discovery and matchmaking;that is, it gives the information required by a search agent todeterminewhether the linkmeets its needs. Link profiles are generallybe structured into ontology-based taxonomies, which provide the firstlevel of discrimination in searching for a desired link.

The profile includes a high-level description about the link and itscharacters, as shown in Fig. 3. The xsd, lm, wsp, and owl are thenamespaces that represent the XML Schema, Link Model, Web ServiceProfile, and OWL, respectively. The default namespace of link profileontology is lp. The link profile ontology provides the followinginformation.

title property provides human-readable labels for the link.textDescription property provides a brief description of the link. Itsummarizes what the link offers, and indicates any additionalinformation that the profile wants to share with the receivers.keyword property provides keywords of the link The emphasizedkeywords indicate terms whose semantics, or meaning, weredefined for the agent through the Semantic Web.has_link property provides a pointer to associate Profile class withLink class. The following axiom states that the has_link property isfunctional, i.e., a profile can only describe at most one link.LinkPowerSet class is the set of all subsets of Link class.

bowl:FunctionalProperty rdf:ID=qhas_linkqNbowl:domain rdf:resource=q#Profileq/Nbowl:range rdf:resource=q#LinkPowerSetq/N

b/owl:FunctionalPropertyNrole property describes the meanings or abstract concepts of thelink. The range of role property is the owl:Thing class. This impliesthe link is an instance of a certain class that is an subclass of owl:Thing, thus it can inherit knowledge from this class .createdBy property records who to create a link. The range of theproperty is wsp:Actor class that is quoted by Web Services Profile.The wsp:Actor class records address and other information toallow a receiver of the profile to contact the issuer directly.LinkCategory class specifies link classification systems, such asProducts, Commercial Services, Information, etc.

4. Link model

Web resources are Web-accessible entities, such as Web pages,Web services, pictures, programs, devices, etc. Their links aredescribed in terms of a link model ontology,3 which describes in

The link profile ontology of OWL-L can be found at http://sparc.nfu.edu.tw/~hsuic/sw/OWL-L/1.0/LinkProfile.owl.

3 The link model ontology of OWL-L can be found at http://sparc.nfu.edu.tw/~hsuic/sw/OWL-L/1.0/LinkModel.owl.

Fig. 3. Semantic structure of the Link Profile.

Fig. 2. The top level ontology of the OWL-L.

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detail both the category and structure of the link. Fig. 4 shows the classinheritance hierarchy and semantic structure of link. The xsd, lp, rdfand owl are the namespaces that represents the XML Schema, LinkProfile, RDF and OWL respectively. The default namespace of linkmodel ontology is lm.

Link class of OWL-L collects the three types of links: Simple Link,Extended Link and Linkbase. The following axiom contains owl:disjointWith statement that asserts Link class extensions of the threeclasses descriptions involved have no individuals in common.

bowl:Class rdf:ID=q#LinkqNbowl:disjointWith rdf:parseType=qCollectionqN

bowl:Class rdf:about=q#SimpleLinkq/Nbowl:Class rdf:about=q#ExtendedLinkq/Nbowl:Class rdf:about=q#LinkBaseq/N

b/owl:disjointWithNb/owl:ClassN

The following sections describe in detail the main parts of the linkmodel. The model comprises three types of links, namely simple link,extended link and linkbase.

4.1. Simple link

A simple link is a link that associates exactly two resources, onelocal resource and one locator (remote resource), with an arc goingfrom the former to the latter. It is similar to the simple unidirectionalhyperlinks of today's HTML. The following OWL-based fragment ofSimpleLink class asserts three limitations:

1. The SimpleLink class is a subclass of Link class.2. If the SimpleLink class has a show property, then the showproperty

only has values of class showType.3. If the SimpleLink class has an actuate property, then the actuate

property only has values of class actuateType.

The showType and the actuateType are both rdf:List types, namelydata lists. The showType is composed of the values new, replace,embed, none, and other. The actuateType comprises onLoad, onRe-quest, none, and other.

href property provides the address that allows an agent to find aremote resource.arcRole property provides a pointer to refer an property of ontologyand acquires additional knowledge about the ending resource. Forinstance, an ending resource might generically denote a “mother”,butmight in the context of a particular arc have the role of “teacher”,and in the context of a different arc have the role of “wife”.Relationship is a general property class that contains all proper-ties. It is the range of arcRole property, this implies arcRole caninherit knowledge from OWL's properties.

show property is used to communicate the desired presentation ofthe ending resource on traversal from the starting resource.actuate property is used to communicate the desired timing oftraversal from the starting resource to the ending resource.

SimpleLink is a subclass of Link, thus it can use a link profile todescribe itself. Obviously, this improves semantic description of XLink.In addition, the arcRole property is similar to role property of theProfile class, and provides abstract concepts for ending resources.

4.2. Extended link

Extended links offer full XLink functionality, such as inbound andthird-party arcs, as well as links that have arbitrary numbers ofparticipating resources. An extended link can be considered as adirected graph: the link participants are the nodes, and the arcs thedirected edges. As a result, their structure can be fairly complex,including properties for pointing to remote resources, classes forcontaining local resources, and semantic technology for specifying arctraversal rules. Before introducing the structure of an extended linkclass, the Locator, LocalResource, and OntoArc classes, which are in theextended link, need to be discussed.

4.2.1. Locator class and local resource classAn extended link indicates remote resources that participate in it

by Locator classes, and indicates its participating local resources byLocalResource classes that appear inside the extended link. Thesemantic structure of Locator and LocalResource classes are depictedin Fig. 5.

title property provides human-readable labels for locators andlocal resources.role property describes the meaning or abstract concepts forlocators and local resources. It is similar to the role property of theProfile class, and the arcRole property of the SimpleLink andOntoArc classes.href property identifies remote resources using URIs and fragmentidentifiers such as XPointer [18] expressions. The Locator classmust have exactly one href property. The following OWL-basedfragment contains an owl:Restriction statement that enforces theabove constraint.

bowl:Class rdf:ID=qLocatorqNbrdfs:subClassOfN

bowl:Restriction owl:cardinality=q1qNbowl:onProperty rdf:resource=q#hrefq/N

b/owl:RestrictionNb/rdfs:subClassOfNb/owl:ClassN

Fig. 4. Semantic structure and category of the Link.

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value property offers the content of the local resource. It must beused in the LocalResource class.ResourceComponent class assigns what is the component of theResourceBag class.ResourceBag is a subclass of rdf:Bag. It is a container, which can beadopted to contain resources. The owl:item property describes arelation between ResourceBag and ResourceComponent, so thateach resource bag is associated with instances of Locator class orLocalResource class. In OWL-L, the resource is identified by an RDFID rather than by a label in the XLink. The ResourceBag refers to anaggregate resource that contains a set of resources with differentIDs. The aggregate resource can replace the set of resources withthe same label in the XLink. Each individual resource in theaggregate resource has different roles to quote different ontologies,and the appropriate arcs are specified to generate the identicaltraversal pairs.WebServices is a subclass of Lactor and collection of all WebServices. This is to emphasize the fact that Web Services is also akind of remote resources. An OWL-S based annotated document of

Fig. 5. Semantic structure of the relevant resources.

aWeb Services can be quoted by href property of Locator in OWL-L.Obviously, OWL-L and OWL-S are semantically interoperable, andthus cooperate with each other to improve the performance andaccuracy of semantic web mining.

4.2.2. OntoArc classAn extended link may indicate rules for traversing among its

participating resources by means of instances of the OntoArc class. Inother words, OntoArc can present and describe the linking relation-ships amongmore than two participating resources. The OntoArc classcan be classified into three subclasses, Outbound, Inbound andThirdParty (refer to Fig. 4). The arcRole, show, and actuate propertiesof OntoArc are similar to those of SimpleLink.

title property provides human-readable labels for the arcRole.from property assigns the starting resource of an arc. The value offrom property, starting resource, is an instance ID of Locator,LocalResource or ResourceBag classes. In contrast with the XLinkresources have to identify by the labels.to property assigns the ending resource of an arc. The value of toproperty, ending resource, is also an instance ID of the Locator,LocalResource or ResourceBag classes.Outbound class has a local starting resource and a remote endingresource goes outbound, that is, away from the linking element.Inbound class has an ending resource is local and a remote startingresource goes inbound.ThirdParty class neither the starting resource nor the endingresource is local, then the arc is a third-party arc. One advantage ofXLink is to offer third-party arcs that allow a separation of contentand link structure. The ThirdParty class is defined as the followingOWL-based fragment.

bowl:Class rdf:ID=qThirdPartyqNbrdfs:subClassOf rdf:resource=qOntoArcq/Nbrdfs:subClassOfN

bowl:RestrictionNbowl:onProperty rdf:resource=q#fromq/Nbowl:allValuesFrom rdf:resource=q#Locatorq/Nb/owl:RestrictionN

Fig. 6. Semantic structure of the Extended Link and LinkBase.

4 The extended link annotation of Pat Jones can be found at http://sparc.nfu.edu.tw/~hsuic/sw/OWL-L/1.0/PatCourse.owl.

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b/rdfs:subClassOfNbrdfs:subClassOfN

bowl:RestrictionNbowl:onProperty rdf:resource=q#toq/Nbowl:allValuesFrom rdf:resource=q#Locatorq/Nb/owl:RestrictionN

b/rdfs:subClassOfNb/owl:ClassN

4.2.3. Semantic structure of extended linkAn extended link is a link that associates an arbitrary number of

resources and arcs. The participating resources, such as Locator,LocalResource, or ResourceBag classes,may be remote or local resources.The participating arcs, such as Inbound, Outbound, or ThirdParty classes,provide rules for traversal among these participating resources. Thesemantic structure of ExtendedLink class is shown in Fig. 6.

The ExtendedLink is a subclass of Link. It must have exactly onecomponents property. All values for components property must beinstances of LinkComponentBag. The ExtendedLink class is defined asthe following OWL-based fragment.

bowl:Class rdf:ID=qExtendedLinkqNbowl:intersectionOf rdf:parseType=qCollectionqN

bowl:Class rdf:about=q#Linkq/Nbowl:Restriction owl:cardinality=q1qN

bowl:onProperty rdf:resource=q#componentsq/Nb/owl:RestrictionNbowl:RestrictionN

bowl:onProperty rdf:resource=q#componentsq/Nbowl:allValuesFrom rdf:resource=q#LinkComponentBagq/N

b/owl:RestrictionNb/owl:intersectionOfN

b/owl:ClassNLinkComponent class assigns what is the component of theLinkComponentBag class.

4.3. LinkBase

XLink allows the definition of links that reside in a separatelocation from the linked resources. This feature is useful whenresources are read-only, or when can not embed linking constructs,for example when creating links that anchor on multimedia contentfiles. Additionally, XLink provides the definition of link repositories,

called linkbases, which are often used to facilitate link managementeasier by gathering together several related instances of theExtendedLink class. Fig. 6 shows the semantic structure of LinkBase.The LinkBase class is a subclass of Link and must have exactly onecomponents property. All values for components property must beinstances of LBaseComponentBag, which is a multiset of theExtendedLink classes. The LinkBase class is defined as the followingOWL-based fragment.

bowl:Class rdf:ID=qLinkBaseqNbowl:intersectionOf rdf:parseType=qCollectionqN

bowl:Class rdf:about=q#Linkq/Nbowl:Restriction owl:cardinality=q1qN

bowl:onProperty rdf:resource=q#componentsq/Nb/owl:RestrictionNbowl:RestrictionN

bowl:onProperty rdf:resource=q#componentsq/Nbowl:allValuesFrom rdf:resource=q#LBaseComponentBagq/N

b/owl:RestrictionNb/owl:intersectionOfN

b/owl:ClassN

5. Annotations of Web resource links with OWL-L

OWL-L comprises three top-level ontologies, OntoLink, LinkProfile and Link Model. To demonstrate the semantic descriptioncapabilities of OWL-L, an extended link of identification by course-Load4 is created to describe the links that are relevant to eachstudent’s course load. This example will be referenced throughoutthis paper. The example refers to the link profile and link modelontologies, which define classes and properties that constitute thefoundation of a link description. These classes and properties arespecialized by creating subclasses, role and arcRole propertiesspecific to the extended link.

5.1. Ontology link of courseLoad

In the following fragment is encoded by OWL-L. We define thecourseLoad is an OntoLink that presents the cLinkPro link profile andis described by the cLinkMod link model. The olink is the namespacethat represent the OntoLink ontology.

Fig. 7. Mapping between extended link and Web resources.

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bolink:OntoLink rdf:ID=qcourseLoadqNbolink:presents rdf:resource=q#cLinkProq/Nbolink:describedBy rdf:resource=q#cLinkModq/N

b/olink:OntoLinkN

The example below is given to explain the components of thecourseLoad. The cLinkMod link model comprises three Locators(mo7, st62 and CS101), one Local resource (GPA), two third-partyarcs (member, and instructor) and one inbound arc (score). Thesecomponents will be encoded by OWL-L and presented in thefollowing sections.

OntoLink: courseLoadProfile:

cLinkPro (has_Link cExtLink) (role ExtendedLink)LinkModel:

cLinkMod (hasProfile cLinkPro)Locator: mo7 (role Mother),

st62 (role Person),CS101 (role Course)

Local resource: GPAThirdParty arc: member (from, CS101) (to, st62) (arcRole,

auditor), instructor (from, CS101) (to, mo7) (arcRole, professor)Inbound arc: score (from, st62) (to, GPA)

5.2. Link profile of courseLoad

The link profile ontology includes the vocabulary to describe thelink profile. Its central concept, Profile class, is a subclass ofLinkProfile and contains the contact information of providers, anextensible set of link characteristics and a semantic description byspecifying the title, textDescription, keyword and role. It also pointsto the described Link. Appendix A presents the OWL-L code of thecLinkPro profile.

5.3. Link model of courseLoad

In the cLinkMod link mode, the hasLink property provides apointer to associate the link model with the cExtLink.

bolink:LinkMode rdf:ID=qcLinkModqNblm:hasLink rdf:resource=q&lm;#cExtLinkq/N

b/olink:LinkModeN

5.3.1. Extended linkA cExtLink class is defined. This is a subClassOf ExtenededLink, and

a subClassOf an anonymous class, with a components property. Allvalues for this property must be instances of CourseInbound,CourseThirdParty, CourseLocalResource or CourseLocator classes.Significatly, OWL-L conceptualizes a specific link as a class, each ofwhose instances is a particular use of the link. Thus, each specific link(such as cExtLink in PatCourse.owl) is a subclass, rather than aninstance of Link. CourseInbound, CourseThirdParty, CourseLocalRe-source, and CourseLocator are defined as subClassOf Inbound,ThirdParty, LocalResource and Locator classes, respectively. Thesesubclasses are limited to the cExtLink components. Appendix Bpresents the OWL-L code of the cExtLink class.

5.3.2. Locator and local resourceThe following OWL-L fragment defines three instances of CourseLo-

cator, each of which uses a href property to specify the remote resourcethat is participating in the link. The circle symbols in Fig. 7 denoteinstances of CourseLocator (such as st62,mo7, andCS101) and the dottedline point to the physical entities. Additionally, we also give each instancea role. The role property can refer to an abstract concept that is anontology class. As shown in Fig. 8, the fy, ha, and cu are the namespacesthat represent the Family, Human, and Course Ontologies, respectively.Each CourseLocator instance refers to a specific class from which it caninherit knowledge. For example, st62 is an instance of CourseLocator,with title, Pat Jones. The title is generallyadopted todescribe themeaning

Fig. 8. An extended link can refer to an additional semantic through the role/arcRole properties.

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for human-readable form. The role refers to the Person class of theHuman ontology, and the href property refers to the URL address of st62.

bCourseLocator rdf:ID=qst62qNblm:titleNPat Jonesb/lm:titleNblm:role rdf:resource=q&hu;#Personq/Nblm:href rdf:resource=qhttp://sparc.nfu.edu.tw/~hsuic/web-

page/home/PatJones/index.htmlq/Nb/CourseLocatorNbCourseLocator rdf:ID=qmo7qN

blm:titleNDr. Jay Smithb/lm:titleNblm:role rdf:resource=q&fa;#Motherq/Nblm:href rdf:resource=qhttp://sparc.nfu.edu.tw/~hsuic/web-

page/home/JaySmith/index.htmlq/Nb/CourseLocatorNbCourseLocator rdf:ID=qCS101qN

blm:titleNComputer Science 101b/lm:titleNblm:role rdf:resource=q&cu;#Courseq/Nblm:href rdf:resource=qhttp://sparc.nfu.edu.tw/~hsuic/web-

page/home/courses/cs101.htmlq/Nb/CourseLocatorN

In the following OWL-L fragment defines the GPA is an instance ofCourseLocalResource.

bCourseLocalResource rdf:ID=qGPAqNblm:titleNgrade-point averageb/ lm:titleNblm:valueN3.5b/ lm:valueN

b/CourseLocalResourceN

5.3.3. OntoArcThe followingOWL-L fragment defines two instances of CourseThird-

Party, eachofwhich has a pair of fromand tovalues that represent sourceand destination resources respectively. The rectangle symbols in Fig. 7denote instances of CourseThirdParty (such as member, and instructor),and the dotted lines point to the physical links. Additionally, eachCourseThirdParty instance is also given an arcRole. The arcRole propertycan refer to an abstract concept, which is a property of the ontology class.As shown in Fig. 8, each CourseThirdParty instance refers to a specific

property fromwhich it can inherit semantics. For example, the instructoris an instance of CourseThirdParty, which has a title “CS101 is taught byDr. Jay Smith”. The arcRole refers to the professor property of the Courseontology.

bCourseThirdParty rdf:ID=qmemberqNblm:from rdf:resource=q#CS101q/Nblm:to rdf:resource=q#st62q/Nblm:arcRole rdf:resource=q&course;#auditorq/Nblm:titleNAll students, auditing the courseb/lm:titleN

b/CourseThirdPartyNbCourseThirdParty rdf:ID=qinstructorqN

blm:from rdf:resource=q#CS101q/Nblm:to rdf:resource=q#mo7q/Nblm:arcRole rdf:resource=q&course;#professorq/Nblm:titleNCS101 teach by Dr. Jay Smithb/lm:titleN

b/CourseThirdPartyN

In the following OWL-L fragment, we define score is an instance ofCourseInbound, which has an ending resource (i.e. GPA) is local and aremote starting resource (i.e. st62) goes inbound.

bCourseInbound rdf:ID=qscoreqNblm:from rdf:resource=q#st62q/Nblm:to rdf:resource=q#GPAq/Nblm:titleNPat Jones's GPAb/lm:titleN

b/CourseInboundN

5.4. Three different approaches to enhance the knowledge representation

OWL-L supports three different approaches to enhance the knowl-edge representation of XLink, namely XML-based metadata, OWL-Lsemantic and ontology-based reasoning. The first approach, calledXML-based metadata, adopts XML-based metadata to describe Webresources. This approach is used to develop existing XLink applications,but still cannot intelligently locate relevant Web resources. The OWL-Lsemantic approach supports OWL-based semantic capabilities thatmakeWeb resources links computer-interpretable. The ontology-basedreasoning approach provides OWL-based ontologies based on

Table 1The comparison of different approaches.

XML-basedmetadata

OWL-L semantic Ontology-based reasoning

Markuplanguage

XML OWL-L OWL

SemanticWeb layer

XML layer Ontology layer Ontology layer

Knowledgerepresentformat

XML-based Ontology-based Ontology-based

Knowledgerepresentlogic

Metadata Description logics Description logics

Inferencecapability

Syntax Semantic Semantic

Limitation Lackintelligence

Can't represent non-monotonic rules

Build on top of OWL-L

Advantage Simple andpopular

Can enhance theknowledgerepresentation of XLink

Can be integrated into OWL-L toenhance the computerreasoning

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description logics to provide sound and decidable reasoning to OWL-L,and therefore enhances the semantic reasoning capabilities of OWL-L.Table 1 shows the features of these three common semantic approachesare summarized and listed in.

5.5. Adding semantic knowledge to OntoLink using Web-based ontologies

The values of the role and arcRole are URI references. They identifysome resources that describe the intended properties. The format of thisresource is not standardized by XLink and therefore is an open field forproprietary extensions of semantics. OWL-L defines the range of the roleproperty in owl:Thing, so that a resource can set a special ontology classby referring to an additional semantic through a role property. Similarly,the range of the arcRole property is also defined in owl:Thing. An arc isthus associated with an arcRole to retrieve additional semantics from aspecific property of ontology class. These ontologies are implemented inOWL, which can seamlessly interoperate and integrate with OWL-L.Consequently, the semantic and knowledge representation capabilitiesof XLink are significantly improved.

As in the previous example, the related information about mo7 canbe obtained from the following four categories. The first category ismaterial derived from the resource itself, supported by XML-basedmetadata. The second category obtains functions from the OWL-Lsemantic approach. The last two categories make inference to obtainadditional facts provided by the OWL-based reasoning approach.

1. XML-based metadata approach

The physical entity is the resource itself that is an inherentmaterial.The mo7 is an RDF ID, and identifies the physical entity located atthe following address: http://sparc.nfu.edu.tw/~hsuic/webpage/home/JaySmith/index.htmlThe above URL represents a HTML page that provides detailinformation for the mo7. Moreover, a physical entity may be anXML document containing a set of property descriptor elements.An element in this XML document is selected by the URI referencecontaining an XPointer fragment identifier: http://sparc.nfu.edu.tw/~hsuic/webpage/home/whub.xml #xpointer(//person[@type=’JaySmith’])

2. OWL-L semantic approach

The mo7 is an instance of CourseLocator, which is a subclass ofLocator. Hence, mo7 can inherit the features and limitations fromthe CourseLocator and Locator classes. For example, agents caninfer thatmo7 is a remote resource rather than a local resource, andthat mo7 must have exactly one from property value.

3. Inference knowledge by role

The role property of mo7 specifies the actor that is the Mother classof the Family ontology. Therefore, mo7 can be regarded as aninstance of the Mother class, which inherits semantic relations andknowledge from the Family ontology. For example, the followingfragment is a children property is declared in the Family ontology.Agents can infer that mo7 may have a son.bowl:ObjectProperty rdf:ID=qchildrenqNbrdfs:domain rdf:resource=q#Motherq/Nbrdfs:range rdf:resource=q#Sonq/N

b/owl:ObjectPropertyN4. Inference knowledge by arcRole

The mo7 is a destination resource of the instructor arc, which isassociated with the professor property of Course ontology throughthe arcRole. The professor property is declared in the Courseontology, illustrated as the following fragment. The property ofOWL can bemapped to the arc of OWL-L. Agents can infer that mo7is an instance of the Teacher class. Additionally, the Functional-Property implies that each course there is at most one teacher forthe professor property. Thus, agents can infer that the course CS101is only instructed by mo7.bowl:FunctionalProperty rdf:ID=qprofessorqNbrdfs:domain rdf:resource=q#Courserq/Nbrdfs:range rdf:resource=q#Teacherq/N

b/owl:FunctionalPropertyN

6. Related work

XLink, as developed by W3C, describes how Web resources arerelated. A link in XLink is associated with metadata that explicitlydescribes the relationships. The main advantage of XLink is that itoffers out-of-line links that allow separation of content from the linkstructure. The content is contained in the XML document, while thelink structure is provided by the linkbase. Many studies on XLink andlinkbases have been performed recently by researchers. These includehypermedia linking [19], link transformation [20,21], link server[21,22], Web navigation [23], query language [24,25] etc. However,they do not address using ontology technologies to perform machineunderstanding in XLink. This study is the first to consider this issue inontologies. A brief overview on existing XLink applications forinformation retrieval is given below.

xlinkit [22] is a lightweight application service that provides rule-based link generations and examines the consistency of distributedWeb contents. A user writes a number of rules or constraints in anXML-based rule language, while the system generates a linkbaselinking elements that meet or violate the specified constraints.XLinkProxy [21] acts as an intermediary server between a browserand a Web server, by forwarding a resource request obtained from abrowser to the appropriate HTTP server. XLinkProxy may add newinformation, such as new links, to the resource before sending it to therequesting browser. XPathLink [24] was developed as a naturalextension of XPath, which allows the application to formulate queriesto XLink links transparently, and to navigate XLink links in a naturalway. The dbxlink scheme [25] has been proposed as an incrementalextension to XLink and XPath, formed as a logical model for networksof linked documents. Based on this logical model, XPath constructscan be used to operate transparently on linked documents.

Those approaches focus on XML-based metadata and syntax, butunlike ontologies, they do not provide rich semantics for knowledgerepresentation. In contrast, OWL-L adopts a Semantic Web approach.OWL is a W3C recommendation and a major technology for theSemantic Web. OWL describes the abstract concepts of a domain interms of. classes and properties. It is a Web-based ontology languagefor the Semantic Web that provides a formal semantic representation

Table 2The comparison of OWL-L and XLink.

OWL-L XLink

Developer This study W3C recommendationstandard

Developmentlanguage

Ontology Web Language (OWL) eXtensible Markup Language(XML)

Languagecategory

Ontology language Markup language

Developmentgoal

Facilitate Web resources links tocomputer-interpretable

Describe the link relationshipsamong Web resources

Model OntoLink, LinkProfile, LinkModel Simple link, Extended Link,linkbase

Knowledgerepresent

Metadata, ontology, description logics Metadata

Resourcediscovery

Keyword-based matching, ontology-based reasoning, OWL-L semantic

Keyword-based matching

854 I.-C. Hsu et al. / Computer Standards & Interfaces 31 (2009) 846–855

and machine-understandable reasoning capabilities. The currentversion of OWL-L builds on OWL by following the layered approachto Semantic Web stack [26].

OWL-L is an OWL-based ontology language, based on XLink, whichdefines a set of classes and properties. The OntoLink, LinkProfile, andLinkModel classes are defined in the top level ontology of OWL-L. TheOntoLink class provides an organizational point of reference fordeclaring Web resource links. The LinkProfile class provides theinformation required for an agent to discover a link, while theLinkModel class provides information that enables an agent to exploita link. Therefore, OWL-L can describe the properties of Web resourcesin an unambiguous and computer-interpretable form, thus facilitatingdiscovery, matchmaking, and composition of Web resources throughtheir semantic descriptions. Table 2 shows significant differencecomparisons between the OWL-L and XLink.

7. Summary and concluding remarks

This paper discusses the limitations of existing XLink relative to theknowledge representations of concerns, and proposes OWL-L toimprove these limitations. Exactly how OWL-based ontologies can beseamlessly integrated with OWL-L to enhance knowledge representa-tions is also demonstrated. OWL-L is an upper ontology for describingWeb resources links, written in OWL. Two aspects of OWL-L arepresented, namely the link profile and the linkmodel, to help softwareagents to locate and compose links automatically.

OWL-L is a novel ontological approach to modeling the structure ofXLink. It makes three main contributions. First, a new OWL-basedontology language, called OWL-L, is developed with the objective ofmaking Web resources links computer-interpretable. OWL-L is firstapproach to address the limitations of XLink using ontologytechnologies. Second, OWL-L provides an abstract concepts frame-work, comprising OntoLink, LinkProfile, and LinkModel classes, to thedescription ofWeb resources links, and then to facilitate to discover orexploit a link. Third, OWL-L supports three different approaches,namely XML-based metadata, OWL-L semantic and ontology-basedreasoning, to present the knowledge representation. Because theoriginal XLink can only support the XLink-based metadata, adding thefeatures from ontology and OWL-L semantic significantly enhancesthe intelligent capabilities comparing to XLink.

Future work should extend Semantic Web Mining [27,28] withOWL-L that can improve the results of Web Mining by exploiting thenew semantic structures in the web. Conversely, Web mining can beapplied to help create the Semantic Web. For example, OWL-Lapplications can be built by Web Mining. Web 2.0 Mashup has beenrecognized as the next generation of web applications [29,30]. Itrequires shared meaning to enable the creation of new applications bycombining or reusing different Web resources. Future work should

also study the integration of OWL-L with Web 2.0 Mashuptechnologies, such as Web feeds and Web API, to locate relevantWeb resources intelligently.

Acknowledgement

The authors would like to thank the National Science Council of theRepublic of China, Taiwan for financially supporting this researchunder Contract No. NSC 96-2218-E-150-004.

Appendix A. Link profile of courseLoad

blp:Profile rdf:ID=qcLinkProqNblp:has_link rdf:about=q&lm;#CourseLoadLinkq/Nblp:titleNCourse Load for Pat Jones.b/lp:titleNblp:textDescriptionNAn OWL-L based extended link description of Course Load for Pat

Jones.b/lp:textDescriptionNblp:keywordNCourseb/lp:keywordNblp:role rdf:resource=q&olink;#ExtendedLinkq/Nblp:createByNbwsp:Actor rdf:ID=qI-ChingqNbwsp:nameNI-Ching Hsub/wsp:nameNbwsp:titleN Department of Computer Science and Information

Engineering b/wsp:titleNbwsp:phoneN213 271 7822 b/wsp:phoneNbwsp:faxN213 271 7821 b/wsp:faxNbwsp:emailN husic@nfu.edu.tw b/wsp:emailNbwsp:physicalAddressN264, Wenhua Rd., Huwei Township, Yunlin,

Taiwan b/wsp:physicalAddressNbwsp:webURLNhttp://sparc.nfu.edu.tw/~hsuic/b/wsp:webURLNb/wsp:ActorNb/lp:createByNb/lp:ProfileN

Appendix B. Extended link

bowl:Class rdf:ID=qcExtLinkqNbrdfs:subClassOf rdf:resource=q&lm;#ExtenededLinkq/Nbrdfs:subClassOfNbowl:RestrictionNbowl:onProperty rdf:resource=q&lm;#componentsq/Nbowl:allValuesFromN

bowl:ClassNbowl:unionOf rdf:parseType=qCollectionqNbowl:Class rdf:about=q#CourseInboundq/Nbowl:Class rdf:about=q#CourseThirdPartyq/Nbowl:Class rdf:about=q#CourseLocalResourceq/Nbowl:Class rdf:about=q#CourseLocatorq/Nb/owl:unionOfNb/owl:ClassNb/owl:allValuesFromN

b/owl:RestrictionNb/rdfs:subClassOfNb/owl:ClassNbowl:Class rdf:ID=q#CourseInboundqNbrdfs:subClassOf rdf:resource=q&lm;#Inboundq/Nb/owl:ClassNbowl:Class rdf:ID=q#CourseThirdPartyqNbrdfs:subClassOf rdf:resource=q&lm;#ThirdPartyq/Nb/owl:ClassNbowl:Class rdf:ID=q#CourseLocalResourceqNbrdfs:subClassOf rdf:resource=q&lm;#LocalResourceq/N

855I.-C. Hsu et al. / Computer Standards & Interfaces 31 (2009) 846–855

b/owl:ClassNbowl:Class rdf:ID=q#CourseLocatorqNbrdfs:subClassOf rdf:resource=q&lm;#Locatorq/Nbrdfs:subClassOfNbowl:Restriction owl:Cardinality=q1qNbowl:onProperty rdf:resource=q#formq/Nb/owl:RestrictionNb/rdfs:subClassOfNb/owl:ClassN

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I-Ching Hsu (hsuic@nfu.edu.tw) received a Ph.D. degree inthe computer science department at the National Chung-Hsing University of Taiwan in 2007. Dr. Hsu has beenworking at Chung-Shan Institute of Science & Technology inthe area of intelligent data integration and modelingtechnologies since 1991. He is currently an assistantprofessor of Computer Science and Information Engineer-ing at National Formosa University, Taiwan. He hasparticipated and directed projects in the area of SemanticWeb, XML, UML and Internet applications development.His current research aims at the creation and study ofSemantic Web for Web 2.0, Sensor Web and information

modeling applications.

Yuan Kwei Tzeng (phd9413@angelica.cs.nchu.edu.tw) is aPh.D. student working with associate Professor Der-ChengHuang in the Department of Computer Science and

Engineering at the National Chung-Hsing University ofTaiwan. Mr. Tzeng received a B.S. degree and a M.S. degreefrom the department of Applied Mathematics, at NationalTsing-Hua University in 1983 and 1985 respectively. Mr.Tzeng has been working at Chung-Shan Institute of Science& Technology in the area of intelligent data integrationtechnologies since 1985. His current research aims at thecreation and study of Semantic Web for modeling andsimulation network security mining applications and in-formation modeling.

Der-Cheng Huang (huangdc@dragon.nchu.edu.tw) is anassociate professor with the Department of ComputerScience and Engineering, National Chung-Hsing Universityof Taiwan. His research interests are in VLSI design, test and

diagnosis SoC and Embedded system design, test anddiagnosis Computer-aided design and verification of VLSIVLSI Digital Signal Processing Microprocessor system de-sign and information modeling.