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Research on the Architecture of Ontology-based Context-aware Application inPervasive Environment
Yongquan Dong"12, Qingzhong Lil,Yuliang ShilI School ofComputer Science and Technology, Shandong University, Jinan, P.R. China
2 School ofComputer Science and Technology, Xuzhou Normal University, Xuzhou, P.R. Chinadongyongquan@,mail. sdu. edu. cn lqz@,sdu. edu. cn liangyus@,sdu. edu. cn
With the prevalence ofpervasive devices, plentyof applications need to support context awareness.Due to the heterogeneity of context information, it isnecessary to provide a set of common vocabularieswith shared semantics. Based on above requirements,the paper presents an architecture ofontology-basedcontext-aware application in pervasive environment.In the architecture context processing is regarded asa separated module which conforms to the principleof software engineering, "seperation of concernsThe architecture enables applications to focus on theinformation that they desire, and reduces the need toworry about how contextual information is retrieved.In order to reuse of the ontology, the paper designstwo level's ontology-generic ontology and domain-specific ontology which unifies semantics of contextinformation. At last an example scenario of homedomain is given to illustrate the rationality andeffectiveness ofthe architecture.
Keywords: Pervasive Computing, Ontology, Context,Context-aware Application.
Pervasive computing paradigm is firstly proposed byWeiser[1-2], which will become "third wave ofcomputing". With the prevalence of pervasive devices,users can hold many electronic devices to facilitate theirdaily lives and work. They have to deal with diversedevices (mobile or fixed) with diverse interfaces andused in diverse environments. It is imperative to enabledevices and applications to automatically adapt tochanges in their surrounding physical and electronicenvironments. Context [3-4] is any information thatcharacterizes a situation related to the interactionbetween users, applications, and the surroundingenvironment. The usage of context requires manyapplications to support its processing. So it is necessaryto design a reasonable architecture to make currentapplications support the processing of context withoutmodifying themselves. From the perspective of
software engineering, the architecture should conformto the principle of "separation of concerns", that is,application designers need not to care about the detailsof how to derive the context that applications canunderstand.
Another problem is the different representation ofcontext. The lack of common context specificationraises some issue of interoperability, reuse. Ontologyprovides a formal, explicit specification of a sharedconceptualization of a domain. Using ontology toprovide the semantics of context information is apromising area.
Based on two above problems, the paper proposes anarchitecture of ontology-based context-awareapplication in pervasive computing. The examplescenario suggests that the architecture is reasonable andapplicable.The remainder of the paper is structured as follows.
Section 2 presents a context layer model. Section 3focuses the architecture of ontology-based context-aware application and introduces the details of each part.Section 4 elaborates on the context ontology, whilesection 5 presents an example scenario to suggest thatthe architecture is reasonable and effective. Finally, insection 6 conclusions are drawn, while an outline offuture plans is provided.
2. Context Layer Model
The generation of context is not one time, but manytimes in which the abstraction of context will begradually enhanced. We divide context information intofour layers. As is shown in Figure 1, the four layerscontain physical context, interpreted context, integratedcontext and applicable context from low level to highlevel. The physical contexts describe contexts which arederived from the environment. The interpreted contextsrepresent contexts which are be transformed and bemapped to contexts that are defined by applying anaccepted vocabulary for simplification. The integratedcontexts describe contexts which relate to an entity in asituation. The applicable contexts are built from highlyspecific contexts whose contents are adapted to theapplication.
1-4244-0971-3/07/$25.00 )2007 IEEE.
Figure 1 Context Layer Model
3. The Architecture of Ontology-basedContext-aware Application
In this section, we present an architecture ofontology-based context-aware application in pervasiveenvironment and introduce the details of each part. Thearchitecture generalizes the foundations laid in[3-5] toprovide a higher abstraction.
The architecture of ontology-based context-awareapplication is shown in Figure 2.
Context-ware Application Layer
Context Mildlleware Layer Cohtext I_feienne Engie
OntoloEoe La text Integrator
Mr gw~~~~~ohtext Interpreter = | e
Ontology Context Collector |Finder
|EryironrLenLt Resource LaErer ( SenLsors, S ervic es, Devices ..
Figure 2 The Architecture of Ontology-basedContext-aware Application
The architecture is divided into three layers that areenvironment resource layer, context middleware layer,and context-aware application layer from bottom to top.The function of each part is described as following.
(1) Environment Resource Layer. It contains manyresources such as sensors, services and devices whichare the sources of the production of context. Theseresources constitute a pervasive environment.
(2) Context Middleware Layer. It focuses on theprocessing of context, which contains contextacquisition, context interpretation, context integrationand context inference etc. In order to support contextsharing, this layer incorporates ontology to unify thecontext semantics meaning. As a separated layer, it willadapt well to any existing application withoutsupporting context.
The layer has many modules each of which achievesspecific functions. In Figure 2, we omit the word"module" in each module.
Context collector module is responsible foracquiring the raw data from the resources inenvironment resource layer. Context interpreter moduleanswers for producing the interpreted contexts andraises the level of abstraction of context information.Context integrator module gathers context informationrelated to an entity, that is, integrated context. If thereare many context collectors which generate samecontext, the context integrator decides which contextcollector to use according to the sensor's accuracy anderror rate. Context inference engine module infersadditional and not directly observable contextinformation which conforms to applicable context anddetermines which application to run according to rulesin knowledge base which is easy to be managed by user.Context storage module receives the physical context,interpreted context, integrated context and applicablecontext in order to maintain a history of all contextswhich can be used to establish trends and predict futurecontext values. Context ontology module provides aunified, well structured scheme for the semanticrepresentation of context information which all thecontext must conform to. Context ontology managermodule dynamically updates context ontology and usesit to interact with peers in pervasive environment.Finder module answers for maintaining a registry ofwhat capabilities existing in the architecture andmanaging the communications between each contextmodule and the application.
Each module has the same interface that willfacilitate easy usage. The interface is as shown inFigure 3. Every module has the notification, callbackand polling interface and its core functions. Notificationinterface is responsible for notifying the interestedapplications when context has changed and conformedto the conditions that the relevant applications havepreset. Callback interface allows applications to obtainall level of context. Polling interface takes charge ofallowing applications to be proactive to theenvironment.
NotificatiZonZ |Callback Foiling
Figure 3 The Structure of Each Module
(3) Context-aware Application Layer. It can utilizethe publish/subscribe mechanism to accept the results ofcontext middleware layer which conform to semanticmeanings of input parameters of applications and canalso use polling mechanism to query all levels ofcontext.
4. Context Ontology
A major problem for context is the heterogeneity ofcontext information, and the lack of a general acceptedcontext-specification raises many issues, such asinteroperability, reuse etc. So ontology is incorporatedinto the architecture. The use of ontology to representcontext has already been proposed by others[6-8]. Inthis paper we generalize the use of ontologies accordingto its layered model.
In order to support interoperability among pervasiveenvironment, it must be done to share each of domainknowledge. Ontology[9-10] provides a formal, explicitspecification of a shared conceptualization of a domain.It describes concepts in a certain domain andrelationships among these concepts. Because of itsconceptual hierarchical structure and strong expressiveability in semantics, ontology plays an important role inknowledge share and reuse. By using ontology, allresources in pervasive environment can share thedomain knowledge so that they can communicate witheach other seamlessly.
4.2 Context Ontology
Context ontology defines a common vocabulary toshare context information in a pervasive computingdomain; and include machine-interpretable definitionsof basic concepts in the domain and relations amongthem.Due