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(mobile) agent-based applications. Experiments that highlight the exibility and accuracy of the

1. Introduction

g servse to applicairelessherefototypesg moberwhelthem

Developing data services using a real, large-scale wireless

(e.g., GSM, GPRS) and mobile devices.

, thatIlarriay an

provided by a certain mobile agent platform,1 and can autono-

ARTICLE IN PRESS

Contents lists available at ScienceDirect

.e

Journal of Network and Computer Applications

Journal of Network and Computer Applications 32 (2009) 846865useful comments. Corresponding author. Tel.: +34 976762340; fax: +34 976761914.

Mobile agent platforms are usually implemented in Java, among other

reasons because of its portability. In these cases, a place is a Java process thatmously travel from place to place (within the same computer orbetween different computers) (Milojicic et al., 2000). Mobile$ This work was supported by the CICYT project TIN2007-68091-C02-02. The

authors are grateful to Yolanda Villate for testing the Locker Rental Service using

the proposed testing system. We also thank the anonymous reviewers for their

E-mail addresses: silarri@unizar.es (S. Ilarri), emena@unizar.es (E. Mena),

jipileca@si.ehu.es (A. Illarramendi).

1

provides certain services (e.g., communication and transportation of agents to

other places).1084-80

doi:10.1network can imply a high cost in wireless communicationsimportant role. Mobile agents are software components thatexecute on an execution environment (traditionally called place)behaviors could be needed to test how the data serviceperforms in that situation. Besides the effort required, it wouldbe impossible to replicate the same situation.

service providers) and moving objects (data service clients)can be used to test wireless data services (Mena et al., 2002;et al., 2004). In the proposed testing system, mobile agents pldistributed and dynamic nature of the underlying mobile networkinfrastructure. Besides, deploying a data service in a real networkfor testing purposes is not usually an option. Among otherreasons, the following can be highlighted:

Many users with real wireless devices following certain

network should be avoided.

Taking into account the previous considerations, a system hasbeen developed, based on mobile agent technology (Milojicicet al., 2000; Lange and Oshima, 1999; Ilarri et al., 2006b), for thesimulation of wireless infrastructures composed of proxies (dataThe attractiveness of computinanywhere and anytime has given ridevelopment of mobile computing aa great pressure over providers of wrst in providing a new service. Tallows developers to test their proexpensive fashion is required. Testintions in a real environment is an ovas time-consuming as developing45/$ - see front matter & 2009 Elsevier Ltd. A

016/j.jnca.2009.01.003proposed system and experiments testing real applications using the system are also shown.

& 2009 Elsevier Ltd. All rights reserved.

ices that are availablen intense research andtions. However, there isdata services to be there, a mechanism thatin a timely and non-ile computing applica-ming task (sometimes,), mainly due to the

Some extensions to the real network functionality could berequired. For example, simulating that certain extra informa-tion (e.g., context-aware information) is sent as part of theusual signaling trafc in the network could be needed.

The data service will probably present aws during the testingphase, which will affect adversely the normal operation of thereal network.

Mobile users should not experience the overload of testing newapplications on the real network.

The double task of simulating a data service for testingpurposes and then implementing it for operation on a reala IIS Department, University of Zaragoza, Mara de Luna 1, Zaragoza 50018, Spainb LSI Department, University of the Basque Country, Apdo. 649, 20080 San Sebastian, S

a r t i c l e i n f o

Article history:

Received 4 April 2008

Received in revised form

2 October 2008

Accepted 30 January 2009

Keywords:

Mobile computing applications

Mobile agents

a b s t r a c t

The interest in mobile com

revenue in the forthcomin

them is a real challenge. In

convenient choice.

In this article, a system

environment is presented.

advantages, such as: (1) it

can be extended to adapt

transparently to the tested

and accuracy of the resultSesystem based on mobile agents to tes

rgio Ilarri a,, Eduardo Mena a, Arantza Illarramejournal homepage: wwwll rights reserved.nd (posed system, based on mobile agents, presents a range of interesting

to test a real wireless data service on a simulated environment; (2) it

e simulation needs; (3) it allows mixing real and simulated elements

ervice; (4) it allows distributed testing, which increases the scalability

5) it is capable of testing services where the mobile devices executehat

promany circumstances the use of tools to test these services is the most

can be used to test mobile computing applications in a distributedars

t, inapplications is on the rise and they are expected to be a great source of

. However, not only developing wireless data services but also testingmobile computing applications$

i b

lsevier.com/locate/jnca

agents can save remote data communications (e.g., they can

ma

LAN), and the communication among BSs is wired. Thus, BSs allow

proxy area with mobile data services. For this task, there issome process, called Proxy Server in this article, running on a

areand

3.1

themoexaob

ARTICLE IN PRESS

Computer Applications 32 (2009) 846865 847the number of computers used for the simulation: The tester canuse one, two, or many computers depending on his/her needs.

Simulation of moving objects that execute agent-based applica-tions. Applications based on (mobile) agents, which areconsidered very useful for mobile computing, can also beeasily tested.

The structure of the rest of this article is as follows. In Section 2,we describe the basic mobile environment infrastructure con-sidered. In Section 3, we explain how moving objects and proxiesare simulated in the developed system. In Section 4, we describethe mechanism used to simulate moving objects that executemultiagent applications. In Section 5, we explain how the testingsystem can be used. In Section 6, we evaluate experimentally thesuitability of the proposed testing system, analyzing its accuracyand exibility. In Section 7, we present some related works.Finally, Section 8 summarizes the main conclusions.

2. Mobile environment infrastructure

In this section, the main elements in a mobile environmentinfrastructure are briey described. Looking beyond the details ofa specic wireless technology, the generally accepted mobileenvironment infrastructure (Pitoura and Samaras, 2001) isconsidered, which is composed of mobile hosts (MHs) and basestations (BSs). Each BS (or mobile support station, MSS) serves allthebetwi(distributed) simulation.Support for distributed testing, which allows to increase theparallelism and scalability. The tested application is not aware ofor partially simulated environment.Use of mobile agent technology to simulate objects that moveamong areas served by different proxies. Mobile agents are thesoftware counterparts of real moving objects, and they avoidnon-intended delays caused by remote communications in at the suitability of certain design decisions. Summing up, thein features of the testing system are:

Plug and simulate. A real data service can be tested on a totallywirtesprocess all the data locally, on the computers that hold such data).The proposed testing system deals with mobile computing

environments that can include both real and simulated movingobjects and proxies. Indeed, it is able to hide the true nature (realor simulated) of moving objects and proxies in a scenario. In thisway, data services will not be aware that some of the movingobjects and proxies they deal with are not real but simulated. So,the system avoids the need for developing a data serviceprototype that deals with a simulated environment and that latermust be adapted to work on a real wireless network. Instead, it isadvocated to develop the nal data service directly, test it in asimulated (or partially simulated) environment until it is ready,and nally execute the same data service in the real-worldwireless environment. With this approach, the developers of

eless data services can build their own scenarios and easilyagents present a range of unique advantages (such as autonomy,exibility, and effective usage of network bandwidth, Lange andOshima, 1999) which makes them very attractive to wireless(Spyrou et al., 2004), pervasive (Cardoso, 2002), and distributedcomputing in general. In a traditional client/server architecture, aclient requests a server to perform a certain task. As opposed tothis approach, a mobile agent can move to a computer that has therequired resources and perform the task itself. Therefore, mobile

S. Ilarri et al. / Journal of Network andMHs within its coverage area or cell. The communicationween a MH and the BS that provides it with coverage isreless (e.g., using cellular network technology or a wirelessthat will behave as the simulated object is required to behave,which includes mobility and any other behavior. Basically, thesimulation of a moving object requires the execution of threetasks: simulating the changes of proxy area, simulating theobjects movements, and simulating the objects functionality.2

2 It should be claried that the proposed system does not aim at emulating the(Mactu

to

exaIn this section, it is described how moving objects and proxiessimulated in the proposed system. A sample scenario with realsimulated moving objects and proxies is also described.

. Simulated moving objects

A simulated object is the representation of a moving object intesting system. Simulated objects can be inspired by real

ving objects, although they could have different features. Formple, in a certain scenario, simulating that a real stationaryject is moving could be required.A simulated object will be implemented as a mobile agentilojicic et al., 2000; Lange and Oshima, 1999; Ilarri et al., 2006b)3.certain communication port, where it can receive servicerequests from objects within its proxy area. In a cellularnetwork, a proxy area could be the union of the coverage areasof one or more BSs. It should be noted that a more specic termcould be used to designate a proxy depending on the serviceit provides; for example, a proxy that provides locationinformation is usually known as a Location Server (see theMobile Location Protocol Specication by the Open MobileAlliance, available at http://www.openmobilealliance.org/tech/afliates/lif/lindex.html).

Therefore, in order to simulate environments to test wireless dataservices, how these two elements can be simulated must beconsidered.

Simulation of moving objects and proxiesthe communication between MHs and xed hosts (FHs) connectedto the Internet. In accordance with the previous description,wireless ad hoc networks are not considered in this work. Fromthe point of view of data services for mobile environments, thefollowing two elements are distinguished:

Moving objects are entities provided with a wireless device thatallows them to access the network. For example, a carequipped with a portable computer connected to the Internetor a person carrying a PDA (Personal Digital Assistant)connected to a wireless hotspot are examples of movingobjects. As moving objects move, the available data servicesthat they can access might change, as different mobile dataservices might be available in different geographical areas.Moving objects can have attached devices such as GPSreceivers or other sensors that feed them with contextinformation. Similarly, they are capable of executing software,such as a web browser or a route guiding system that showsthe location of the moving object on a map. Depending on theirfeatures, moving objects can have different functionalities.

Proxies are computers that provide moving objects under theiral hardware of the moving objects but the tasks they perform. If it is important

simulate that the processing capability of a moving object is limited, for

mple, then some delays can be easily introduced in the moving objects code.

proxy,3 as shown in Fig. 1. Thus, mobile agents will always interact

ARTICLE IN PRESS

REAL SITUATION

ject:

S. Ilarri et al. / Journal of Network and Com848locally with their proxies in the same way that moving objectsinteract with theirs. This is a way of distributing the simulationload across several computers that, as an additional benet,avoids the introduction of articial delays in the simulationThese three tasks are explained in more detail in the following.An example of moving object that simulates a user using a webbrowser will be shown in Section 5.2.

3.1.1. Simulation of the changes of proxy area

The proposed testing system benets from the resemblancebetween moving objects and mobile agents to simulate a changeof proxy area by a moving object as a trip of the mobile agent thatsimulates that moving object, from the old proxy to the new

the mobile data service

Proxy Server

proxy to receive Interactions with the

the coverage of a different proxyThe moving object travels under

Proxy2

Proxy1

Proxy Server

Fig. 1. Handoff of a moving ob(caused by real network delays among proxies). So, no remotecommunications between the mobile agents and their proxies arerequired. In other words, a communication between an object andits current proxy is represented in the testing system as a localcommunication between the corresponding simulated object (i.e.,the mobile agent) and the proxy where it resides. An alternativesolution where the mobile agent does not execute on its proxywould require the use of the network to support the communica-tions between the simulated object and the proxy. Moreover,sometimes a service of a real proxy is not available remotely andthe only way to get access to it, apart from using a real deviceunder its coverage, is sending a mobile agent to that proxy; as anexample, see in Section 7 the text relative to Satoh (2002).

In a real situation, moving objects discover dynamically thenetwork infrastructure by using signals received from nearbyproxies (i.e., each moving object knows which proxy provides itwith coverage). As mentioned at the beginning of Section 3.1, amobile agent will implement the functionality of a simulatedmoving object. Such a mobile agent is initialized with a ProxyCatalog (proxies in the scenario and their coverage areas). Duringthe simulation of the objects movements (see Section 3.1.2), themobile agent will query the Proxy Catalog to keep track of the

3 It is assumed that not only simulated proxies but also real proxies allow

mobile agents to move there (i.e., they provide an execution environment for

them); in this way, it is possible to build scenarios with simulated objects and real

proxies at the same time.proxy that would provide the object with coverage if it were real.When the mobile agent detects that the simulated object that itrepresents leaves the area of the current proxy and enters the areaof a different proxy, the mobile agent moves there. Therefore, asindicated before, the mobile agent is always on the proxy thatprovides coverage to the simulated object; if the simulated objectis out of coverage, the corresponding mobile agent will remain onits current proxy until the object gets coverage, and then it willmove to the new proxy.

It should be stressed that the movement of a mobile agent isonly expected to take a small amount of time (e.g., agents in themobile agent platform SPRINGS, Ilarri et al., 2006b, need lessthan 1 second to travel even in a very highly overloaded LAN with500 mobile agents moving and communicating simultaneouslywith no stop). Moreover, most mobile agent platforms are

locally with the new Proxy Server

The mobile agent travels to the new proxy to interact

SITUATION IN THE SIMULATION

Proxy1 Proxy2

real and simulated scenarios.

puter Applications 32 (2009) 846865implemented in Java, whose class loading mechanism providescode caching: Therefore, the code of an agent will only betransmitted to a certain node once and will be later reused byother agents of the same class that arrive there. The small delaythat the trip of a mobile agent may incur is not signicant, and sousing mobile agents is benecial, according to the experimentspresented in Section 6.1.

It is interesting to highlight that, when a change of proxy areaoccurs, the previous proxy will only notice that a moving objectunder its area has stopped using its services. Similarly, the newproxy will only notice that a new moving object under its areastarts using its services. The mobile agent should communicatewith the Proxy Server on its current proxy in the same way that areal moving object would. In this way, from the point of view ofthe tested data service, there would be no difference between areal and a simulated object.

3.1.2. Simulation of the objects movements

A mobile agent begins its execution on the proxy that providescoverage to the initial location of the simulated object that suchan agent represents. Then, the mobile agent can simulate, forexample, that it follows a predened path by processing afunction that computes the next location from the currentlocation, speed and direction. The obtained location could beused for a variety of purposes (e.g., to simulate that the objectcommunicates its location to its proxy) or simply to detect when achange of proxy area occurs.

ARTICLE IN PRESS

Fig. 2. GPS simulator to control simulated moving objects manually.

S. Ilarri et al. / Journal of Network and Computer Applications 32 (2009) 846865 849It should be noted that the use of mobile agents allows creatingobjects with the required behavior, such as: (1) simple simulatedobjects that follow a predened trajectory; (2) objects whosemovements are controlled remotely (e.g., a GPS simulator4 isshown in Fig. 2 that allows to control a simulated object remotelyby turning the knob on the left and the speed selector on theright); or (3) objects that react to the environment by changing

their behavior (e.g., they move slowly or fast depending on trafcconditions).

3.1.3. Simulation of the objects functionality

The different functionalities of the simulated object (providedto real objects by hardware appliances or software applications)that are interesting for testing are simulated. If possible, eachbehavior will be represented by the real code of the application totest. If some behavior cannot be used without changes (e.g., themoving object is supposed to access a database and there is noreal database available for testing), then it will be implemented asa thread of the mobile agent that will simulate that behavior. Forexample, a mobile user browsing the web with his GPS-enabledwireless device can be simulated by a mobile agent with twothreads (see Fig. 3): One would be a GPS simulator that computesthe current location based on the (wanted) users trajectory, andthe other would request web pages following the (wanted) useraccess pattern. If a location-dependent service must be tested,another thread could be in charge of communicating the userscurrent location to his/her proxy. It should be indicated that eachthread should be encapsulated in a software agent, as explained inSection 4, but it is preferred to keep the description simple here.

Mobile agents can simulate the behavior of almost anyprogram or appliance attached to real mobile devices. Whichfeatures of these elements are simulated depends on the goals ofthe simulated scenarios (only interesting features should be

coverage area of a real proxy, and (3) to test with real objects newfunctionalities not provided by real proxies.5

4 The authors of this article thank SailSoft (http://www.sailsoft.nl) for

providing the Visual Basic source code of GPSSIMUL. Some of the code was

adapted to use it in the developed system.To detect real objects, the Proxy Agent of the simulated proxysends Proxy Assistant Agents to the real proxies whose coveragearea intersects with the area of the simulated proxy. An example isshown in Fig. 5, where Proxy4 is not real (it is simulated on asimulated). When one of the interesting functionalities is theexecution, on the moving object, of an application based on agents(or, similarly, when the moving object executes several threads),some challenges arise due to the difculty of packaging theseresources as a mobile agent for simulation (see Section 4).

3.2. Simulated proxies

A simulated proxy is dened as a certain conceptualization of aproxy. Simulated proxies can be inspired by real proxies, but theycan also have different properties. For example, to test certain dataservices, a real proxy could be required to have a differentcoverage area. A simulated proxy will be implemented by creatinga place (execution environment for mobile agents, as explained inSection 1) with a Proxy Agent on a xed computer. The ProxyAgent is a static agent that will provide the wanted functionalityof a Proxy Server. As an example, a proxy that provides location-dependent web pages (Acharya et al., 1995) will be shown inSection 5.2.

These two elements, the place and the Proxy Agent, can resideon any computer on the xed network (real proxies are wiredconnected). The URLs of such xed computers are stored in theProxy Catalog (see Section 3.1.1) to identify the simulated proxies.URLs of real proxies are also stored in the Proxy Catalog, in orderto allow the simulated objects to be aware of the real networkinfrastructure; otherwise, they could not detect the real proxies.

In Fig. 4a, an example of a Proxy Catalog is shown (simulatedproxies are those with dashed areas), where the followinginformation is stored for each proxy in the sample scenario: itsidentier, its URL (IP address and communication port on whichrequests frommoving objects can be received), its location, and itscoverage area. It should be noted that both real and simulatedproxies are identied similarly by a URL. Different proxies can besimulated on different or on the same xed computer (as shownin Fig. 4b). When a simulated moving object needs to commu-nicate with a proxy, it rst obtains the URL of such a proxy byquerying the Proxy Catalog, and then it communicates with it as itwould in a real environment. So, the access to the data service isindependent of whether the proxy is real or simulated.

In order to simulate communication delays, the ProxyAgentscan be parameterized to delay adequately when communicatingwith simulated objects (to simulate wireless network delays) andwith other proxies (to simulate xed network delays). They canalso simulate temporal disconnections from their moving objects,if needed. For more details about the simulation of delays, seeSection 5.1.

The complete functionality of a proxy cannot be simulatedbecause, for example, a simulated proxy cannot provide coverageto real moving objects. However, a scenario where a simulated(therefore non-existing) proxy detects real moving objects canstill be dened, as long as those real objects are under thecoverage of a real proxy. This scenario could be interesting due tothree main reasons: (1) to simulate that real objects change ofproxy area as they move within the coverage of a single real proxy,(2) to simulate the existence of areas without coverage inside the5 For example, it could be interesting to simulate that certain information

about the objects (e.g., their locations) are stored by the proxies, which may not be

the real case.

ARTICLE IN PRESS

ject

ComGPS simulator

MobileWeb browser simulator

Place

agent

Fig. 3. A simulated object (a) and a real obProxy

Proxyserver

a

S. Ilarri et al. / Journal of Network and850certain xed computer) and Proxy3 is the one that actuallyprovides coverage to the real moving object shown in the gure.Proxy Assistant Agents send to their Proxy Agent informationabout moving objects entering the area of the correspondingsimulated proxy: Every Proxy Assistant Agent obtains thisinformation by interacting with the Proxy Servers on theircorresponding real proxies. For this, a Proxy Server must supportat least queries that retrieve information about the objects withinits area. The testing system can simulate the availability of anyinformation about the real objects (even if this information doesnot exist in the real world as, for example, the Proxy AssistantAgent could generate it randomly or according to some otherstrategy). In Fig. 5, the Proxy Assistant Agent on Proxy3 sends theinteresting data of the real moving object to the Proxy Agent of thesimulated Proxy4 (which executes on a certain xed computer).

3.3. Simulation of a sample scenario with real and simulated objects

and proxies

In this section, a sample scenario that could be used to test acertain wireless data service is shown. In Fig. 6a, as in Fig. 4,simulated proxies are those with a dashed coverage area (Proxy2,Proxy5, and Proxy6), and the rest of the proxies (Proxy1, Proxy3,and Proxy4) are real; the simulated objects are also shown

Proxy Server

Proxy4

Fig. 4. Proxy Catalog example: simulateWebbrowser

(b) communicating with the same proxy.

ProxyServer

Proxy3(poposhoalstesreaapppomeThoncovrepanpro

protio

PA

d scb

r Applications 32 (2009) 846865puteliceCar1, policeCar2, policeCar3, policeCar5, policeCar15,liceman1, policeman2, policeStation3, and policeStation19). Ituld be noted that, for the sake of generality, static objects areo considered as moving objects if they are interesting for theted data service (e.g., if they can be clients of the service). Thel scenario is presented in Fig. 6b, where some real objectsear that were not in Fig. 6a (car15, car38, policeCar4, andliceStation2). The three simulated proxies have been imple-nted as places (with a Proxy Agent) on two xed computers.e simulated objects have been implemented as mobile agentsthe places of the proxies (real or simulated) that provideerage to them in the wanted scenario; e.g., policeCar5 isresented by a mobile agent executing on the real proxy Proxy1,d policeCar15 is simulated by a mobile agent on the simulatedxy Proxy6 on the second xed computer.The proposed testing system allows mixing real and simulatedxies and moving objects transparently. The following situa-ns can be distinguished (see Table 1):

A real object with a real proxy. A real object interacting with areal proxy requires no simulation, and both can be part of apartially simulated scenario.A real object with a simulated proxy. It is possible to simulateproxies that provide coverage to real objects (whenever they

Proxy1

Proxy Server

roxygent

ProxyAgent

ProxyAgent

Fixed computer 1

Proxy6Proxy5Proxy2

Fixed computer 2

enario (a) and real scenario (b).

ARTICLE IN PRESS

2 (real

TabMix

Rea

Sim

ComProxyS. Ilarri et al. / Journal of Network andare under the coverage of any real proxy, as explained inSection 3.2).A simulated object with a real proxy. A simulated object isrepresented by a mobile agent, which executes on the place ofits real proxy.

Proxy

AgentAssistant

serverProxy

Proxy

Agent

Proxy3 (real)

Proxy1 (real)

Assistant Re

Proxy4 (if it were real)

Proxy Agent

Fig. 5. A simulated proxy detec

Fig. 6. Simulation example: test sce

le 1ing real and simulated moving objects and proxies.

Real proxy Simulated proxy

l objectp p

As long as a real proxy detects it

too

ulated objectp pA mobile agent on

the real proxy

A mobile agent on the computer

that simulates the proxy

It itradeinssimfolprooncoufolthrsimret

al mo

ting

polic

P

Prox

policpolic

narServer Proxy

Proxyserver Proxy

Fixed computer supporting simulated Proxy4

Proxy Agent

)

puter Applications 32 (2009) 846865 851A simulated object with a simulated proxy. This case is like theprevious one, but with the mobile agent executing on the placeof the simulated proxy.

s also possible to simulate that a real moving object follows ajectory different from the real one. This would be equivalent toning a simulated object whose features and behavior arepired by such a real object. For example, it is possible toulate that a certain wired device is a moving object thatlows a trajectory or that it accesses a data service on a certainxy: A mobile agent can be created to represent such a devicethe wireless network. More importantly, the wired device itselfld actually access the data service by interacting (e.g.,lowing the orders of a user) with the corresponding proxyough its mobile agent in the simulation. For example, it can beulated that a desktop computer is a moving object whichrieves from its current (real or simulated) proxy information

AgentAssistant

ving object

a real moving object.

car38

policeStation2

policeCar4

car15Proxy4

policeman1

policeCar5

eCar1

policeCar15

Fixed computer 2

Proxy Server

Proxy Server

Proxy Server

Proxy1Proxy Agent

Fixed computer 1

roxy2 Proxy5

Proxy3

Proxy6

y Agent Proxy Agent

policeman2

policeStation3policeStation19

eCar2eCar3

io (a) and real scenario (b).

(which will be termed Moving Object Agent, MOA). It should be

with an internal thread that simulates a user using a web browserwill be shown in Section 5.2.

4.1.

Theis cappsim

muPDAis, t

app

agetha

T

ARTICLE IN PRESS

Computer Applications 32 (2009) 846865stressed that the approach described in this section also applies,similarly, to moving objects hosting static agents or threads. Evenif the agents are static in the original application code, they areneeded to be mobile in the simulation. Thus, an IA must movewith its MOA, which can be more easily implemented if the IA is aabout other nearby (real or simulated) moving objects; thisinformation can then be used by the desktop computer (e.g., toshow it to a user). In this way, it is possible to access wirelessservices using a xed computer.

4. Simulating multiagent applications executing on a movingobject

In this section, the strategy proposed for the simulation ofmoving objects that execute multiagent applications is described.From the point of view of a simulation, among the mostcomplicated applications that moving objects can execute arethose based on mobile agents. Mobile agents have been proposedas a very interesting technology to build applications for mobileenvironments (Spyrou et al., 2004; Cardoso, 2002). Therefore, it ishighly interesting to consider how the testing system couldhandle this case. As explained in Section 1, mobile agents cantravel across computers. Thus, they can decide to interrupt theirexecution on its current moving object, travel to anothercomputer or moving object (where the execution is resumed),and come back in the future even if the original moving object isthen under the coverage of a different proxy. Therefore, thesimulation of moving objects becomes a difcult task if there is aninterest in simulating that they are executing multiagent applica-tions. If a simulated moving object is required to execute anapplication composed of agents, because it is interesting orneeded for the tests, then such an application should theoreticallyexecute inside the mobile agent that simulates the moving object,which is obviously not possible.

Alternatively, the functionality of each agent could besimulated by using a new thread within the mobile agent thatsimulates the moving object. This approach is very costly because:(1) the behavior of the multiagent application must be re-implemented using threads; and (2) some of the agents couldbe mobile and require to perform movements to other computersor devices to resume their execution there, which cannot besimulated easily without using mobile agents (e.g., they couldneed to access real data on those remote computers/devices).Instead, a solution based on reusing the original code of themultiagent application is advocated, which allows to perform asimulation as close to reality as possible with a minimum effort.Agents that should execute on the simulated moving object willbe reused directly, and a certain coordination mechanism willallow them, when needed, to travel attached to the mobile agentthat simulates the moving object where they reside.

One additional advantage of this approach is that, in thesimulated scenario, mobile agent applications running onsimulated moving objects will behave exactly as expected. Thus,some of those agents could even move to real computers outsidethe simulation scenario, access a real database, and come back tothe simulation scenario with the results. With this approach, thetime required to prepare a simulation is greatly reduced.

In this section, it is analyzed how to realize this plug-and-simulate proposal while keeping the original applications mobileagents (which will be called Internal Agents, IAs) unaware of thereal or virtual nature of the moving object where they execute

S. Ilarri et al. / Journal of Network and852mobile agent. Similarly, each thread must be encapsulated in amobile agent, so the simulation can benet from the coordinationprotocol described in this section; an example of a moving object(1) Detection of the need for traveling. The MOA computes the nextlocation of the moving object that it represents (e.g., from apredened trajectory) and realizes that it implies a change ofproxy area.

(2) Request of movement. The MOA requests its IAs to travel to thedestination proxy.

(3) Packaging for the journey. The IAs get ready for the journey bywaiting until the current transaction (dened in this article asa sequence of operations that cannot be interrupted by ajourney) ends.6

(4) Traveling. The MOA and IAs move to the new proxy. The orderin which they travel is not important, they can travel inparallel.

(5) Notication of arrival. After traveling, each IA sends anacknowledgement to its MOA indicating that the trip wasperformed correctly. A notication of arrival could fail if theMOA is still traveling to the destination; in such a case, thenotication will be retried until it gets through.

(6) End of travel. When the MOA arrives at the destination, it waitsuntil it receives notication of arrival from all its IAs. Then, thetravel is considered nished. In case the MOA or any IA ndssome problem traveling, the movement will be retried asmany times as needed; the simulation fails only if themovement has not succeeded after a predened timeout.

(7) Resume execution. The MOA sends a notication to its IAs forthem to resume the execution, and it will also continue itsown execution.

6 It should be noted that the termination of the main thread of an autonomousin trunagen

anot

threCoordination protocol

he different stages of the coordination mechanism developedhe proposed testing system to simulate moving objectsning multiagent applications are the following (see Fig. 8):4.2.unat purpose, a set of classes has been dened that provide suchnts with the required features (see Section 5.1) in such a wayt their original functionality will not be disturbed (they will beware of being running in a simulated scenario).themthal, extending the functionality of the agents in the originallication code in order to allow the MOA to coordinate within the simulation scenario is required (see Section 4.2). Fortravgoaroxy area by the simulated moving object), its associated IAsst move too, in the same way that agents executing on a realwill remain on it when that PDA moves geographically. Thathe MOA has to coordinate itself with its IAs when it needs toel, making sure that they move with it as well. To achieve thisWof pulated, several mobile agents must be used in the simulation:mobile agent that is used to simulate the moving object, thatalled MOA, and the agents used in the original multiagentlication, that are called IAs. The equivalence between real andulated scenarios is summarized in Table 2.hen an MOA moves to another proxy (to simulate the changeAsimReal and simulated scenario

s shown in Fig. 7, when a moving object containing agents ist cannot be forced (e.g., the agent could be communicating remotely with

her at that moment). Instead, the MOA requests an IA to travel and the IAs

ad will detect the pending request and perform the trip as soon as it can.

ARTICLE IN PRESS

ComputeS. Ilarri et al. / Journal of Network andA situation where an IA departs from its moving object and comesback later (see Fig. 9 for a sample scenario, where an IA travelsfrom one simulated moving object to another) is implementedmore easily with a mobile agent platform that provides locationtransparency (Stefano and Santoro, 2002), as SPRINGS (Ilarri et al.,2006b) does. Thus, such a platform automatically keeps track ofthe proxy where each MOA is executing and ensures that a

comtraforsim

strmaaut

5. Using the testing system

ProxyAgent provides the functionality needed by any simulatedproxy (see Section 3.2). In our implementation, it basically

InternMultiagent app

PDAProxy

Fig. 7. Moving objects executing multiagent applicati

Table 2Simulation of objects executing multiagent applications: MOA and IAs.

Real Simulated

Moving object (e.g., a PDA) Mobile agent MOA

Place on moving object Place on proxy

Agents on moving object Mobile agents IAs

Detection of the need for traveling

Request of movement

IAs packagingMOA traveling

IAs traveling

Notification of arrival

Waiting notifications

Resume execution

Fig. 8. Activity diagram for the coordination protocol.allows a moving object to register and de-register from theproxy, keeps information about the objects registered, andprovides logging facilities.

InternalAgent provides the functionality of an IA, that is, itimplements the elements needed by the coordination protocoldescribed in Section 4.2. It should be noted that the agents andthreads executing on a moving object are implemented asmobile agents for simulation purposes (see Section 4).This section shows how the testing system can be used. First,the main classes in the system and how they can be adapted tospecic simulation requirements are described. Next, an exampleshows how existing code can be reused for testing.

5.1. Classes in the system

In Fig. 10, the main classes of the system are shown (the coreclasses are shaded). The most important ones are the three classeswhich correspond to the need of implementing the functionalitiesof proxies (see Section 3.2) and MOAs/IAs (see Section 4.1):

Anme

essed that some mobile agent platforms, such as SPRINGS,nage failed communications and trips transparently (retryingomatically if needed).chamunication eventually succeeds even if the target agent isveling to another computer. Using other mobile agent plat-ms that do not provide location transparency, solving theulation issues that arise in these cases would be morellenging. Regarding the steps 5 and 6, it should also beMoving Object Agent

Internal Agents (IAs)

(MOA)

Proxy

al agentslication

ons: real scenario (a) and simulated scenario (b).r Applications 32 (2009) 846865 853MovingObjectAgent provides the basic functionality of an MOA.This includes the implementation of the coordination protocolexplained in Section 4.2 as well as the simulation of move-ments (according to a certain mobility model, as indicatedbelow) and handoffs. See Section 3.1 for details about thesimulation of moving objects and Section 4.2 for an explana-tion of the coordination protocol needed to allow thesimulated moving object to execute mobile agents.

other couple of classes in the diagram that are worthntioning are indicated in the following:

MobilityModel is an abstract class which represents a mobilitymodel (Camp et al., 2002; Schindelhauer, 2006) for aMovingObjectAgent. Two mobility models considered as partof the system are a trajectory-based mobility model and arandom mobility model (represented in the gure by the

Inmoageplaageproclause(e.

ARTICLE IN PRESS

Proxy2 Proxy2

),

oth

ComMOA2 represents Sim. Obj. 2(under the coverage of Proxy2(which has one internal agent),to query data there

Fig. 9. Example of an internal agent that moves toIA

MOA1 represents Sim. Obj. 1Step 1IA travels to Sim. Obj. 2

Initial state

Proxy3

MOA2

Sim. Obj. 2Sim. Obj. 2

MOA2

Proxy3MOA1

Proxy1

IASim. Obj. 1 Sim. Obj. 1

Proxy1

MOA1

S. Ilarri et al. / Journal of Network and854classes TrajectoryMobilityModel and RandomMobilityModel,respectively).DelaySimulator provides the functionality needed to simulatenetwork delays. Several subclasses are provided by default(e.g., GSMSimulator and GPRSSimulator) to simulate delays inspecic network environments. An instance of DelaySimulatoris associated to a ProxyAgent, in order to delay communicationshaving its proxy as destination. This class implements adelaying method parameterized with the size of the datacommunicated (in KBs) and the name of the source proxy (thissecond parameter is not applicable in the case of wirelesscommunications from a moving object to a destination proxy).Such a method must be called whenever a communication issusceptible of being delayed.

Fig. 10, an additional class appears: SpringsAgent. In mostbile agent platforms, a programmer implements a mobilent by specializing a specic class provided in the API of thetform. In the current implementation of the system, the mobilent platform SPRINGS (Ilarri et al., 2006b) is used, whichvides a base class SpringsAgent that must be extended by thesses in the system. Depending on the mobile agent platformd (Trillo et al., 2007), the name of this class could be differentg., MobileAgent in the mobile agent platform Grasshopper and

1 Proxy Agent

Mobility Model

contains*

*

1

InternalAgent Moving Object Agent

Trajectory Mobility Model Random Mobility Model

Springs Agent

Fig. 10. Class diagram foobject (now under Proxy2)

Sim. Obj. 2

Sim. Obj. 1 Sim. Obj. 1

Sim. Obj. 2

Sim. Obj. 2 is under Proxy3, IA returns to its initialSim. Obj. 1 is under Proxy2

with the results

Step 2 Step 3

Proxy2 Proxy2

Proxy1

Proxy3

MOA1

Proxy1

Proxy3

MOA1

MOA2

IA

MOA2

IA

er simulated moving object and comes back later.

puter Applications 32 (2009) 846865Aglet in Aglets); also depending on the platform, a different baseclass may need to be considered for a Proxy (e.g., StationaryAgentin Grasshopper), as it is a static agent. According to the experienceof the authors with different mobile agent platforms, onlysyntactic changes are usually required (the name of the agentclass, the name of the main method of the agent, the method theagent must call to move to another place, etc.), although this canalso depend on the main features of the platform (e.g., if it is basedon remote calls or message passing).

The tester of a data service should extend the classesMovingObjectAgent and ProxyAgent to dene the moving objectsand proxies needed for his/her specic simulation tests (andreusing the original code of the data service to test). For example,a subclass of MovingObjectAgent representing a simulated carcould add the required code to communicate periodically, to itsproxy, information about its surroundings (e.g., number of nearbycars); a subclass of Proxy could receive this information andprocess it in order to compute the number of cars within a certainarea. In this way, the developers of wireless data services can testfeatures that are not even available in a real network. The tester ofthe data service must also make sure that each agent/thread thatshould execute on a simulated moving object extends the classInternalAgent.

It can also be interesting to specialize the DelaySimulator classto implement the simulation of network delays in the wanted

uses* 1

Simulation framework

LAN Simulator

Delay Simulator

GSM Simulator GPRS Simulator

r the testing system.

ARTICLE IN PRESS

Need/problem Solution

f needed,

lize Inter

req

ry cl

mobile agent in the

d In

the

te su

uenc

ol. In

lread

ent)

exa

Middle/high. Some modications in the original code may

ior (

ted i

Computer(1) The tester needs to simulate some

special functionality on the proxies

Specialize ProxyAgent. Implement, i

auxiliary classes

(2) The tester needs to simulate some

special functionality on the moving

objects

SpecializeMovingObjectAgent. Specia

if an additional thread of behavior is

Implement, if needed, other auxilia

(3) The tester needs to reuse an

application based on mobile agents

Specialize MovingObjectAgent. Each

existing application must also exten

attach itself as an internal agent of

MovingObjectAgent (that must crea

agent), checking with a certain freq

required by the coordination protoc

is not possible (because the agent a

that is not a superclass of InternalAg

used instead (see Section 5.2 for an

(4) The tester needs to simulate moving

objects with different concurrent

Encapsulate each concurrent behav

InternalAgent and proceed as indicaTable 3Guidelines for using the testing system.

S. Ilarri et al. / Journal of Network andconditions. In particular, it is possible to integrate the proposedtesting system with a low-level network simulator. For example,in the context of this work, some tests have been carried out withthe popular network simulator ns-2 (http://www.isi.edu/nsnam/ns/). It is possible to dene a network in ns-2 (dening the nodes,links, bandwidth, latencies, error rates, etc.) and interact with thesimulation from Java code, in order to obtain the delays to apply tothe network communications. Finally, the tester of the dataservice can also implement mobility models suited to his/herneeds by subclassing MobilityModel.

In summary, Table 3 provides some guidelines to consider totest a mobile data service with the proposed testing framework,along with an assessment of the difculty of the different tasks.An example of use of the testing system, which shows how thebasic classes in the framework can be extended, is presented inthe next section.

5.2. Use case: testing location-dependent web proxies

This section illustrates, with a simple example, how existingcode can be reused in the proposed testing system. The scenariopresented in this section is motivated by the idea of location-

behaviors

(5) The tester needs to reuse an

application, based on mobile agents,

implemented in a platform different

from the one used in the testing system

(currently, SPRINGS)

A version of the testing system impleme

same mobile agent platform is needed

(6) The tester knows nothing about

mobile agents

A basic knowledge of mobile agents is re

will benet from learning the way a pla

how agents are created, how they move

communicate. Moreover, he/she needs t

a trip of an agent implies a change of ex

environment (so the local resources are

anymore and the values of static variabl

and that the agents variables must be S

marked as transient

(7) The tester encapsulates code in a

mobile agent but if fails

Probably something clashes with the ag

the code tries to set a SecurityManager a

does not allow it to be changed): AvoidternalAgent and

corresponding

ch an internal

y if a trip is

case subclassing

y extends a class

, wrapping can be

mple)

be required to check periodically the need of traveling and

clean the state of the agent before a trip (e.g., close open

les). This may be difcult if the original agent performs

long transactions of work that cannot be interrupted by a

trip, in which case the transaction should be divided into

smaller tasks. Otherwise, it is simple

thread) in an

n the row above

Middle/high. See the row aboveuired

assesdepdepreumenthemovlocaclas

(1)

nted

quire

tform

, and

o und

ecut

not

es m

erial

ent p

nd t

the cDifculty

other Low. It only depends on the difculty of implementing the

required functionality (if not implemented already): Its

integration is straightforward

nalAgent also

(see row 4).

Low. It only depends on the difculty of implementing the

required functionality (if not implemented already): Its

integration is straightforward

Applications 32 (2009) 846865 855endent web pages (Acharya et al., 1995), whose contentsend on the location of the user. The existing code that will besed is Lobo (http://lobobrowser.org/), a web browser imple-ted in Java. Although Lobo is open-source, no modication tooriginal code will be performed. The goal is to simulateing objects that execute the Lobo browser to accesstion-dependent web pages. Regarding proxies, the followingses are dened:

A class SimpleURLRewriterWebServer is implemented. An in-stance of this class is initialized with two parameters: a portnumber and a URL country extension (e.g., es for Spain, it forItaly, us for the United States, etc.). It creates a thread thatlistens to the communication port specied (using a socket) toread URL requests (e.g., a request GET http://www.google.comHTTP/1.0) from clients (web browsers using that instanceof SimpleURLRewriterWebServer as HTTP proxy). When arequest is received, it changes the country extension ascongured (e.g., if a request for http://www.google.com isreceived and the extension set is it, then the URL is changed tohttp://www.google.it). Then, it opens a socket to that URL andread the contents of the corresponding web page, which is

using the Middle. It is usually easy to re-implement the testing

system with a different mobile agent platform, as its

functionalities can be wrapped in appropriate classes

using the new platform. It can be more difcult if the

platform to use considers a different architecture or

communication model (e.g., if synchronous calls have to

be emulated via message passing)

d. The tester

is deployed,

how they

erstand that

ion

accessible

ay be lost),

izable or

Low. These are basic aspects and easy to learn. No in-

depth knowledge or extensive experience is required

latform (e.g.,

he platform

onict

Low. These problems are not expected to be frequent and

the solution is usually easy, as long as the original code

can be modied or provides a exible API

(2)

Reg

(1)

ARTICLE IN PRESS

Computer Applications 32 (2009) 846865856 S. Ilarri et al. / Journal of Network andreturned to the client that initially performed the request. Thus,it implements an HTTP proxy that automatically returns webpages to the browser as they would be received if the URLs weretyped in the corresponding country (many web servers todayperform this country-based redirection automatically if the URLtyped has a com extension). As the instance of theSimpleURLRewriterWebServer class can be congured withany extension, it is possible to simulate that the Lobo webbrowser is executing in a different country by setting its HTTPproxy as the URL and port of the corresponding SimpleURLRe-writerWebServer instance.A class ProxyLocDepWebPages is implemented that extendsProxyAgent to customize the desired behavior of the proxies inthe testing system. The basic structure of this class is shown inFig. 11. Basically, it creates an instance of SimpleURLRe-writerWebServer whose parameters are determined by thename of the proxy (for simplicity, the mapping between theproxy name and the port and extension is hard-coded, but amore exible approach can be easily adopted) and adds amethod getHTTPproxyPort that will allow a moving object toquery the port of the HTTP proxy they should connect to. Itshould be noted that class SimpleURLRewriterWebServer is anauxiliary class used to implement the wanted functionality ofproxies. However, it can also be used in isolation to set up anHTTP proxy that can be used from any web browser.

arding moving objects, the following classes are dened:

Lobo is a program, so a class LoboClient is rst implemented tosimulate a user using the Lobo web browser. This class simplynavigates to different URLs stored in a text le, waiting acertain amount of time between page requests. Thus, this is a

(2)

(3)

Fig. 11. Basic structure of Proxy

7

preD

impl

shousimple class that makes use of the Lobo Browser API toautomate navigation.A class called LoboClientWithoutFiles is implemented, whichextends LoboClientwith amethod called readURLswhich returnsa vector with the URLs in the le and is able to process theseURLs using the information contained in the vector instead ofusing directly the le. This class is needed because using thefunctionality of LoboClient from a mobile agent implies that it isnot possible to rely on a local le. Thus, when the mobile agentmoves to another computer, the le is not accessible anymore.A class called LoboClientWithoutFilesIA extends InternalAgentto implement the behavior of a user navigating periodicallythrough a set of URLs (Fig. 12). It encapsulates an instance ofLoboClientWithoutFiles, which performs most of the work. Acall to the method travelIfNeeded, implemented by Interna-lAgent, is performed from time to time (when a transactionnishes, according to the explanation in Section 4.2) to makethe IA follow its MOA (if needed). The main method of theagent is called main in SPRINGS and it species the life of theagent. It should be noted that implementing the extrafunctionality as an internal agent is not required: Instead, itis possible to implement the extra functionality in a subclassof MovingObjectAgent. However, the extra functionality hasbeen considered here as an additional thread of the movingobject, and therefore is implemented as an IA. The preDepar-ture method is automatically called in SPRINGS when theagent is about to move to another context.7

LocDepWebPages.

Methods of SpringsAgent that are implemented by InternalAgent, such as

eparture, postDeparture, postArrival, preArrival, etc., may be overridden by the

emented subclasses, but in that case the new implementation of the method

ld include a call to the corresponding method in the superclass.

ARTICLE IN PRESS

Com(4)S. Ilarri et al. / Journal of Network andA LoboClientWithoutFilesMOA class extends MovingObject-Agent to add the behavior of browser navigation using Lobo.The basic structure of this class is shown in Fig. 13. Basically,the ProxyAgent is queried about its HTTP proxy when ahandoff is performed, and the port number is communicatedto the internal agent LoboClientWithoutFilesIA. To commu-nicate with another agent, the syntax of SPRINGS for remotecalls is used, which allows a location-transparent commu-nication by just specifying the name of the target agent (inthis case, both agentsthe MOA and the IAwill be on thesame computer). Moreover, SPRINGS identies the targetmethod using Java reection, and so any of the agentsmethods can (by default) be invoked remotely.

Fig. 12. Basic structure of Loputer Applications 32 (2009) 846865 857As explained, no changes in the original code are performed. Onlythe new functionalities required (an HTTP proxy to perform URLredirections) need to be implemented. Then, a GUI allows thedeveloper to dene and execute scenarios consisting of movingobjects and their trajectories and proxies and their proxy areas.For both moving objects and proxies, the corresponding imple-menting class (that must be a subclass of MovingObjectAgent orProxyAgent, respectively) can be specied in the GUI, such thatthe needed instance is built (using Java reection) when the testerdecides to run the testing system. Fig. 14 shows a scenarioconsisting of two cars under the coverage of different proxies andthe Lobo browser associated to each object at a particular timeduring the execution of the testing system (car1 is in Spain and

boClientWithoutFilesIA.

ARTICLE IN PRESS

ComS. Ilarri et al. / Journal of Network and858car2 is in Italy). Both cars request the same URL (http://www.nokia.com) but the contents of the web pages shown bythe two browsers are different. It should be noted that the Lobobrowser appears on the screen associated to the terminal wherethe proxy that provides coverage to the moving object isexecuting. Therefore, a user on that terminal could interactdirectly with the Lobo browser. Depending on his/her purposes,the tester could not require the Lobo browser to appear on thescreen.

6. Evaluation of the system

In this section, the proposed testing system is evaluated. Forthis purpose, four experiments are carried out. The rst oneproves the benets of a simulation approach based on mobileagents to distribute the testing load efciently over a set ofcomputers and minimize the communication overhead. Thesecond one shows an example that indicates how the testingsystem can be used to test applications based on mobile agentseasily, thanks to the agent coordination protocol proposed. Finally,the last two experiments prove the suitability of the proposedtesting system to evaluate pre-existing data services. Thecomputers used in the simulations are Pentium IV 3.6GHz, 2GBRAM, with Linux 2.6.14, connected through a LAN.

The proposed testing system has been implemented using Javaand the mobile agent platform SPRINGS (Ilarri et al., 2006b). Java

Fig. 13. Basic structure of Lobputer Applications 32 (2009) 846865is used because: (1) as explained in Section 1, most mobile agentplatforms are implemented in Java; and (2) many wireless dataservices to test will be probably implemented in Java. Thus, Javaalso offers many facilities to develop applications for distributedand mobile computing; portability, safety and robustness are themain reasons to use Java for wireless development, according tohttp://developers.sun.com/mobility/getstart/. However, mobiledata services implemented in a different programming languagecould also be possibly evaluated by the system by using, forexample, JNI (Java Native Interface) to enable the interactionbetween Java code (of the testing system) and application code (ofthe mobile data service to test) written in another programminglanguage.

6.1. Evaluation of the simulation approach based on mobile agents

In this section, the goal is to measure the accuracy of theproposed simulation strategy (based on mobile agents), bycomparing it with other alternative approaches. Simulating thatthe proxies are able to detect the locations of moving objectswithin their coverage is wanted in this experiment. In thesimulation, each proxy stores the locations of such objects, whichare received every second (considering the objects trajectories)from the mobile agents that represent them.

The accuracy of a simulation is measured in this test in terms ofthe difference between the expected (real) locations of the objects

oClientWithoutFilesMOA.

ARTICLE IN PRESS

Computer Applications 32 (2009) 846865 859S. Ilarri et al. / Journal of Network and(i.e., the locations of the objects in the real scenario) and thelocations known to the underlying infrastructure (i.e., thelocations received by the proxies) over time. Although measuringthe location error is only signicant in a scenario where theobjects communicate locations, it indicates whether the simu-lated objects are able to communicate with the proxies as quicklyas required by the scenario that must be tested. In other words,

Fig. 14. Snapshot during the testing of m

Fig. 15. Network infrastructure considered for accuracy evaluation.the location error in this experiment indicates how close to reality

oving objects with Lobo browsers.the simulation performs.To perform this test, the network infrastructure shown in Fig. 6

has been slightly modied in order to minimize the areas withoutcoverage and consider only simulated proxies (see Fig. 15). Toavoid the problem of objects getting out of coverage (it is notpossible to compare a real location with a location that isundetectable by the network infrastructure), the objects areconstrained to move within a rectangular area (marked inFig. 15) that is fully within coverage; a change in the directionof movement is forced for objects that are about to get out ofthat area.

For this experiment, 1500 objects moving at 60mph arecreated. First, predened trajectories are generated according tothe following strategy: Every object is initialized with a randomlocation and orientation, and the orientation will change ran-domly (with a 30% probability) every 5 seconds to simulate erraticand unpredictable movement patterns. These trajectories areconsidered the real data for the tests, and therefore they are storedon les for later comparison with the simulation results.

Three alternatives that could be used to simulate a movingobject are compared: (1) using a mobile agent that represents themoving object (as it is advocated in this article); (2) using a threadthat communicates the location of its moving object using remoteinvocations; and (3) using the IBM Location Transponder (http://www.alphaworks.ibm.com/tech/transponder, published on April2002), a real-time location data reporter which paces thetransmission of location updates to match the time instants whensuch updates should be generated.8 For each of these possibilities,two cases are considered: (1) All the proxies in the network

8 With the Location Transponder, remote communications are also used.

However, it implements a deadline-based strategy for location updates with the

goal of providing a better accuracy.

ARTICLE IN PRESS

latio

sted objeOPT1) m2) re

Com860interepossibly interested objectinfrprocomsevpertesttheprocon

(1)

(2)Fig. 16. Accuracy of the simuS. Ilarri et al. / Journal of Network andastructure are simulated on the same computer, and (2) thexies are simulated using ve different computers in a LAN (oneputer must host two proxies). Each experiment is carried outeral times and, for each time instant, the average location errorobject (in meters) is computed; the results for 100-seconds are shown in Fig. 16. The similarity of the results obtained indifferent experiment repetitions (not shown in the gure)ves that the number of repetitions is enough. The followingclusions are drawn:

Distributed simulations are useful. As shown in Fig. 16, theworst results are obtained when a single computer is used tosimulate all the moving objects and proxies (it becomes abottleneck). In this situation, the experimental results showthat there is no signicant difference between using mobileagents, remote communications, or the IBM Location Trans-ponder. Therefore, only one line is shown for this case in thegure.Mobile agents allow to perform distributed simulations with the

best accuracy. In Fig. 16, it can be seen that there is a signicantimprovement in the simulation when mobile agents are usedin a distributed environment. On the one hand, they allow todistribute the simulation of the moving objects. On the otherhand, a mobile agent simulating a moving object cancommunicate locally with its proxy, avoiding remote commu-nications. This explains that, in a distributed environment, theapproach based on mobile agents outperforms the strategybased on remote communications and the one using the IBMLocation Transponder (both represented by a single line in the

Theappdistshoto pThistimasprosimalteThuimp199com

6.2.

base

Teva

"stock quotes" available target objec

target area

New data on

Fig. 17. Sample application and scenario: broadcasting new dactIONS:obile agent retrieving the interesting datamote invocation retrieving all data on "stock quotes"n using different strategies.

puter Applications 32 (2009) 846865gure, due to the similarity of the results obtained). It shouldbe stressed that although the execution of a mobile agent isstopped during a trip (and resumed once the movement toanother computer has nished), the additional delay intro-duced is small and clearly pays off. Thus, many delays will besaved because the simulated moving object will interactlocally with its proxy.

se experimental results prove the benets of the simulationroach considered, based on the use of mobile agents toribute the simulation efciently over a set of computers. Ituld be stressed that the simulation time (the total time needederform a simulation) is independent of the strategy adopted.is because a simulation is carried out considering the real

e scale, as the goal is to evaluate a real data service executing itit would be executed in a real environment. However, theposed approach based on mobile agents and distributedulations achieves an accuracy clearly superior to the otherrnatives (i.e., it performs a simulation much closer to reality).s, it minimizes the communication overhead, which isortant when distributed simulations are performed (Kumar,2), and distributes the overload efciently over the set ofputers involved.

Evaluation of the coordination protocol: testing an application

d on mobile agents

he exibility of the proposed system enables its use toluate applications based on mobile agents. In this experiment,

t

ta available (a) and requesting relevant data (b).

a s

(se

ob

evaluated by using it to test two pre-existing wireless data

(Villate et al., 2002). In summary, the use of lockers provides

ARTICLE IN PRESS

communications.

Comquerying computers located on the xed network), it broadcaststhe availability of new data on that topic to the movingobjects located within a certain target area around its location(see Fig. 17a). In this way, any other object within the area(interested in the topic) can query the reporting object (targetobject) to get the relevant data. On the interested object, there is amobile information agent in charge of retrieving these data. Forquerying, two possibilities can be considered (see Fig. 17b):

The mobile information agent moves to the object holding thedata. On arrival in the target object, it analyzes and lters thedata on the interesting topic. Then, it comes back only with aportion of the data (the data that it considers relevant to theinterested object).

The mobile information agent retrieves all the data on theinteresting topic from the target object through a remoteinvocation. Then, the data obtained are analyzed and ltered.witWhA teacaddaremuwesceup(toradthequrelpersinstawicomwaresrect functioning of the proposed agent coordination protocole Section 4.2).A scenario where different moving objects (mobile devices)tain information on different topics that they want to shareh their peers (e.g., sports news or stock quotes) is considered.en an object obtains new data about a certain topic (e.g., byappcorample scenario is shown to illustrate that a mobile agent-basedlication can be easily evaluated. This experiment evaluates theFig. 18. Comparison of the query time when using mobile agents and remote

S. Ilarri et al. / Journal of Network andest has been carried out in order to evaluate the convenience ofh of these approaches. The ProxyAgent class was specialized tothe capability of keeping track of the objects within the targetas; in this way, a moving object can query the set of objects itst notify. InternalAgent andMovingObjectAgent (see Section 5.1)re also specialized to implement the wanted behavior. In thenario, a wireless bandwidth of 40 kbps and a random delay ofto 1 second affecting wireless communications is consideredsimulate unreliability). Additionally, it is assumed that theius of a target area is half a mile and that 100KB of new data oninteresting topic can be retrieved each time a target object is

eried. Only a certain percentage of those data will be reallyevant to the interested object. In the experiment, thiscentage is modied and the average query time (time elapsedce the appropriate actions to retrieve the remote data arerted until those data are available on the interested object)th both strategies (a mobile agent and remote querying) arepared. For each percentage of relevant data, the experiment

s repeated several times, obtaining similar results. The averageults are shown in Fig. 18, where it can be observed that:

resandass

tha

higSeb

siminsnicwever, when the user arrives in Zaragoza, the LRS considerst it is better not to move the locker there (since there is not ah wired communication delay between Zaragoza and Sanastian). Access times from Zaragoza to San Sebastian areatHoReal data have been obtained in a scenario where a useriding in the USA travels to Spain, concretely to San Sebastianthen to Zaragoza. When the user arrives in San Sebastian, his

ociated locker travels there too. Consequently, while the user isSan Sebastian, le requests imply access to the local proxy.wireless device users with the following advantages. Firstly,lockers alleviate the device exposure problem (wireless devicesare more vulnerable and fragile than stationary devices, becausethey can be easily stolen, lost or damaged). Secondly, data storedin a locker are available for the agents at the proxy, even when thewireless device is disconnected, thus providing a solution for theavailability problem (wireless devices might stay disconnected forlong periods of time). So, specic tasks are carried out in the xednetwork, with data stored in a locker, instead of on the userdevice, in this way relieving the media problem (wirelesscommunications are often unstable, asymmetric and expensive).Thirdly, lockers can follow user movements, moving from proxy toproxy, but always residing close to the current location of the user,also relieving the media problem. Finally, due to the use of agenttechnology in their implementation, lockers constitute an auton-omous and auto-managed storage space.services: a locker rental service (Villate et al., 2002) and alocation-dependent query processor (Ilarri et al., 2006a).

6.3.1. Testing a locker rental service (LRS)

This service offers its users the possibility of keeping their datain a secure and safe space (located on a proxy) called locker Using a mobile agent is benecial in most cases. Thus, it canperform data ltering on the target objects, reducing theamount of data communicated over the network.

Only when the amount of data that it lters out is low (morethan 90% of data on the topic must be communicated), theextra overhead of the wireless transmissions of the mobileagent does not pay off. Even in that case, the difference isminimal.

Obviously, the query time with the remote approach isindependent of the percentage of relevant data, as all the dataon the topic must be transmitted anyway over the network(ltering is performed only on the client side).

It should be noted that, in the scenario presented in this section,different situations may arise. For example, an internal agent canmove from one moving object to another; when coming back(with the results retrieved) to its original moving object, thatobject may be under the coverage of a different proxy (andtherefore the computer hosting the mobile agent that representsit may be a different one). Moreover, an IA may also move due to arequest from its current MOA (an MOA must travel with its IAs),whether the IA is on its original moving object or visiting another.All these situations are managed transparently in the testingsystem, as explained in Section 4.

6.3. Evaluating real services using the testing system

In this section, the suitability of the proposed testing system is

puter Applications 32 (2009) 846865 861ilar to local access times, as wired communication costs areignicant with respect to the (much higher) wireless commu-ation costs (a GSM network is used).

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S. Ilarri et al. / Journal of Network and Computer Applications 32 (2009) 846865862The previous scenario has been simulated using three xedcomputers to represent the proxies in the USA, San Sebastian andZaragoza, respectively. The corresponding ProxyAgents simulatedifferent delays among them (delays between those threegeographical places using the xed network) and the wirelessnetwork delay with the user device. The user device wasrepresented as a mobile agent that implements a simulatedmoving object traveling through the three proxy areas. Themodules of the data service have been used with no signicant

Fig. 19. Locker rental service: differchange.9

In Fig. 19, the X-axis shows the user requests at each location(10 accesses to les of about 190KB) and the Y-axis shows theaccumulated time of interaction between the user device and thelocker (in seconds). It can be seen that the times are very similarwhether they are obtained by simulation or not. Thus, by usingthe testing system, the results are similar to those obtained byrunning the data service using real proxies in different countries.

6.3.2. Testing a location-dependent query service (LOQOMOTION)

LOQOMOTION (LOcation-dependent Queries On Moving ObjecTsIn mObile Networks) is a software architecture, based on mobileagents, whose goal is the processing of location-dependentqueries in distributed environments (Ilarri et al., 2006a), i.e.,queries for which the answer depends on the locations of objects.It deals with contexts where not only the user issuing the querycan change her/his position, but the objects involved in the querycan move as well. An example of location-dependent query is:show me the taxi cabs within two miles around my currentposition. These queries are considered as continuous queries, i.e.,queries whose answers must be updated continuously, asopposite to instantaneous queries in which only a single answeris obtained for a query.

In Fig. 20, it is shown the precision of LOQOMOTION whensimulating the scenario of Fig. 6 in different ways: (1) all theelements (proxies and objects) are simulated on the same xedcomputer (the proposed testing system is used on a single

9 With the exception that they use the Proxy Catalog of the simulated scenario.computer); (2) one xed computer is used to simulate each proxy(the proposed testing system is used in a distributed environmentwith six xed computers); and (3) an alternative to the proposedtesting system that tries to replicate a real scenario where eachelement is an independent device: no mobile agents and one xedcomputer for each element are used, i.e., six xed computer forproxies and eleven xed computers more for moving objects(represented as processes communicating with proxies usingremote calls). In these three simulations, each moving object

es due to the use of the simulator.comstoOb5 s

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woart

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LOQmunicates GPS location data to its proxy, and each proxyres GPS location data of moving objects under its coverage.jects move at 30mph and the answer is refreshed everyeconds.The results shown in Fig. 20 focus on the location of a certainving object retrieved by a query for a 1-minute test. Thecision of LOQOMOTION (in terms of location error) over timethe three simulations is very similar.10 Therefore similar resultsobtained when using the proposed testing system (avoidingng many hardware resources) with respect to using anroach closer to reality (independent devices and more hard-re cost).

Related work

In this section, other interesting works are reviewed. The threerks that are considered more related to the proposal in thisicle are the following:

The integrated mobile computing environment and simulationtestbed MCE (Rajagopalan et al., 1995) bears some similaritieswith the work presented in this article. Thus, it stresses theimportance of minimizing code changes during the evaluationof algorithms, protocols and applications, so that they can betested on the simulation testbed and then used directly on a

10 The imprecision incurred around time 25 seconds is due to a handoff ofmonitored moving object, which in this case decreases the accuracy of

OMOTION.

Inrel

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N for di

Computethe simulation of moving objects executing applications basedon mobile agents are not considered.In Satoh (2002, 2003), mobile agents move among access pointsin a wireless network to emulate the movements of mobiledevices. In this way, a mobile agent can use the serviceprovided by its access point in the same way that a mobiledevice would. However, the goal is not to simulate mobileenvironments, not even moving objects. Instead, packaging anapplication as a mobile agent that travels across access pointsis proposed to avoid having to move physically the mobiledevice to access a wireless data service. Moreover, the issuesthat arise when the application itself is based on (mobile)real network. A distributed simulation infrastructure is alsoconsidered. In particular, host mobility is simulated bycheckpointing and process migration in a UNIX environment,whereas in the proposal in this article mobile agents are usedin any heterogeneous environment with Java. Unfortunately,the approach presented in Rajagopalan et al. (1995) is notevaluated. Moreover, the focus is more on testing protocols andalgorithms (e.g., routing algorithms) than on the evaluation ofdata services for mobile computing. Therefore, issues such as17 computers, no agents

6 computers, agents1 computer, agents

010

Fig. 20. Precision of LOQOMOTIOS. Ilarri et al. / Journal of Network andagents are not studied.In Kubach et al. (2001), both the mobility and informationaccess patterns of mobile users are simulated, providing anenvironment for evaluating mechanisms to obtain location-dependent information. As opposed to the approach presentedin this article, only a certain behavior of the moving objects issimulated: their requests to query location-dependent infor-mation. Moreover, evaluating the real data service on asimulated environment is not considered.

the rest of this section, some other relevant works in threeated areas are mentioned:

Simulation of mobile networks. The goal of the proposed systemis to allow developers to test high-level data services and notto evaluate network protocols. Therefore, this work is ortho-gonal to those dealing with the simulation of wirelessnetworks at a protocol level (Zeng et al., 1998; Kojo et al.,2001; Samfat et al., 1995). In fact, such works could be used toimprove the simulation of the proposed system at that level ofdetail (e.g., as mentioned in Section 5.1, it is possible tointegrate it with ns-2). This category includes also works that,to evaluate network protocols, not only simulate mobile

Kaufman (2003) and Hu and Lee (2005). These works focusmainly on the simulation of moving objects as generators ofspatio-temporal data for performance evaluation, rathernetworks but also the movements of the users; for example,Samfat and Molva (1997).Simulation of moving objects. These works are also comple-mentary to the one presented in this article, as they can beused to simulate movements of objects in the proposedsystem: On the one hand, several works propose different mobilitymodels for moving objects (Schindelhauer, 2006; Campet al., 2002), such as random walk, for different contexts.Moving objects in the proposed testing system can beprogrammed with any wanted behavior. Therefore, they canconsider a predened trajectory or any other mobilitymodel (see Section 5.1).

On the other hand, and based on the previous models and/or other simulation requirements, several projects aim atgenerating location data for benchmarking purposes; e.g.Saglio and Moreira (2001), Theodoridis et al. (1999), Pfoserand Theodoridis (2003), Brinkhoff (2002), Myllymaki and

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r Applications 32 (2009) 846865 863than on testing the functionality of high-level data services.As opposed to these works, the system proposed in thisarticle allows the simulation of other behaviors of themoving objects (e.g., their interactions with proxies).Moreover, the possibility of mixing real and simulatedobjects is not considered in these works, and distributedsimulations are not supported either. Nevertheless, theseworks can also be considered complementary to theproposal in this article: They can be used to simulate thetrajectories followed by the moving objects in the testingsystem.of software agents in a simulated environment. Some

rks consider simulations to evaluate mobile agent systemset al., 2004; Liotta et al., 2002; Uhrmacher et al., 2000;rmacher and Kullick, 2000); these approaches allow tose the original agent code in the simulation, which isilar to the goal of reusing the original data service code inproposed testing system. In the context of multiagent

tems, there are also works that highlight the importance oftributed simulations (as in this article) to avoid overloadingingle centralized computer with simulation tasks (Uhrma-r et al., 2000; Uhrmacher and Kullick, 2000; Ewald et al.,06; Riley and Riley, 2003). Finally, it is interesting to indicatee works that also use mobile agents for simulation

en

Th

synchronization needed between the original application

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Comagents and the agents used in the simulation. The proposed testing system is able to hide the true nature

(real or simulated) of moving objects and proxies inthe wireless network. So, it allows developers to test a realdata service in (completely or partially) simulated wirelessenvironments.

A sample use case has been presented that illustrates how thesystem can be extended. Moreover, the system has been testedexperimentally regarding two different aspects. On the one hand,its main components have been evaluated: the simulationapproach based on mobile agents and the agent coordinationprotocol proposed to test applications based on mobile agents.These tests have proven that distributed simulations using mobileagents achieve the best precision and that the agent coordinationprotocol enables the use of the system to simulate moving objectsthat execute applications based on mobile agents. On the otherhand, the whole testing system has been evaluated by using it tosimulation and allows accessing (real) services not availableremotely.

Several computers can be used to simulate proxies and movingobjects easily, which allows to increase the parallelism andaccuracy of the simulation. The data service is not aware of thenumber of computers used.

It also allows the simulation of objects that execute multiagentapplications. A suitable coordination protocol provides thetesrellowsystem can be easily extended by the service tester toimplement moving objects and proxies suited to his/her needs(reusing the original code of the data service).Moving objects are simulated using mobile agents, and proxiesare simulated as execution environments for mobile agents(places). The use of mobile agents increases the accuracy of theeded), a system for testing these services has been developed.e main benets of the proposal presented in this article are:

It allows the developers of wireless applications to test theirservices with minimal changes in the application code. TheperneDue to the difculty of testing wireless data services in a realvironment (e.g., many real wireless devices managed by manysons within the coverage of a real wireless network could be8.ver that introduces virtual costs for service invocations.

Conclusionsserpurposes (Stone et al., 1996; Wilson et al., 2001). In Stone et al.(1996) mobile agents are used to reduce the network trafc(they interact with their local proxy) and distribute simulationtasks for multiplayer games over a network. In ABELS (Wilsonet al., 2001), mobile agents are used to link efcientlyindependent simulations that can be executing on differentcomputers (as opposed to the work in this article, whereinmobile agents are core elements of the simulations).

Finally, it is interesting to indicate a recent seminal work (Spieet al., 2008) whose goal is to evaluate the performance of anexecutable process description modeled with an extension of BPEL(Business Process Execution Language) proposed to take intoaccount situations where some services are offered by devices in aubiquitous environment. A straightforward simulation approach isproposed, where the devices are replaced with a web application

S. Ilarri et al. / Journal of Network and864t real data services, showing that the results of the tests areiable. Finally, the developed system could be integrated with a-level network simulator such as ns-2.References

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