Extending Network Life by Using Mobile Actors in Cluster-based Wireless Sensor and Actor Networks

Transcription

1 Extendng Networ Lfe by Usng Moble Actors n Cluster-based Wreless Sensor and Actor Networs Nauman Aslam, Wllam Phllps, Wllam Robertson and S. Svaumar Department of Engneerng Mathematcs & Internetworng Dalhouse Unversty, Halfax, Nova Scota, Canada, B3J2X4 {naslam, wllam.phllps, bll.robertson,svas}@dal.ca Abstract Wreless sensor actor networs (WSANs) consst of a large number of resourceconstraned nodes (sensors) and a small number of powerful resource rch nodes (actors). Ths paper nvestgates the case where sensors are organzed nto clusters and moble actors are used for mantanng an energy effcent topology by perodcally manpulatng ther geographcal poston. We present an elegant technque that allows actor nodes to fnd an optmal geographcal locaton wth respect to ther assocated cluster heads such that the overall energy consumpton s mnmzed. The smulaton results demonstrate that the technque proposed n ths paper sgnfcantly mnmzes energy consumpton and extends the networ lfetme compared wth tradtonal cluster-based WSN deployments. Keywords: wreless sensor actor networs, moble actors, energy conservaton, clusterng, networ lfetme. 1. Introducton Wreless Sensor Actor Networs (WSANs) have emerged as the-state-of-the-art technology n data gatherng from remote locatons by nteractng wth physcal phenomena and relyng on collaboratve efforts by few resource rch and a large number of low cost devces [1]. In WSANs sensors are low cost resource constraned devces that have lmted energy, communcaton and computaton capabltes. Once deployed, sensor nodes collect the nformaton of nterest from ther on board sensors, perform local processng of these data ncludng quantzaton and compresson, and forward the data to a base staton (BS) through a cluster head or gateway node. Actors, on the other hand are resource rch nodes that are equpped wth better possessng power, hgher transmsson powers, more energy resources and may contan addtonal capabltes such as moblty. Despte the tremendous technologcal advancements n the feld of wreless sensor networ (WSNs), energy conservaton s stll one of the fundamental challenges n WSN system desgn. In a sensor node, energy s consumed n sensng, computaton and processng, however the wreless transcever consumes a sgnfcant amount of energy as compared to all other sources. Most of the WSN deployments are based on Please use the followng format when ctng ths chapter: Aslam, N., et al., 2008, n IFIP Internatonal Federaton for Informaton Processng, Volume 264; Wreless Sensor and Actor Networs II; Al Mr; (Boston: Sprnger), pp

2 Lfe by Usng Moble Actors n Cluster-based Wreless Sensor and Actor Networs 75 an applcaton specfc data gatherng.e. a large number of sensor nodes send ther data to the BS. Clusterng s employed as a standard approach for achevng energy effcency and scalable performance n WSNs [2]. Sensor nodes can be organzed herarchcally by groupng them nto clusters, where the data s collected and processed locally at the cluster head. Sngle hop clusterng protocols follow two-ter archtecture, consstng of sensor nodes and cluster heads. In ter one communcaton, each sensor node sends t data to ts local cluster head. In ter two the cluster heads send the processed data to the BS. In protocols where the cluster head selecton s performed randomly, the dstrbuton of cluster heads s not well controlled. Most protocols based on the two-ter archtecture use drect communcaton for sendng cluster head data to the BS. If a cluster head s located far away from the sn, ts energy consumpton would be greater than the one located closer to the sn for smlar traffc patterns from cluster members. Intutvely, deployment of resource rch actor nodes could off set some of the hgher transmsson costs ncurred by cluster heads n data transmsson to the BS. In such a case, each elected cluster head wll assocate wth an actor node whch wll forward cluster head s data to the BS. Snce random clusterng nvolves electon of new cluster heads n each round therefore deployment of statc actor nodes wll not be energy effcent. Such neffcency may result from varatons n dstances between the actor node and ts assocated cluster heads and ther respectve resdual energy levels. The concept of moblty solves ths dlemma by effectve locaton management of actor nodes thus provdng the ablty for enhanced energy savngs. Fgure 1 llustrates a scenaro where moble actors are used for relayng cluster head s data to the BS. Ths paper proposes a methodology where moble actor nodes relocate to a new locaton n each round such that the energy consumed by assocated cluster heads s mnmzed. A weghted cost based functon that accounts for the resdual energy levels of assocated cluster heads s used to fnd the new of locaton the actor nodes. Fgure 1: Example of Sensor Feld wth Cluster Heads and Moble Actors

3 76 N. Aslam et al. The concept of moble nodes n WSN s relatvely new and has been proposed n varous contexts. One of the earlest proposed researches was concept of data mules [3]. Data mules are referred as moble enttes that collect data from sensors as they pass by the sensor nodes buffer the collected data and drop the data off to the wred access pont. Data mules were prmarly amed at collecton of non real-tme data. Authors n [4] used moble nodes for data collecton n sparsely connected networ. Frst nodes are parttoned nto several groups wth respect to ther data generaton rates and locaton. Then the schedule for each group s created usng the travelng salesman problem such that there s no data loss or overflow. Message Ferryng s ntroduced n [5, 6] where a set moble nodes (message ferres) provde communcaton servce for nodes n sparse connected networs. In the message ferry approach, each node communcates only wth the message ferres, therefore all communcaton cost s reduced to a short dstance sngle hop communcaton at the expense of ncreased message delay. Ths paper adopts a rather smple approach by explotng the actor moblty to acheve balanced energy dsspaton by cluster heads. The rest of the paper s organzed as follows. Secton 2 presents system model and assumptons. Secton 3 presents detals of our proposed technque for locaton management of moble actors. Smulaton results are dscussed n Secton 4. Fnally, our man conclusons and future research drectons are hghlghted n Secton System Model and Assumptons We mae the followng assumpton for our sensor networ: 1. The networ conssts of a large number of resource constraned nodes (referred as sensng nodes) and small number of resource rch nodes (referred as actor nodes) wth enhanced communcaton capacty, ablty to recharge ther energy source and moblty. 2. We assume that all nodes have means to fnd out ther geographcal poston. 3. The nodes are dspersed n a 2-dmensonal space followng a unform dstrbuton. 4. The base staton s located outsde the deployment regon and has no energy constrants. 5. Once deployed the sensor nodes reman statonary and contnue to operate untl they completely exhaust ther batteres. 6. We use the energy model presented n [7]. In ths model the energy requred to run the transmtter or recever electronc crcutry Eelect s 50 pj/bt and ε amp the energy to run transmtter amplfer s 100 pj/bt/m 2. The energy expended by a node n transmttng an l-bt message a dstance of d s gven by, E le lε d 2 = + Tx elect amp (1) The energy expended n recevng an l-bt message s gven by, E = le (2) Rx elect

4 Lfe by Usng Moble Actors n Cluster-based Wreless Sensor and Actor Networs Locaton Management of Moble Actors In ths secton we present our ratonale and algorthm for locaton management of moble actors. The presented technque s based on the motvaton that all cluster heads should have a balanced energy dsspaton rate. Ths objectve s acheved by ntroducng a cost functon that allows the actor nodes to adjust ther geographcal locaton n such a manner that the overall energy of assocated cluster heads s mnmzed. To further elaborate, let s consder an example llustrated n Fgure 2. Four cluster head A, B, C and D are assocated wth an actor node. The locaton of each node s shown below n 'x' and 'y' coordnates. Fgure 2a shows the ntal locaton of actor node and ts assocated cluster heads. Snce dstance from each cluster clusters head to the actor node s dfferent, t s mpossble have comparable energy consumpton for all cluster heads. Intutvely, fndng a centrod as llustrated n Fgure 2b wth respect all cluster head locatons would provde a poston where actor node s at the mnmum dstance from each cluster head. However, fndng a centrod would stll leads to uneven energy consumpton rate f the resdual energy levels of all assocated cluster heads are dfferent. To account for the resdual energy levels we ntroduce a weghted cost based functons that allows actors node to fne tune ts locaton based on the resdual energy level. Fgure 2b also shows the actor node s updated locaton where t moves closer to the node A that has mnmum resdual energy. Now we descrbe calculaton of cost factor. Table 1 outlnes the notaton used n the followng sectons. A A (10,90) Er = 0.1 J D (70,60) Er = 0.3 J Locaton after fne-tunng (10,90) Er = 0.1 J (34,60) D (70,60) Er = 0.3 J B (15,30) Er = 0.3 J Actor Node (50,10) (a) Intal node locatons C (80,10) Er = 0.4 J Locaton based on Centrod B (15,30) Er = 0.3 J (47,43) C (80,10) Er = 0.4 J (b) Locaton of actor node based on cost functon Fgure 2: Example llustratng locaton management of actor node usng weghted cost functon

5 78 N. Aslam et al. Table 1: Notatons Used Set of actor nodes th node n A A a Target locaton coordnates of a ( X, Y ) Set of cluster heads th node n CH CH Locaton of th cluster head (x, y ) Resdual energy of th cluster head e Cost factor for th cluster head We assume that cluster heads are elected usng any generc sngle hop clusterng protocol such as [2, 8, 9]. Once elected, each cluster head wll lsten for the Actor Advertsement Messages (sent by actor nodes). Each cluster head then fnds a least cost actor node and sends an Assocaton Message to the selected actor node ndcatng ts locaton coordnates and resdual energy level. Upon recevng the Assocaton Message the actor node nows that t wll forward cluster heads data to the BS. Once the actor node receves Assocaton Message from all t calculates the cost factor for each cluster head as follows, ch ε Where, ( emax e ) + e ε = ( e e ) + e max mn max 2 2 mn 2 2 mn e = max( e ) for CH e = mn( e ) for CH (3) The calculated cost factors are used to fnd the new locaton coordnates Y from equaton (4) and (5) respectvely. X and d dx d dy 2 ε ( x x ) = 0 (4) 2 ε ( y y ) = 0 (5) Based on the dscusson related to Fgure 2 above, each moble actor node utlzes the cost functon to optmally adjust ts coordnates. Ths s done so by tang nto account the resdual energy levels of ts assocated cluster heads and ther

6 Lfe by Usng Moble Actors n Cluster-based Wreless Sensor and Actor Networs 79 communcaton cost to the actor node. The pseudo code of the steps used for locaton management n cluster based envronments s outlned n Algorthm-I. Algorthm-I: Begn ch CH 1. Receve Actor Advertsement Messages 2. Fnd least cost Actor Node 3. Send Assocaton Message a A 4. Send Actor Advertsement Message 5. Whle Tmer T s vald 6. Receve Assocaton Messages from cluster heads 7. end Whle 8. Calculate weghted energy cost ε for all assocated cluster heads 9. Calculate new X and Y 10. Move to new poston (X, Y ) End 4. Smulaton Results Ths secton presents the performance analyss of our proposed technque. We assume deal condtons at the physcal level such that the probablty of wreless nterference s neglgble. The networ smulaton model was bult usng MATLAB. We used the clusterng algorthm presented n [8]. We also assume that the networ conssts of a small number of actor nodes (ranges from 1% to 3.5% of the networ sze). The smulaton proceeds n data collecton rounds. Snce each data collecton round conssts of cluster set up and actor node selecton phase. In each round, each non cluster head node sends one data pacet to ts cluster head. The cluster head performs n-networ processng by aggregatng the data receved from ts member nodes and forwards ths data to the actor node. Fnally, the actor node forwards data receved from ts assocated cluster head to the BS. We also assume that the actor nodes do not transmt any data whle they are movng from one locaton to another. Moblty feature s only used to optmze the energy conservaton. Two performance metrcs are used. The frst one s denoted by networ lfetme as used n [2, 8, 9]. Ths metrc represents networ lfetme n data collecton rounds from the nstant the networ s deployed to the moment when the frst node runs out of energy. The second performance metrc s the average cluster head energy consumed per round. Table 2 summarzes the mportant smulaton parameters used. It can be observed from Fgure 3 and 4 that by usng moble actors the networ lfetme s ncreased n more than 100% as compared to the drect transmsson case. It s also worth notcng the sharp slope for moble actor s case whch s a result of balanced energy dsspaton n the sensor nodes.

7 80 N. Aslam et al. Table 2: Smulaton Parameters Sensor Deployment Area BS Locaton 100 x 100 m (50,175) m Number of Nodes (N) 200, 400 Data Pacet Sze Control Pacet Sze Intal Energy of Actors Intal Energy of Sensors Number of Actor Nodes 4000 bts 25 bytes 20 J 0.5 J 1 % to 3.5 % of N E 50 n J/bt elect ε 100 pj/bt/m 2 mp Number of Actve Nodes Drect Transmsson Case Moble Actors Case Number of Data Collecton Rounds Fgure 3: Networ lfe (200 Nodes Networ)

8 Lfe by Usng Moble Actors n Cluster-based Wreless Sensor and Actor Networs 81 Number of Actve Nodes Drect Transmsson Case Actor Node Case Number of Data Collecton Rounds Fgure 4: Networ lfe (400 Nodes Networ) We also nvestgated the optmal number of actor nodes for the two networ szes of 200 and 400 nodes. Fgure 5 and 6 show how networ lfe s effected as a result of varyng the number of actor nodes. The x-axs denotes the number of moble actors n the system and the y-axs denotes the number of rounds when the frst node runs out of ts energy. The number of moble actors s vared from 1% of networ sze to 3.5% of the networ sze. It can be observed from both Fgure 5 and 6 that a value of 2% (4 n 200 nodes networ and 8 n 400 nodes networ) provdes the best results. When the number of moble actors s 1% of the networ sze the number of assocated cluster heads per actor node s large. Hence t may lead to stuatons where the resultant topology cannot be further optmzed. Therefore addng a small number of actor nodes further ncreases energy savngs resultng n enhanced networ lfetme. Conversely, a relatvely large number of actor nodes wll results n more overhead messages. Hence t s desrable to have a mnmum number of actor nodes that provde optmal coverage per cluster head. From the smulatons we conclude that the best value for networ lfe tme s gven by when the number of moble actors s 2% of the networ sze.

9 82 N. Aslam et al. 800 Number of rounds to frst node death Nodes Scenaro Number of actor nodes Fgure 5: No. of actors vs. networ lfe n frst node death (200 Nodes Networs) Number of rounds to frst node death Nodes scenaro Number of actor nodes Fgure 6: No. of actors vs. networ lfe n frst node death (400 Nodes Networs) Fgure 7 llustrates the mean cluster head energy consumed per round for moble actors and drect communcaton scenaro. The statstcs are collected usng 100 ndependent runs. It s clearly observed that the cluster heads n former case consume far less energy (almost 50%) because of shorter dstance transmsson to the moble

10 Lfe by Usng Moble Actors n Cluster-based Wreless Sensor and Actor Networs 83 actors as compare to the drect communcaton case. Hence the overall effect n networ lfe tme gan s qute sgnfcant Average cluster head energy consumed per round (J) Average cluster head enegry consumed per round (J) Average CH energy consumed/round (J) Average CH energy consumed/round (J) Fgure 7: Hstogram for average cluster head energy consumed per round (J). (a) 400 Nodes Drect communcaton case (b) 400 Nodes wth moble actors (c) 200 nodes wth drect communcaton case (d) 200 Nodes wth moble actors. 5. Conclusons and Future Wor In ths paper we have presented a smple and elegant technque to use moble actor nodes for extendng the networ lfe tme. We have shown that by deployng a small number of resource rch moble nodes the networ lfe tme can enhance n excess of 100% as compared to sngle hop clusterng. We presented a weghted cost based functon that allows moble actors to fne tune ther locaton by consderng the resdual energy levels of assocated cluster heads. We ntend to nvestgate effects of dfferent moblty models n our future research.

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