A Coverage-Preserving Node Scheduling Scheme for Large Wireless Sensor Networks

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1 A Coverage-Preservng Node Schedulng Scheme for Large Wreless Sensor Networks D Tan Multmeda Communcatons Research Laboratory Unversty of Ottawa 800 Kng Edward Avenue x2146 dtan@ste.uottawa.ca Ncolas D. Georganas Multmeda Communcatons Research Laboratory Unversty of Ottawa 800 kng Edward Avenue x6225 georganas@dscover.uottawa.cal ABSTRACT In wreless sensor networks that consst of a large number of lowpower, short-lved, unrelable sensors, one of the man desgn challenges s to obtan long system lfetme, as well as mantan suffcent sensng coverage and relablty. In ths paper, we propose a node-schedulng scheme, whch can reduce system overall energy consumpton, therefore ncreasng system lfetme, by turnng off some redundant nodes. Our coverage-based off-duty elgblty rule and backoff-based node-schedulng scheme guarantees that the orgnal sensng coverage s mantaned after turnng off redundant nodes. We mplement our proposed scheme n NS-2 as an extenson of the LEACH protocol. We compare the energy consumpton of LEACH wth and wthout the extenson and analyze the effectveness of our scheme n terms of energy savng. Smulaton results show that our scheme can preserve the system coverage to the maxmum extent. In addton, after the node-schedulng scheme turns off some nodes, certan redundancy s stll guaranteed, whch we beleve can provde enough sensng relablty n many applcatons. Categores and Subect Descrptors F.2.2 [Nonnumercal Algorthms and Problems]: Geometrcal problems and computatons Routng and layout, network problems. General Terms Algorthms, Measurement, Performance, Relablty, Expermentaton, Theory. Keywords Coverage, Node Schedulng, Wreless Sensor Networks. Permsson to make dgtal or hard copes of all or part of ths work for personal or classroom use s granted wthout fee provded that copes are not made or dstrbuted for proft or commercal advantage and that copes bear ths notce and the full ctaton on the frst page. To copy otherwse, or republsh, to post on servers or to redstrbute to lsts, requres pror specfc permsson and/or a fee. WSNA 02, September 28, 2002, Atlanta, Georga. Copyrght 2002 ACM /02/ $ INTRODUCTION Recently, the dea of wreless sensor networks has attracted a great deal of research attenton due to wde-ranged potental applcatons that wll be enabled by wreless sensor networks, such as battlefeld survellance, machne falure dagnoss, bologcal detecton, home securty, smart spaces, nventory trackng, etc. [1-4]. A wreless sensor network conssts of tny sensng devces, deployed n a regon of nterest. Each devce has processng and wreless communcaton capabltes, whch enable t to gather nformaton from the envronment and to generate and delver report messages to the remote base staton (remote user). The base staton aggregates and analyzes the report messages receved and decdes whether there s an unusual or concerned event occurrence n the deployed area. Consderng the lmted capabltes and vulnerable nature of an ndvdual sensor, a wreless sensor network has a large number of sensors deployed n hgh densty (hgh up to 20nodes/m 3 [5]) and thus redundancy can be exploted to ncrease data accuracy and system relablty. In wreless sensor networks, energy source provded for sensors s usually battery power, whch has not yet reached the stage for sensors to operate for a long tme wthout rechargng. Moreover, sensors are often ntended to be deployed n remote or hostle envronment, such as a battlefeld or desert; t s undesrable or mpossble to recharge or replace the battery power of all the sensors. However, long system lfetme s expected by many montorng applcatons. The system lfetme, whch s measured by the tme untl all nodes have been draned out of ther battery power or the network no longer provdes an acceptable event detecton rato, drectly affects network usefulness. Therefore, energy effcent desgn for extendng system lfetme wthout sacrfcng system relablty s one mportant challenge to the desgn of a large wreless sensor network. In wreless sensor networks, all nodes share common sensng tasks. Ths mples that not all sensors are requred to perform the sensng task durng the whole system lfetme. Turnng off some nodes does not affect the overall system functon as long as there are enough workng nodes to assure t. Therefore, f we can schedule sensors to work alternatvely, the system lfetme can be prolonged correspondngly;.e. the system lfetme s prolonged by explotng redundancy. In ths work, we present a novel node-schedulng scheme, whch s used to confgure node work status and schedule Ths work was supported n part by the SENSE Proect, Communcatons and Informaton Technology Ontaro (CITO), a Center of Excellence. 32

2 the sensor on-duty tme n large sensor networks. Our desgn has been drven by the followng requrements: frst, because t s nconvenent or mpossble to manually confgure sensors after they have been deployed n hostle or remote workng envronments, selfconfguraton s mandated. Second, the desgn has to be fully dstrbuted and localzed, because a centralzed algorthm needs global synchronzaton overhead and s not scalable to largepopulated networks. Thrd, the algorthm should allow as many nodes as possble to be turned off n most of the tme. At the same tme, t should preserve the ntal sensng coverage wth mnmum sensng hole, or blnd ponts. It s deal f the workng nodes can cover the same montored area as the orgnal one. Fourth, the schedulng scheme should be able to mantan the system relablty,.e., certan redundancy s stll needed. In the proposed approach, each node n the network autonomously and perodcally makes decsons on whether to turn on or turn off tself only usng local neghbor nformaton. To preserve sensng coverage, a node decdes to turn t off when t dscovers that ts neghbors (sponsors) can help t to montor ts whole workng area. To avod blnd pont, whch may appear when two neghborng nodes expect each other s sponsorng, a backoff-based scheme s ntroduced to let each node delay ts decson wth a random perod of tme. In ths work, we mplement the proposed scheme as an extenson of the exstng data gatherng protocol, LEACH [10]. We compare the energy consumpton wth and wthout the extenson and analyze the effectveness of our algorthm n terms of energy savng. Our smulaton results show that the system lfetme n the extended LEACH can be prolonged n the energy model nherted from LEACH, wthout reducng the overall sensng coverage, as well as mantanng certan system relablty. The rest of ths paper s organzed as follows: secton 2 revews the related work n the lterature. In secton 3, we ntroduce the detals of the proposed scheme. Secton 4 dscusses mplementaton detals and presents the smulaton results. Secton 5 concludes the paper. 2. RELATED WORK Mnmzng energy consumpton and maxmzng the system lfetme has been a maor desgn goal for wreless sensor networks. In the last few years, researchers are actvely explorng advanced power conservaton approaches for wreless sensor networks. On the one hand, devce manufacturers have been strvng for low power consumpton n ther products. In [6-7], low power transcever archtectures and low power sgnal processng systems are dscussed separately. In [8], an energy-scavengng technque, whch enables self-powered nodes usng energy extracted from the envronment, s presented. In [9], a low power data converter, sgnal processng, RF communcaton crcuts are ntegrated nto one chp. On the other hand, protocol desgners are seekng an energy effcent communcaton archtecture, whch nvolves all levels from the physcal layer to the applcaton layer [4]. For nstance, drected dffuson [11] and LEACH [10] are two typcal data communcaton protocols proposed for wreless sensor networks. In drected dffuson, routes (called gradents) that lnk sources of nterestng data to snks are formed when nterest s dssemnated throughout the network. When the source has data of nterest, t sends the data along the gradent paths back to the snks. Energy s saved by renforcement-based adaptaton to the emprcally best path, cachng and n-network data aggregaton. LEACH s a clusterng-based protocol that utlzes a randomzed rotaton of a local cluster-head to evenly dstrbute the energy load among sensors n the network. It also uses localzed coordnaton to enable scalablty and robustness for dynamc networks and ncorporates data fuson nto the routng protocols to acheve energy conservaton. Our work s dedcated to schedulng nodes by usng applcaton knowledge (.e., t belongs to one branch of applcaton layer protocols accordng to the categores n [4]) and does not address the data communcaton problem. It can be mplemented as the extenson of any energy effcent data communcaton protocol n wreless sensor networks. In [12], a probng-based densty control algorthm s proposed to ensure long-lved, robust sensng coverage by leveragng unconstraned network scale. In ths protocol, only a subset of nodes are mantaned n workng mode to ensure desred sensng coverage, and other redundant nodes are allowed to fall asleep most of the tme. Workng nodes contnue workng untl they run out of ther energy or are destroyed. A sleepng node wakes up occasonally to probe ts local neghborhood and starts workng only f there s no workng node wthn ts probng range. Geometry knowledge s used to derve the relatonshp between probng range and redundancy. In ths algorthm, desred redundancy can be obtaned by choosng the correspondng probng range. However, ths dervaton s based on the assumpton that all the nodes have exactly the same sensng range. It s hard to fnd a relatonshp between probng range and desred redundancy, f nodes have dfferent sensng ranges. Furthermore, the probng-based off-duty elgblty rule can not ensure the orgnal sensng coverage and blnd ponts may appear after turnng off some nodes, whch s verfed n our experment. In [13], Chen et al. proposed an algorthm to turn off nodes based on the necessty for neghbor connectvty. They ntend to reduce the system energy consumpton wthout sgnfcantly dmnshng the connectvty of the network. In [14], Xu et al. proposed a scheme n whch energy s conserved by lettng nodes turn off ther communcaton unt when they are not nvolved nto sendng, forwardng or recevng data phase. Also, node densty s leveraged to ncrease the tme that communcaton unt s powered off. In [15], an algorthm, called Geographcal Adaptve Fdelty (GAF) was proposed, whch uses geographc locaton nformaton to dvde the area nto fxed square grds. Wthn each grd, t keeps only one node stayng awake to forward packets. These three node-schedulng schemes turn off nodes from communcaton perspectve wthout consderng the system s sensng coverage. In fact, n wreless sensor networks, the man role of each node s sensng. Unusual event could happen at any tme at any place. Therefore, f we only turn off nodes, whch are not partcpatng n data forwardng, certan areas n the deployng area may become blnd ponts. Important events may not be detected [12]. Besdes dmnshng the number of actve nodes, there are other network topology control technques, whch also ntend to ncrease power effcency and extend network lfetme. [16-18] produces a mnmum-energy communcaton subnetwork by adustng transmsson power. The subnetwork s computed dstrbutedly at each node usng local neghbor locaton nformaton [16-17] or drectonal nformaton [18]. Instead of controllng the transmsson power level, node-schedulng schemes power off some redundant nodes n the network and therefore can acheve further energy conservaton. 33

3 P 1 N θ φ N k P 2 N k l N l S (b) S (d) S (a) ( φ and θ (c) 3. NODE SELF-SCHEDULING ALGORITHM Generally, the node-schedulng problem s composed of two subproblems. Frst, what s the rule that each node should follow to determne whether t should turn tself off or not? Second, when should nodes make such decson? In ths secton, we wll descrbe our node-schedulng scheme n these two aspects, respectvely. 3.1 Coverage-based Off-duty Elgblty Rule Sponsored Coverage Calculaton basc model As dscussed above, the man obectve of ths algorthm s to mnmze the number of workng nodes, as well as mantan the orgnal sensng coverage. To acheve ths goal, we calculate each node s sensng area and then compare t wth ts neghbors. If the whole sensng area of a node s fully embraced by the unon set of ts neghbors,.e. neghborng nodes can cover the current node s sensng area, ths node can be turned off wthout reducng the system overall sensng coverage. In ths secton, we wll descrbe how a node determnes that ts neghbors can cover ts sensng area gven ther locaton nformaton. To smplfy the problem, we assume that all nodes have the same sensng range and each node knows ts sensng range r. In the later dscusson, we wll show how to modfy ths smplfed model f nodes have dfferent sensng ranges. A node s sensng area s a crcle centered at ths node wth radus r, f all nodes le on a 2-dmensonal plane. The scheme we wll descrbe s also applcable to a 3- dmensonal space. We denote node s sensng area as. To facltate the calculaton, we only consder the neghbors whose dstance from the current node s equal to or less than the sensng range r as shown n defnton 1. Defnton 1: Neghbor. The neghbor set of node s defned as N ( = { n ℵ d(, r, n } where ℵ s node set n the deployment regon, d(, denotes the dstance between node and node. Thus, for node, the off-duty elgblty rule can be expressed as (,.e., the unon of ts neghbors sensng areas s a superset of node s sensng area. The expresson s equvalent to ( ) (. By observaton, we know that the crescentshaped ntersecton n Fgure 1(a) ncludes a sector as llustrated n Fgure 1(b). Although the area of the sector s smaller than that of the crescent, t s much easer to calculate the area of the sector rather than that of the crescent, because the area of a sector can be represented by ts central angle accurately and untng S Fgure 1: Sponsored Coverage Calculaton-Basc Model k S (e) ( two sectors s equvalent to mergng two central angles. Therefore, although node can cover a crescent-shaped regon wthn node s sensng area (Fgure 1(a) shadow regon), node wll only admt that node can help t montor a sector-shaped regon (Fgure 1(b) shadow regon) f node s turned off. To help the further descrpton, we defne ths sector as a sponsored sector. Defnton 2: Sponsored sector. Suppose nodes and are neghbors, and both sensng areas and touch at pont P 1 and P 2. As llustrated n Fgure 1(b), the sector, bounded by radus P 1, radus P 2 and nner arc P 1 P 2, s defned as the sponsored sector by node to node, and s denoted as S. The central angle of the sector s denoted as θ. The drecton of node referred to node s denoted as φ. Lemma 1: If S, then ( ). ( Proof : ( ) S ( ( ( ) ( ( ). S ( > Lemma 1 ensures that nvestgatng whether the neghbors can cover the current node s sensng area s equvalent to checkng whether the unon of sponsored sectors (called sponsored coverage) contans the current node s sensng area, whch n turn, s equvalent to calculatng whether the unon of central angles can cover the whole 360 o as llustrated n Fgure 1(e). If the condton S s satsfed, we call the ( neghborng nodes are off-duty sponsors of node. From geometry calculaton, the central angle s gven as d(, θ = 2 arccos. Snce 0 < d(, r, t s easy to know 2 r that the range of the central angle s 120 θ <180. Obvously, a node must have at least three neghbors to get a chance to be turned off Sponsored Coverage Calculaton extenson model In the ntal dscusson, we assume that each node has the same sensng range r, and each node knows ts geographcal locaton. In ths part, we wll extend the basc model and provde soluton for the cases that nodes can obtan neghborng nodes drectonal nformaton from ncomng sgnals or nodes have dfferent sensng ranges. 34

4 A. Explotng drecton nformaton In ths case, we stll assume that every node has the same sensng range. As llustrated n the Fgure 1(b-d), n order to merge two central angles, we need to calculate the magntude and the drecton θ φ of each central angle (.e. and ) frst. It s easy to get that d(, θ = 2 arccos and y y. φ > = arctg 2 r x x Obvously, node locaton nformaton s needed for calculatng the θ φ actual values of and. However, we have known that the θ range of s [120,180 ). If we take the lower bound, 120, as the safe value, t s not necessary to calculate the actual value for θ φ. Now, the problem s how to know. Technques to φ estmate drecton,.e. from ncomng sgnals have already been dscussed n the IEEE antennas and propagaton communty as the Angle-Of Arrval (AOA) problem. Ths can be accomplshed by usng more than one drectonal antenna, as mentoned n [11]. If the rado communcaton unt n a sensor node has such drectonal nformaton estmaton capablty, the above model can stll be used θ by smply takng as 120, even wthout knowng node locaton nformaton. In [12], Ye et al. beleve that avalablty of drectonal nformaton from drectonal antennas s not currently practcal n wreless sensor networks. We don t ntend to dscuss the avalablty problem here and ust present ths extended model, as one possble soluton. B. Dfferent sensng range In ths case, nodes have dfferent sensng ranges, whch may be caused by two reasons. Frst, nodes have dfferent ntal sensng ranges. Second, a node s sensng range s changed durng ts lfetme. For nstance, the power level may have an mpact on the sensng range. Node s and ts neghbor node s current sensng range are denoted as r and r respectvely. There are many dfferent cases how the nodes and ther neghbors are located. For nstance, Fgure 2 presents four of them. In order to stll be able usng central angles to calculate sponsored coverage, here, we only consder two cases as shown n Fgure 3 (a-b). (c) N S (b) (a) ( ( ( ) ( )) S S S > (d) S > ( ) Fgure 2: layout of neghborng nodes-extended Model (2) d ( a) r θ d r b ) S > ( Fgure 3: two consdered cases-extended Model (2) Case 1: node s sensng area completely contans node s sensng r + d (, ) r area, whch happens whenever holds. In ths case, node can be turned off wthout further calculaton. Case 2: The sensng areas of both nodes touch at two ponts, and the ntersecton area ncludes a sector centralzed at node. Ths case d(, r r r d (, happens whenever both and are true. In ths case, the central angle s ( d (, + r ) = = r θ 2 θ 2 arccos 2 r d (, In summary, when nodes have dfferent sensng ranges, a node s neghbor set defnton s modfed as N ( = { n ℵ ( d(, r r r d(, ) ( r + d(, r ), n } Obvously, the basc model descrbed prevously s a specal case of ths extenson when r =r =r. 3.2 Node Schedulng Scheme Based On Elgblty Rule In our schedulng scheme, the operaton s dvded nto rounds. Each round begns wth a self-schedulng phase, followed by a sensng phase. In the self-schedulng phase, nodes nvestgate the off-duty elgblty rule descrbed n the prevous secton. Elgble nodes turn off ther communcaton unt and sensng unt to save energy. Nonelgble nodes perform sensng tasks durng the sensng phase. To mnmze the energy consumed n the self-schedulng phase, the sensng phase should be long compared to the self-schedulng phase. How on-duty nodes collect data and communcate s the ssue of the data gatherng protocols and s out of the scope of ths paper. The self-schedulng phase conssts of two steps. Frst, each node advertses ts poston and lstens to advertsement messages from other nodes to obtan neghborng nodes poston nformaton. Second, each node calculates a neghbor sponsorng sensng area, compares t wth ts own and decdes whether t s elgble for offduty or not. The detals of these two steps are ntroduced as follows Neghbor Informaton Obtanng Step To obtan neghbor node nformaton, a smple approach s that each node broadcasts a Poston Advertsement Message (PAM), whch contans node ID and ts current locaton, at the begnnng of each round. Because only neghbors wthn a node s sensng range are consdered n the elgblty rule, n order to mnmze energy consumpton, each node transmts PAM wth the mnmum power as long as t reaches ts sensng range. Such transmsson power control scheme ensures that only nodes wthn the transmtter s sensng range can receve ts PAM. If nodes have dfferent sensng ranges, PAM should also nclude the current sensng range of the transmtter as well. 35

5 3.2.2 Back-off Based Self-schedulng Step After fnshng the collecton of neghbor nformaton, each node evaluates ts elgblty for turnng off by calculatng the sponsored coverage, as descrbed n the prevous secton. However, f all nodes make decsons smultaneously, blnd ponts may appear, as shown n Fgure 4. Node 1 fnds ts sensng area can be covered by node 2,3 and 4. Accordng to the off-duty elgblty rule, node 1 turns tself off. Whle at the same tme, node 4 also fnd ts sensng area can be covered by node 1,5 and 6. Belevng node 1 wll keep workng, node 4 turns tself off too. Thus, a blnd pont occurs after turnng off both node 1 and node 4, as n Fgure 4(d) (a) orgnal sensng area covered by node1-6 3 (c) node 4 turns tself off by the on-duty elgblty rule (b) node 1 turns tself off by the on-duty elgblty rule (d) blnd pont appears blnd pont Fgure 4: Blnd Pont Occurrence To avod such a problem, we ntroduce a back-off scheme. We let each node start ts determnaton after a random back-off tme perod T d and broadcast a Status Advertsement Message (SAM) to announce ts status, f t s elgble for turnng off. Neghborng nodes recevng a SAM wll delete the sender s nformaton from ther neghbor lsts. Thus, the nodes that have a longer backoff delay wll not consder the nodes that have decded to be turned off before. Assumng W s the sze of random back-off tme choce, the probablty of node 1 and node 4 selectng the same random number s 1/W. Although a large W can reduce the probablty to a suffcent small value, there s stll a chance that node 1 and node 4 could select the same random number. To avod a blnd pont further, we let each node wat for a short perod tme T w after sendng the SAM out, f t s elgble for turnng off, nstead of turnng off ts communcaton unt mmedately. Ths ready-tooff perod should be enough for node 1 to receve SAM from node 4, or vce verse. If one SAM s receved durng the ready-to-off perod and the transmtter s one of ts off-duty sponsors, the node wll renvestgate ts off-duty elgblty. If the elgblty doesn t hold any more, the node returns t status from ready-to-off to on-duty. Otherwse, the node turns tself off after T w. The nodes, whch have decded to serve as on-duty ones, don t re-evaluate ther offduty elgblty once the decson has been made. The status transton graph s shown n Fgure 5. nelgble on-duty ready-tooff ready-onduty After Td/ nvestgate the rule next round s comng receve SAM/ nelgble elgble off-duty After Tw Fgure 5: FSM for self-schedulng phase 4. PERFORMANCE EVALUATION AND SIMULATION In ths secton, we present some expermental and smulaton results as the performance evaluaton of our algorthm. 4.1 Performance Evaluaton of the elgblty rule Frst, we evaluate the performance of coverage-based off-duty elgblty rule by expermental results Comparson wth probng-based off-duty elgblty rule In [12], another rule s proposed to determne f a node can turn tself off or not. The basc dea s: f the node detects that there s a workng node wthn ts probng range, t wll turn tself off. The probng range s a confgurable parameter correspondng to redundancy desred by the user. Ths probng-based off-duty elgblty rule s smple, and does not need node geographcal locaton. However, t cannot preserve the orgnal sensng coverage. Furthermore, t reles on the assumpton that every node has the exactly same sensng range to keep the relatonshp between probng range and desred redundancy. To compare t wth our coverage-based off-duty elgblty rule, we carry out some experments n statc networks. We deploy 100 nodes n a square space (50m by 50m). Nodes x- and y-coordnates are set randomly. Each node has a sensng range of 10 meters and knows who are ts neghbors and where the neghbors are located. We let each node decde whether to turn t off or not n a random sequence. The decson of each node s vsble to all the other nodes. The nodes, whch make decsons later, cannot see the nodes that have been turned off before. After all nodes have make decsons, the number of off duty nodes s counted and the current sensng coverage by onduty nodes s compared wth the orgnal one where all nodes are actve. To calculate sensng coverage, we dvde the space nto 1m 1m unt cells. We assume an event occurs n each cell, wth the event source located at the center of the cell. We nvestgate how many orgnal nodes and how many on-duty nodes can detect every event. If an event cannot be detected by any on-duty node, but s wthn the range of the orgnal sensng coverage, we call the event source cell a blnd pont. The occurrence of blnd ponts means that the correspondng off-duty elgblty rule cannot preserve the orgnal sensng coverage. We also compute the average sensng degree before and after turnng off nodes. Table 1 shows the expermental results when we apply the coverage-based and probngbased [12] off-duty elgblty rules n 100 random topologes, respectvely. 36

6 As we can see, by applyng our coverage-based off-duty elgblty rule, 53 nodes can be turned off on the average. The sensng degree s reduced from 10 to 4. No blnd pont appears n any topology after turnng off some nodes. The probng-based off-duty elgblty rule makes almost the same number of nodes be turned off as ours when Elgblty Rules Probng range Number of off-duty nodes Table 1: Comparson of two off-duty elgblty rules Orgnal sensng degree the probng range s set as 4 meters. However, blnd ponts appear n 26 topologes n that case. Another observaton s that larger probng range results n more nodes beng turned off and more sensng coverage beng reduced when the probng-based off-duty elgblty rule s used. Obtaned Sensng Degree Number of topologes wth blnd ponts Average number of blnd ponts per topology Proposed N/A N/A < [12] On-duty node number vs. node densty We change node densty by varyng the sensng range from 6 to 13 and the deployed node number from 100 to 300 n the same 50m 50m deployed area. Fgure 6 shows a 3D surface plot of the off-duty node number n dfferent sensng range and deployed node number. From t, we can see that ncreasng the number of the orgnal deployed nodes and ncreasng the sensng range wll result n more nodes beng turned off, whch s consstent wth our expectaton Sensng Coverage vs. node densty We also nvestgate the change of obtaned sensng degree over node densty. As shown n Fgure 8, although the range of the ntal sensng degree s vared from 3 to 48, the obtaned sensng degree s almost stable at 3 or 4 after turnng off some nodes. Therefore, the coverage-based off-duty elgblty rule also effectvely controls the network redundancy. 95 r=8 r=10 r=12 on-duty node number deployed node number Fgure 6: off-duty node number vs. node densty However, on-duty node number doesn t reman constant over dfferent deployed node number when the sensng range and the deployed area are fxed. Instead, t ncreases as the deployed node number ncreases as llustrated n Fgure 7. Ths s due to the ncreasng of edge nodes (located at the boundary of the deployed area). Accordng to our off-duty elgblty rule, edge nodes have no chance to be turned off because all the other nodes are located on one sde of the edge nodes. Intutvely, ncreasng edge nodes wll ncrease the on-duty node number, however, expermental result shows that our coverage based off-duty elgblty rule stll effectvely lmts the on-duty node number. When the deployed node number s ncreased from 100 to 300, the number of on-duty node only ncreases about 30%. Fgure 7: on-duty node number vs. deployed node number Fgure 9-11 presents the same effectveness but from the dfferent vew: the percentage of the deployed area that can be montored by at least D on-duty nodes. We stll dvde the space nto 1m 1m unt cells as mentoned n secton An event occurs n each cell, wth the event source located at the center of the cell. We nvestgate the rato of the cell number reached by at least D on-duty nodes to the total number of cells when sensng range s 8, 10 or 12 meters, respectvely. As llustrated n the fgures, most of the area, above 88%, can be covered by at least 3 on-duty nodes. Almost 100% cells can be reached by at least one on-duty node. And about 97% cells can be montored by at least 2 on-duty nodes. Furthermore, our experments show that ncreasng the deployed node number and the sensng range leads to more coverage (D=1), because less sensng holes exst n the orgnal network. In addton, the two curves (D=1, orgnal D=1) are exactly the same n all the fgures, whch mples that the coverage-based off-duty elgblty 37

7 rule completely preserves the orgnal sensng coverage wthout any blnd ponts. In fact, the results presented n ths secton are relatvely deal compared to the real tme smulaton, because all deployed nodes are scheduled n the sequence, whch means there are no cases when two nodes make off-duty decsons at exactly the same tme. Second, the decson s vsble to all the other nodes. However, n the real smulaton, decson s announced by sendng messages, whch may be lost n the way. Thrd, each node has already known complete neghbor nformaton. By usng effectve MAC protocol (collson avodance), satsfyng effect, whch s very close to deal one, can be acheved as shown n Fgure coverage percentage(%) sensng_range = 10 m D=1 D=2 D=3 D=4 orgnal D=1 coverage ercentage(%) Fgure 8: sensng degree reducton vs. node densty sensng range =8 D=1 D=2 D=3 D=4 orgnal D= deployed node number Fgure 10: coverage vs. deployed node number(r=8) 4.2 Smulaton Results In ths secton, we descrbe the mplementaton of our proposed nodeschedulng scheme as an extenson of LEACH [10]. Our man purpose s to analyze the energy effcency of our proposed scheme by comparng the energy consumpton wth and wthout the extenson Smulaton Envronment We mplement the proposed scheme as an extenson of an exstng data gatherng protocol, LEACH [10]. Although the proposed scheme can be combned wth any other data gatherng protocols, we select LEACH because ts NS-2 smulaton code s avalable n the Internet [20] and t s one of the earlest and most famous communcaton protocols sutable for wreless sensor networks. Furthermore, t has a smlar tmelne as our proposed scheme. LEACH (Low-Energy Adaptve Clusterng Herarchy) s a clusterng-based communcaton protocol proposed by the MIT LEACH proect. In LEACH, nodes are organzed nto local clusters, wth one node actng as the local base staton or cluster-head. All the other nodes must transmt ther data to the cluster heads, whle the cluster-head nodes must receve data from all the cluster members, perform sgnal processng functons on the data (e.g., data deployed node number Fgure 9: coverage vs. deployed node number(r=10) coverage percentage(%) sensng range=12 D=1 D=2 D=3 D=4 orgnal D= deployed node number Fgure 11: coverage vs. deployed node number(r=12) aggregaton), and then transmt data to the remote base staton. Beng a cluster head s much more energy-ntensve than beng a noncluster-head node. In order to evenly dstrbute the energy load assocated wth a cluster-head and avod dranng the battery of any one sensor, cluster-head poston s rotated randomly among all the nodes. The medum access protocol n LEACH s also chosen to reduce energy dsspaton n non-cluster-head nodes. Snce a cluster head node knows all the cluster members, t can act as a local control center and create a TDMA schedule that allocates tmeslots for each cluster member. Ths allows the nodes to reman n the sleep state as long as possble. In addton, usng a TDMA schedule for data transfer prevents ntra-cluster collsons. In LEACH, the operaton s also dvded nto rounds, whch are composed of a cluster set-up phase n whch the clusters are formed, and a steady-state phase n whch sensors collect data from the envronment and transfer data to the cluster-heads and then to the base staton. To extend LEACH wth our node-schedulng scheme, a straghtforward way s to nsert the self-schedulng phase of our scheme before the LEACH cluster set-up phase. At the begnnng of each round, all the nodes self-determne whether to turn themselves 38

8 off or not and off-duty nodes wll not partcpate n the cluster formng and steady-state phase followed. The advantage of such tmelne s that our node-schedulng scheme s embedded nto the LEACH seamlessly wthout any modfcaton of ts orgnal workflow. The tmelne of the mplementaton s llustrated n Fgure 12. round schedulng clusterng sensng tme Fgure 12. Tmelne of LEACH wth extenson An ssue of concern before our smulaton s the mpact of collsons to our algorthm, because collsons would cause data transmsson falure, therefore, lost of neghborng nformaton, and reducton of the off-duty node number. The LEACH protocol uses a new MAC protocol type, MacSensor, whch s a combnaton of Carrer-Sense Multple Access (CSMA), Tme-Dvson Multple Access (TDMA), and a smple model of Drect Sequence Spread Spectrum (DS-SS). DS-SS and TDMA schema are used after clusters have been formed, whle n the cluster set-up phase, a non-persstent CSMA scheme s used, where nodes sense carrer frst before transmsson. If the carrer s currently busy, the node sets a random back-off tme to try agan. Snce our node-schedulng scheme s nserted before the cluster set-up phase, the CSMA scheme s nherted and used. Although ths scheme cannot completely elmnate collsons, t does suppress collsons effectvely n our smulaton. As llustrated n Fgure 13, where the average on-duty node numbers n 100-second real-tme smulaton of 5 random topologes are almost the same as the deal values obtaned n secton Therefore, n ths work we do not address how contenton affects our algorthm. Ths subect s expected to be one of our future works. Fgure 13 also mples the correctness of our mplementaton. Fgure 13: On-duty node number n real-tme smulaton Energy Consumpton Ths secton evaluates the ablty of our node-schedulng scheme to save energy, therefore, ncrease system lfetme by comparng the energy consumpton per node n the orgnal and extended LEACH. In terms of energy conservaton, we cannot only evaluate the energy savng n the data-gatherng phase, because node-schedulng tself also consumes energy n transmsson of PAM and SAM messages as well as computaton, whch should not be gnored. If the cost of the node-schedulng phase domnates the overall energy consumpton n each round, t s better not to turn off nodes. In the orgnal LEACH protocol, energy s manly consumed n two parts: data transmsson for clusterng formng (E c ) and data gatherng (E g ). Whle n the extended LEACH, extra energy s needed n node schedulng phase, whch s denoted as E t. Assumng the number of data gatherngs n each round s N g, then the energy dsspaton of each round n the orgnal LEACH s E = E c + N g E g, whle the energy dsspaton per round n the extend LEACH s E = E t + E c + N g E g. As long as E < E, energy savngs can be acheved by node-schedulng. In fact, the energy coeffcents n E and E are affected by many factors: the sze of sensng range, the length of report message, the number of data gatherngs n each round, and the power consumpton model, etc. Therefore the potental for energy savng s the combnaton effect of multple factors. In our smulaton, we use the same energy parameters and rado model as dscussed n [21], whch ndcates that the transmsson energy consumpton s 2 Eelec k + ε frss amp k d : d < dcrossover and the recepton ETx( k, d) = 4 Eelec k + εtwo ray amp k d : d dcrossover energy consumpton s ERx = Eelec k, where E elec s the energy consumed for the rado electroncs, ε and frss amp ε for a two ray amp power amplfer. Rado parameters are set as E = 50nJ/bt, ε = 10pJ/bt/m 2, ε = pJ/bt/m 4, d frss amp two ray amp crossover = 87m. We only consder the data aggregaton, whle gnore other processng energy consumpton. The energy for performng data aggregaton s 5nJ/bt/sgnal. Once a node s turned off, the energy t consumes s neglgble. The smulaton s carred out n a network wth 100 nodes, each wth a sensng range of 10 meters. Nodes are placed randomly n a rectangular regon whose area s 50m 50m. The remote base staton (or snk node) s located at the low left corner,.e. orgn pont (0,0). The ntal energy of all nodes s 2J. Each sensor sends a 2000-bt report message to the base staton wth a 0.5s tme nterval. The tme duraton of each round s 10 seconds. Fgure 14 llustrates the energy dsspaton curve per node n the orgnal LEACH and the extended LEACH n random network topology when N g =20. The energy dsspaton n the extended LEACH s slower than the orgnal one. Fgures 15 and 16 show an ncrease of the system lfetme n the same smulaton settng. Here we use two metrcs to evaluate the system lfetme: the total number of nodes alve over tme and the system sensng coverage over tme (the rato of the area montored by on-duty nodes to the deployed regon). As llustrated n Fgure 15 and 16, although the extended LEACH does not outperform the orgnal one n term of frst node dead tme, the number of nodes alve and the system sensng coverage drop more quckly n the orgnal LEACH than n the extended one. In the result, t takes approxmately 4378 seconds for the last node to de n the extended LEACH, whle 1412 seconds n the orgnal LEACH. And t takes approxmately 2055 seconds for the sensng coverage to drop 20% (reach 80%) n the extended LEACH, whle 1285 seconds n the orgnal one. elec 39

9 Energy dsspaton per node (Joule) orgnalleach extended Leach Tme(second) Fgure 14: Energy dsspaton curve per node when ℵ =100, R =50m 50m, r=10m,n g =20 when sensng coverage reaches 80%(s) extended LEACH orgnal LEACH 4 8 Ng Fgure 17: system lfetme vs. N g when ℵ =100, R =50m 50m, 5000 r=10m Sensng Coverage Percentage(%) Orgnal Leach Extended Leach Tme(second) Fgure 15: Sensng coverage over tme when ℵ =100, R Number of nodes stll alve =50m 50m, r=10m,n g =20 Orgnal Leach Extended Leach Tme(second) Fgure 16: Number of nodes alve over tme when ℵ =100, R =50m 50m, r=10m,n g =20 when sensng coverage reaches 80%(s) extended LEACH orgnal LEACH deployed node number Fgure 18: system lfetme vs. node densty when ℵ =100, R =50m 50m, r=10m,ng=20 Furthermore, we also change the number of data gatherngs n each round from 4 to 20 wth the ncrement of 4, and compare the tme when system coverage drops below 80% n the orgnal and extended LEACH. Fgure 17 shows that the system lfetme wth extended LEACH s always longer than, and s about 1.7 tmes of the orgnal one. Fgure 18 plots the system lfetme as a functon of node densty. It can be seen that the system lfetme ncreases as the node densty ncreases n extended LEACH. In contrast, the system tme decreases as the node densty ncreases n orgnal LEACH. 5. CONCLUSIONS In ths paper, we proposed a coverage-preservng node-schedulng scheme, whch can reduce energy consumpton, therefore ncrease system lfetme, by turnng off some redundant nodes. We presented a basc model for coverage-based off-duty elgblty rule and then extend t to several dfferent scenaros. Ths knd of off-duty elgblty rule guarantees that the orgnal sensng coverage can be mantaned to the extent possble. To further preserve sensng coverage n a real tme envronment, we ntroduce a back-off scheme 40

10 n whch nodes delay by a random tme perod, before nvestgatng the elgblty rule, and wat for a short tme, f they decde to turn off. Dong so prevents nodes sponsorng each other, therefore avods blnd ponts. Expermental results show that enough redundancy stll remaned although some nodes were turned off. We mplemented ths scheme as an extenson to the LEACH protocol, whch s an exstng data communcaton protocol for wreless sensor networks. We compared the energy consumpton n the orgnal LEACH and the extended LEACH and analyzed the effectveness of our scheme n terms of energy savng. Prelmnary smulaton results n the rado model and energy parameters proposed by the LEACH desgner show the potental of such energy savng and system lfetme ncrease. 6. REFERENCES [1] D. Estrn, R. Govndan, J. Hedemann and S. Kumar, Next Century Challenges: Scalable Coordnaton n Sensor Networks, Proc. of ACM MobCom 99, Washngton, August [2] J. Kahn, R. Katz, and K. Pster, Next Century Challenges: Moble Networkng for Smart Dust, Proc. of ACM MobCom 99, August [3] A. Cerpa, J. Elson, D. Estrn, L. Grod, M. Hamlton and J.Zhao, Habtat Montorng: Applcaton Drver for Wreless Communcatons Technology, 2001 ACM SIGCOMM Workshop on Data Communcatons In Latn Amerca and the Carbbean, Costa Rca, Aprl 2001 [4] I.F.Akyldz, W. Su, Y.Sankarasubramanam, E.Cayrc, Wreless Sensor Networks: A Survey, Computer Networks, March 2002 [5] E. Shh, S. Cho, N. Ickes, R. Mn, A. Snha, A. Wang, A.Chandrakasan, Physcal Layer Drven Protocol and Algorthm Desgn for Energy-Effcent Wreless Sensor Networks, ACM SIGMOBILE Conference on Moble Computng and Networkng, July 2001, ROME, Italy [6] A. Porret, T. Melly, C. C. Enz, and E. A.Vttoz, A Low- Power Low-Voltage Transcever Archtecture Sutable for Wreless Dstrbuted Sensors Network, IEEE Internatonal Symposum on Cruts and Systems 00, Geneva, [7] M. J. Dong, K. Geoffrey Yung, and W. J. Kaser, Low Power Sgnal Processng Archtectures for Network Mcrosensors, 1997 Internatonal Symposum on Low Power Electroncs and Desgn, Dgest of Techncal Papers (1997). [8] J.M. Rabaey, M. J. Ammer, J. L. da Slva,D.P.S.Roundy, PcoRado Supports Ad Hoc Ultra-Low Power Wreless Networkng, IEEE Computer Magazne, July [9] G. Asada, M. Dong, T. S. Ln, F. Newberg, G. Potte, W. J. Kaser, Wreless Integrated Network Sensors: Low Powers Systems on a Chp, Proc of the 24th IEEE European Sold- State Crcuts Conference, Elsever, [10] W.R.Hezelman, A.Chandrakasan, and H.Balakrshnan, Energy-Effcent Communcaton Protocol for Wreless Mcro Sensor Networks, IEEE Proceedngs of the Hawa Internatonal Conference on System Scences, January [11] C. Intanagonwwat, R. Govndan and D. Estrn, Drected Dffuson: A scalable and Robust Communcaton Paradgm for Sensor Networks, ACM MOBICOM 00, [12] F. Ye, G. Zhong, S. Lu, L. Zhang, Energy Effcent Robust Sensng Coverage n Large Sensor Networks, Techncal Report. [13] B. Chen, K. Jameson, H. Balakrshnana, R. Morrs, Span: An Energy-Effcent Coordnaton Algorthm for Topology Mantenance n Ad Hoc Wreless Networks, MOBICOM 01, [14] Y. Xu, J. Hedemann, D. Estrn, Adaptve Energy- Conservng Routng for Multhop Ad hoc Networks, Techncal Report 527, USC/ISI, Oct [15] Y. Xu, J. Hedemann, D. Estrn, Geography-nformed Energy Conservaton for Ad Hoc Routng, MOBICOM 01, [16] C.Intagagonwwat, R.Govndan, and D.Estrn, Drected Dffuson: A Salable and Robust Communcaton Paradgm for Sensor Networks, Proc. of the AMC Mobcom 00, Boston, MA, [17] V. Rodoplu and T.H.Meng, Mnmum Energy Moble Wreless Networks, IEEE JSAC, Vol. 17, No.8, August [18] L. L, and J.Y.Halpern, Mnmum-Energy Moble Wreless Networks Revsted, IEEE Internatonal Conference on Communcatons ICC 01, Helsnk, Fnland, June [19] R. Wattenhofer, L.L, P.Bahl, Y.-M.Wang, Dstrbuted Topology Control for Power Effcent Operaton n Multhop Wreless Ad Hoc Networks, Proc. of the Thrd Workshop on Moble Multmeda Communcatons (MoMuC-3), Prnceton, NJ, [20] W.Henzelman, A.Chandrakasan, and H.Balakrshnan, uamps ns Code Extensons, uamps/leach. [21] W.Henzelman, Applcaton-Specfc Protocol Archtectures for Wreless Networks, PhD thess, June 2000, MIT. 41