An Energy-Efficient Communication Scheme in Wireless Cable Sensor Networks

Transcription

1 This fll text aer was eer reiewed at the direction of IEEE Commnications Society sbject matter exerts for blication in the IEEE ICC 11 roceedings An Energy-Efficient Commnication Scheme in Wireless Cable Sensor Networks Xiao Chen eartment of Comter Science Texas State Uniersity San Marcos, TX Neil C. Rowe eartment of Comter Science U. S. Naal Postgradate School Monterey, CA Abstract Nowadays wireless sensor networks (WSNs) hae attracted a great deal of stdy de to their low cost and widerange alications. Most of the sensors sed so far are oint sensors which hae a disc-shaed sensing region. In this aer, we stdy a new tye of sensor: the fiber otic cable sensor. Unlike a traditional oint sensor, this tye of sensor has a rectanglar sensing region with a rocessor installed on it to do rocessing and commnication. ike wireless sensor networks with oint sensors, energy-efficient commnication is still an imortant isse in wireless sensor networks with cable sensors becase of the need to efficiently se limited resorces. To address the isse, we roose a Cable Mode Transition (CMT) algorithm, which determines the minimal nmber of actie sensors to maintain K- coerage of a terrain as well as K-connectiity of the network. Secifically, it allocates eriods of inactiity for cable sensors withot affecting the coerage and connectiity reqirements of the network based only on local information. Before resenting CMT, we first show the relationshi between coerage and connectiity, then the eligibility algorithm ermitting a cable sensor to decide whether to stay actie. CMT calls the eligibility algorithm to schedle cables. Simlation reslts show that or scheme is efficient in saing energy and ths can rolong the lifetime of wireless cable sensor networks. I. INTROUCTION Wireless sensor networks (WSNs) hae attracted a great deal of stdy de to the low cost of sensors and their widerange alications. WSNs roide a new class of comter systems and exand hman ability to remotely interact with the hysical world. Most of the sensors sed so far are oint sensors which hae a disc-shaed sensing region. Recently a new kind of sensor has become aailable for detecting seismic signals [4]. It ses a fiber otic cable which is actiely qeried by sending otical signals down it. Each cable has a rocessor on it to rocess data and commnicate with other rocessors on other cables. The cable can be tens of kilometers long so one cable can roide extensie coerage. Comaring with the tradition oint sensors, this kind of sensor has a long length of coerage area which makes it ideal for borders and roads. It will be helfl in detecting excaation behaior sch as digging tnnels and lanting exlosie deices on the roads as well as roiding a secre border with 24/7 sreillance for illegal immigrants. ike oint sensors, wireless commnication among cables is more flexible and conenient than wired commnication in case the toology of the network changes de to cable moement or failres. Also, as in the wireless oint sensor networks, energy-efficient commnication is ery imortant in wireless cable sensor networks de to the limited ower resorce in cable rocessors and the inconenience of charging their batteries freqently. In this aer, we redce energy consmtion in wireless commnication by roosing CMT: A Cable Mode Transition algorithm, which determines the minimal nmber of actie sensors to maintain K-coerage of a terrain as well as K- connectiity of the network. Here, K-coerage means that eery oint in the terrain is coered by at least K cable sensors and K-connectiity means that if K 1 cable sensors fail the network is still connected. The CMT algorithm allocates eriods of inactiity for cables withot affecting the coerage and connectiity reqirements of the network based only on local information. Secifically, we make the following contribtions: This is the first stdy on energy-efficient commnication for wireless cable sensor networks. Conditions hae been fond for coerage to imly connectiity sing cable sensors. Or Cable Mode Transition algorithm is a distribted rotocol that only reqires local information. Simlations are condcted to erify the efficiency of the roosed scheme. The rest of the aer is organized as follows: Section II is the reliminary, Section III formlates the roblem, Section IV describes the energy-efficient commnication scheme CMT in detail, Section V shows the simlation reslts, Section VI mentions the related work, and Section VII concldes the aer and oints ot the ftre direction. II. PREIMINARY In this aer, the cable seismic sensors we se are different from traditional oint sensors. They are different in both sensing and commnication. The sensing region of a oint sensor can be modeled as a disc with a sensing range s and its commnication region can be modeled as a disc with a commnication range r as shown in Fig. 1(a). For a cable sensor deloyed straight with length in Fig. 1(b), its sensing range is the maximm distance that can be sensed orthogonal to the cable, and its sensing region is a rectangle U.S. Goernment work not rotected by U.S. coyright

2 This fll text aer was eer reiewed at the direction of IEEE Commnications Society sbject matter exerts for blication in the IEEE ICC 11 roceedings s r Rect() R w (a) A traditional oint sensor (b) A cable sensor Fig. 2. w is ineligible if K =1 Fig. 1. Traditional oint sensor s. cable sensor if R denoted as Rect(). That is, any object coming across this region can be detected by the cable. In some figres of this aer, the rectanglar sensing region of a cable is reresented by a shaded area. Actally, the sensing region of a cable shold hae ronded ends that are semicircles of radis since coerage extends ast the ends of the cable for distance. Here we make the simlification based on the fact that there will be significant noise in the icinity of the cable ends. The commnication between cables is done by the rocessors on the cables. Each cable shold hae a rocessor to collect data and commnicate with other rocessors on other cables. The rocessor on cable has a commnication region of a disc with a commnication range R and is reresented by a dot. In this aer, we assme the terrain to be coered is a large 2-dimensional conex and a cable is always deloyed straight becase it is easy to roe that it coers the most sensing area if deloyed in this way. III. PROBEM FORMUATION We formlate the energy-efficient commnication roblem as follows: gien a 2-dimensional conex terrain A, and a coerage degree K secified by the alication, we mst allow as many rocessors as ossible to trn off most of the time and at the same time, enogh rocessors mst stay awake to garantee that A is K-coered and the backbone formed by awakened rocessors is K-connected. In this roblem, there are two isses: coerage and connectiity. First we want to know if they are related. If one imlies the other, then if we satisfy the stronger one, the other is also satisfied. Obiosly connectiity does not imly coerage becase if a network on a terrain is connected, it can haen that not eery oint in the terrain is coered by some sensor. On the other hand, if a terrain is flly coered by a sensor network, is the network connected? The following two theorems show the conditions for coerage to imly connectiity with 1-coerage and K-coerage. The conditions are tre regardless of the locations of rocessors on cables. e to the limited sace, the roofs of all theorems in this aer can be fond in [2]. Theorem 1: For a set of cables haing sensing range and length that 1-coer a 2-dimensional conex terrain A, the commnication grah made by rocessors is connected if the commnication range R Theorem 2: A set of cables that K-coer a 2-dimensional conex terrain A forms a K-connected commnication grah IV. THE CMT AGORITHM From the aboe, we know that if a terrain is K-coered and if the commnication range R , then the network is K-connected. In this section, we resent the cable mode transition (CMT) algorithm to determine the minimal nmber of actie cables to maintain K-coerage secified by the alication as well as K-connectiity. The idea was insired by []. The difference is that their aroach alies to traditional oint sensors with a disc-shaed sensing region and or aroach alies to cable sensors with a rectanglar sensing region. A cable can be in one of the three modes with the energy consmtion from the highest to the lowest: ACTIVE, SNOOPY and SEEP. In the ACTIVE mode, a cable actiely senses and commnicates with other rocessors on other cables; in the SNOOPY mode, each cable collects HEO messages from its neighboring rocessors and checks its eligibility to determine its new mode; and in the SEEP mode, a cable slees to sae energy. The CMT algorithm describes the rles of cable mode transition to sae energy. Before resenting CMT, we introdce the eligibility algorithm it calls whose role is to make each cable check its eligibility to stay actie. A. K-coerage eligibility algorithm Each cable exectes an eligibility algorithm to determine whether it is necessary to stay actie. Gien a reqested coerage degree K, a cable is ineligible to stay actie if eery location within its coerage region is already K-coered by other actie cables in its neighborhood. For examle, in Fig. 2, cables and are actie and cable w is ineligible if K =1, bt eligible if K>1. Before resenting the eligibility algorithm, we define the following concets: The sensing region of cable : contains all the oints sch that <. An intersection oint of two cables and : denoted by, is an intersection oint of the sensing rectangles of and. An intersection oint of a cable and terrain A: denoted by A, is an intersection oint of the sensing rectangle of cable and terrain A. Note that the intersection oints of two cables and between a cable and a terrain A are different from reglar definitions. Here they are formed by the sensing rectangles of cables, not

3 This fll text aer was eer reiewed at the direction of IEEE Commnications Society sbject matter exerts for blication in the IEEE ICC 11 roceedings s r s r (a) q Fig. 3. (a) The intersection oints of cables and (b) The intersection oints between a cable and a terrain A V Fig. 4. The largest distance of two neighboring cables cables themseles. As shown in Fig. 3(a)(b), the intersection oints of two cables and and between cable and terrain A are, q, r and s. Also note that when deloyed on a terrain, two cables may not be arallel or erendiclar to each other. They can intersect with any angle. Bt for conenience s sake and withot affecting the reslts, cables are drawn arallel or erendiclar to each other in Fig. 3. Theorem 3: A conex terrain A is K-coered by a set of cables C if 1) there exist in terrain A intersection oints between cables or between cables and A s bondary; 2) all intersection oints between any cables are at least K-coered; and 3) all intersection oints between any cable and A s bondary are at least K-coered. Theorem 3 conerts the roblem of finding the coerage degree of a terrain to the simler roblem of finding the coerage degrees of all the intersection oints in the terrain. A cable is ineligible to stay actie if all the intersection oints inside its sensing rectangle are at least K-coered. To find all the intersection oints inside its sensing rectangle, a cable needs to know the locations of all the cables in its sensing neighbor set, SN(). SN() shold inclde all the actie cables that are within the maximm distance between the rocessor on and the rocessor on another cable that attaches to. As shown in Fig. 4, that distance is the distance between rocessor on and on another cable, which is (2) 2 +(2) 2 = Cables can find their neighbors throgh exchanging Hello messages. If the actal commnication range R sed by a cable is greater or eqal to , the Hello message from each cable only needs to inclde its own id and location. If R< , a cable may not find all its neighbors throgh sch Hello messages in one ho. Then, more cables will stay actie becase of its limited information. This is roed by the simlation in the next section. Also if R< , the network is not garanteed to be connected as indicated by Theorem 2. The reslting algorithm for a cable to check whether it is eligible to stay actie or not is shown in Fig.. Algorithm A (b) q U Algorithm Eligibility: determine if a cable is eligible to stay actie gien a coerage degree K 1: /* find all intersection oints within Rect() */ 2: IP = { ( w OR A) AN, w SN() AN <}; 3: /* find all oerlaing cables */ 4: OC = { =}; : if IP =then 6: if OC K then 7: retrn INEIGIBE; 8: else 9: retrn EIGIBE; 1: end if 11: end if 12: for each oint IP do 13: /* comte s coerage degree */ 14: cd() = { SN() AN < } 1: if cd() <K then 16: retrn EIGIBE; 17: end if 18: end for 19: retrn INEIGIBE; Fig.. The K-coerage eligibility algorithm Eligibility has three arts: first, cable finds all the intersection oints (of cables and between cables and terrain A) within its rectanglar sensing region and ts these oints into the intersection oint set IP. Next, cable tries to find ot if there are any oerlaing cables which are the cables haen to be laced in the same location as itself. If so, these cables are t into the oerlaing cable set OC. If there is no intersection oint within s sensing rectangle and the nmber of oerlaing cables OC is at least K, cable is ineligible. Otherwise, it is eligible. Finally, cable calclates cd(), the coerage degree of eery intersection oint within its sensing region. If the coerage degree is less than K, cable is eligible; otherwise, it is not eligible. The comtation comlexity of the eligibility algorithm is O(N 3 ) where N is the nmber of cables in the sensing neighbor set. B. Cable mode transitions Now the CMT algorithm is resented in Fig. 6. A cable can transit among SEEP, ACTIVE and SNOOPY modes. Initially all cables are ACTIVE. If the terrain coerage goes oer the reqired degree of the alication, redndant cables will find themseles ineligible to stay actie and go to the SEEP mode ntil no more cables can be trned off withot casing insfficient degree of coerage. If oer time the failre of a cable makes the terrain coerage fall below the reqired degree, some cables will find themseles eligible and go to the ACTIVE mode. The times set by the join and withdraw timers are randomly generated to aoid collisions among mltile cables wanting to join or withdraw simltaneosly.

4 This fll text aer was eer reiewed at the direction of IEEE Commnications Society sbject matter exerts for blication in the IEEE ICC 11 roceedings Cable Mode Transition (CMT) Algorithm 1: If a cable is in the SEEP mode and its slee timer exires, it trns on, starts a snooy timer and goes to the SNOOPY mode. 2: If a cable is in the SNOOPY mode and receies either HEO, JOIN, or WITHRAW message, it calls the eligibility algorithm (in Fig. ) to see if it is eligible to stay actie. If it is, it starts a join timer; else it goes to the SEEP mode. After the join timer starts and if it becomes ineligible (e.g. becase of a JOIN message from a commnicating neighbor), it cancels the join timer. If the join timer exires, the cable broadcasts a Join message and goes to the ACTIVE mode. If the snooy timer exires, it starts a slee timer and goes to the SEEP mode. 3: If a cable is in the ACTIVE mode and receies a HEO message, it dates its neighbor table and calls the eligibility algorithm (in Fig. ) to see if it shold remain actie. If it shold not, it starts a withdraw timer. Before the withdraw timer exires and if it becomes eligible (e.g. becase of a WITHRAW message from a commnicating neighbor), it cancels the withdraw timer. If the withdraw timer exires, it broadcasts a Withdraw message, starts a slee timer and goes to the SEEP mode. Fig. 6. Cable mode transition algorithm V. SIMUATIONS In this section, we condct simlations to ealate or energy-efficient commnication scheme. Or simlator is selfwritten becase there are no aailable ones for cable sensors. We ealate the effectieness and roerties of or scheme by itself de to the lack of others at this oint. In the first simlation, we want to see the relationshi between the nmber of cables deloyed and the aerage coerage degree achieed. To measre coerage, we diide the entire terrain into 1 1 atches. The coerage degree of a atch is aroximated by measring the nmber of actie cables that coer the center of the atch. We se terrains of 3 3 and. In each terrain, we try two kinds of cables: 1 (sensing range and length 1) and (sensing range and length ). Note that we do not hae a nit for these data, which can make the simlations more adatie to the real alications. What matters here is the relatie size of the cables to the terrain. The cables are randomly deloyed onto the terrain in all directions. We start from cables to and go to cables with an increment of in each ste. The degree of each location is calclated and the final coerage degree of the terrain is the aerage of all the locations. Figs. 7(a)(b) show the reslts. From the reslts, we can see that (1) in a larger terrain, the coerage is lower with the same nmber of cables; (2) in a terrain, with the increase of the nmber of cables, the coerage goes ; (3) with the same nmber of cables, the cable has a higher coerage than the 1 cable Aerage coerage degree (a) Aerage coerage degree sing 1 cables in a 3 3 terrain Fig Aerage coerage degree (b) Aerage coerage degree sing cables in a terrain Aerage coerage degree achieed sing different nmbers of cables 1-coerage 2-coerage 3-coerage (a) Actie 1 cables needed in a 3 3 terrain for 1, 2, 3-coerage 1 1-coerage 2-coerage 3-coerage (c) Actie 1 cables needed in a terrain for 1, 2, 3-coerage Fig coerage 2-coerage 3-coerage (b) Actie cables needed in a 3 3 terrain for 1, 2, 3-coerage 1 1-coerage 2-coerage 3-coerage (d) Actie cables needed in a terrain for 1, 2, 3-coerage needed for 1, 2, and 3-coerage becase it is longer and ths has a larger coerage region. In the second simlation, we want to find ot how many cables are actie ot of all deloyed to achiee coerage degrees 1, 2, and 3 sing the CMT algorithm. Again we se the 1 and cables and deloy them on two terrains 3 3 and. Interrain3 3 sing the 1 cables, we start from 1 cables becase this is the nmber to garantee 1, 2, and 3-coerage for the whole terrain. Similarly in terrain 3 3 sing cables, the starting nmber is 1 cables. And in terrain, the starting cable nmbers for 1 and are 2 and 1 resectiely for the same reason. The reslts are shown in Figs. 8(a)(b)(c)(d). From the reslts, we can see that (1) more cables need to be actie to hae higher coerage; (2) the nmber of actie cables does not increase with the increase of cable nmbers. That means, or scheme does not wake more cables becase the coerage goal has already been achieed. In the third simlation, we want to exlore the effect of different commnication ranges to the nmber of actie cables sing the CMT algorithm. We still deloy the 1 and cables on the 3 3 and terrains. To garantee that a terrain is 1-coered, we se different nmbers of cables

5 This fll text aer was eer reiewed at the direction of IEEE Commnications Society sbject matter exerts for blication in the IEEE ICC 11 roceedings Commnication range (a) Actie 1 cables needed with (b) Actie cables needed with different commnication ranges in a different commnication ranges in a 3 3 terrain 3 3 terrain Commnication range Commnication range (c) Actie 1 cables needed with (d) Actie cables needed with different commnication ranges in a different commnication ranges in a terrain terrain Fig Commnication range needed for different commnication ranges for each setting. For examle, for terrain 3 3 with 1, we se 9 cables becase this is the nmber to make sre that the terrain is 1-coered. For terrain 3 3 with, we se cables. For terrain with 1 and, the nmber of cables sed is 23 and resectiely. To make cases general, we t a rocessor on each cable in a random location. We start with a commnication range R that is greater or eqal to , then decrease it gradally. The reslts are shown in Figs. 9(a)(b)(c)(d). From the reslts we can see that if the commnication range is greater or eqal to , many cables can be trned off becase this is the range that they can flly detect their neighbors and find ot if eery location in their sensing region is 1-coered by other actie cables. For examle, in the 3 3 terrain with the 1 cables, only 22 cables need to be actie ot of 9 cables deloyed to 1-coer the whole terrain. The saing is 76%. Then we redce the commnication range. At the beginning, the nmber of actie cables remains the same becase the commnication range is still greater than the distance between two rocessors on cables. Then with the frther decrease of the commnication range, each cable cannot flly detect all its neighbors een if they are there. From the limited neighbors that a cable can detect, it will more and more likely decide to stay awake becase not all the locations within its sensing region can be coered by its detected neighbors. After a certain oint, for examle, in the terrain with the cables, when the commnication range redces to 2 or 1, no cable can be trned off becase each cable thinks it is the only one to coer the locations in its sensing region. In smmary, the simlation reslts show that the CMT scheme can make as many cables as ossible to go to slee withot affecting the connectiity of the network according to the reqired coerage degree K from the alication. VI. REATE WORK In the literatre, for oint sensors, there are seeral aers working on saing ower by determining a minimal nmber of actie sensors to maintain coerage as well as connectiity. In terms of the relationshi between coerage and connectiity, an imortant reslt was roed by Zhang and Ho [6], which states that if the commnication range R c is at least twice the sensing range R s, a comlete coerage of a conex area imlies connectiity of the actie sensors. Wang et al. [] generalize the reslt in [6] by showing that, when the commnication range R c is at least twice the sensing range R s,akcoered network will reslt in a K-connected network. Then the athors t forward the coerage configration rotocol (CCP) that can dynamically configre the network to roide different coerage degrees reqired by alications. Carle and Simlot [1] roose another mechanism for energy-efficient connected area coerage when all sensor nodes hae the same sensing range and the commnication range eqals the sensing range. The goal of the algorithm is to se one of the existing rotocols (e.g. ai and W s algorithm [3]) to select an areadominating set of nodes of minimm cardinality, sch that the selected set coers the gien area. VII. CONCUSION AN FUTURE WORK In this aer, we roosed a distribted algorithm called CMT to redce commnication energy consmtion in a new tye of sensor network: the fiber otic cable sensor network, by ermitting rocessors take trns to go to slee withot affecting the coerage and connectiity of the network. Simlation reslts showed that or scheme is efficient in saing energy and can ths rolong the lifetime of the network. In the ftre, we will discss oint coerage and barrier coerage roblems sing cable sensor. ACKNOWEGMENTS This research was sorted by NSF grant CBET This work reresents the oinions of the athors only and does not necessarily reresent the iews of the U.S. Goernment. REFERENCES [1] J. Carle,. Simlot, Energy efficient area monitoring by sensor networks, IEEE Comter, ol. 37, No. 2, 4, [2] X. Chen, N. C. Rowe, Energy efficient commnication of cable sensors, Technical reort, eartment of Comter Science, Texas State Uniersity, San Marcos, 1. [3] F. ai, J. W, istribted dominant rning in ad hoc wireless networks, Proc. of IEEE International Conference on Commnications, 3. [4] N. Rowe, Efficient deloyment of fiber-otic cable seismic sensors, Proc. of SIPE, Aril, 1. [] X. R. Wang, G.. Xing, Y. F. Zhang, C. Y., R. Pless and C. Gill, Integrated coerage and connectiity configration in wireless sensor networks, Proc. of the 1st ACM SenSys, 3, [6] H. Zhang, J. C. Ho, Maintaining sensing coerage and connectiity in large sensor networks, Technical reort UIUC, UIUCCS-R-3-231, 3.