An Adaptive Sleep Strategy for Energy Conservation in Wireless Sensor Networks

Transcription

1 An Adaptve Sleep Strategy for Energy Conservaton n Wreless Sensor Networks Guseppe Anastas, Marco Cont, Maro D Francesco Abstract - In recent years, wreless sensor network deployents for real lfe applcatons have rapdly ncreased. However, energy consupton stll reans the an ltaton of ths technology. As councaton typcally accounts for the aor power consupton, transcever actvty should be nzed, n order to prolong the network lfete. To ths end, we have developed an Adaptve Staggered sleep Protocol (ASLEEP) for effcent power anageent n wreless sensor networks targeted to perodc data acquston. The proposed protocol dynacally adusts nodes sleep schedules to atch the network deands, even n te-varyng operatng condtons. In addton, t does not requre any a-pror knowledge of the network topology or traffc pattern. ASLEEP has been extensvely studed wth sulaton and easureents n a real testbed. In both cases, the results obtaned show that, under statonary condtons, the algorth effectvely reduces the energy consupton of sensor nodes, by dynacally adustng ther duty-cycle to the current traffc needs, thus ncreasng sgnfcantly the network lfete. Wth respect to slar nonadaptve solutons, t also reduces the average essage latency and ncreases the delvery rato. Under te-varyng condtons the algorth s able to react quckly and adapt the duty-cycle of sngle nodes to the new operatng condtons whle keepng a consstent sleep schedule aong sensor nodes. I. INTRODUCTION A wreless sensor network (WSN) conssts n a large nuber of tny sensor nodes deployed over a geographcal area. Each node s a low-power devce that ntegrates coputng, wreless councaton, and sensng capabltes. Sensor nodes are thus able to sense physcal envronental nforaton (e.g., teperature, hudty, vbratons, acceleratons) and process the acqured data locally, or send the to one or ore collecton ponts, usually referred to as snks or base statons. Hence, WSNs can be vewed as an ntellgent dstrbuted nstruentaton, whch can be effectvely used n any dfferent contexts []. The set of potental WSN applcatons s extreely large. However, ontorng applcatons can partcularly beneft fro sensor networks as they allow a long-ter data collecton at scales and resolutons that are dffcult, f not possble, to acheve wth tradtonal technques [29]. Typcally, data loggers are used to collect data. These devces are large and expensve, and requre a recordng and analyss equpent n place. Ths not only lt the accuracy of the recorded data, but can also dsturb the natural behavor of the observed phenoena. In addton, these devces consue a lot of energy and requre frequent huan nterventons. Instead, sensor nodes are very sall and cheap devces so that they can be eployed for fne-graned data collecton. They can self organze nto sensor networks and cooperate to perfor an assgned task wthout huan nterventon. Ths akes WSNs partcularly sutable for scenaros where the huan presence s dangerous or possble (.e. a checally contanated feld). In addton, the ease of deployent and the ablty of unattended operatons ake WSNs capable to replace ordnary nstruentaton also n ndustral, edcal and agrcultural scenaros. In recent years the nuber of sensor network deployents for real-lfe applcatons has rapdly grown up and, based on recent studes [33] [34], ths trend s expected to ncrease draatcally n the next years, anly n the felds of logstcs, autoaton and control. However, energy consupton stll reans the an obstacle to the dffuson of ths technology, especally n applcaton scenaros where a long network lfete s requred. The key ssue s that sensor nodes are generally powered by batteres whch have lted capacty and, often, cannot be replaced nor recharged, due to envronental or cost constrants. Therefore, effcent energy conservaton strateges ust be devsed at sensor nodes n order to prolong the network lfete. If we break down the energy expendture of a sensor node we can see that the rado subsyste typcally consues uch ore than the sensng and processng coponents. In addton, whle beng dle, the rado transcever consues approxately the sae power as n the transt or receve odes [38]. On the other hand, t consues sgnfcantly less power when t s put n sleep (low power) ode. Thus, the ost effectve approach to energy conservaton s dutycyclng, whch conssts n puttng the rado n sleep ode durng dle perods. Sensor nodes alternate between sleep and wakeup perods, and they have to coordnate each other by agreeng on an approprate sleep schedule n order to ake councaton feasble and effcent []. Unfortunately, desgnng effcent duty-cyclng schees s not straghtforward. Frst, duty-cyclng ntroduces addtonal Ths work s funded partally by the European County n the fraework of the Meory proect, and partally by the Italan Mnstry for Educaton and Scentfc Research (MIUR) n the fraework of the FIRB ArtDeco proect. Guseppe Anastas s wth Dept. of Inforaton Engneerng, Unv. of Psa, Italy, (e-al: g.anastas@et.unp.t) Marco Cont s wth IIT-CNR, Psa, Italy (e-al: arco.cont@t.cnr.t) Maro D Francesco s wth Dept. of Inforaton Engneerng, Unv. of Psa, Italy, (e-al: aro.dfrancesco@et.unp.t)

2 delays n the essage delvery process, as essages cannot be transtted untl the next hop wakes up. Moreover, latency requreents are hghly dependent on the applcaton. For exaple, obect trackng or event detecton requre quck response to the observed phenoena, so hgh latences ay not be tolerated. Desgnng energy effcent solutons whch at the sae te provde low latency n essage delvery s thus a challengng task. Second, ost duty-cyclng schees use fxed paraeters..e., the wakeup and sleep perods are defned before the deployent and cannot be changed durng the operatonal phase. Fxed duty-cyclng schees requre rather sple coordnaton echanss but, typcally, have non-optal perforance. Adaptve schees are thus requred to adust the sleep/wakeup perods, dependng on the observed operatng condtons. In ths paper we present an Adaptve Staggered sleep Protocol (ASLEEP) whch autoatcally adusts the actvty of sensor nodes, achevng both low power consupton and low essage latency. Ths protocol s targeted to data collecton applcatons (e.g. envronental ontorng [29], [45]), n whch sensor nodes have to perodcally report to a snk node. Wth respect to other slar approaches, our schee provdes two an advantages. Frst, t s not ted to any partcular MAC (Medu Access Control) protocol, so that t can be used wth any sensor platfor. Second, t s able to quckly adapt sleep/wakeup perods of each sngle node to the actual operatng condtons (e.g., traffc deand, network congeston, lnk qualty, node densty etc.), resultng n a better utlzaton of the energy resources, hence n a longer network lfete as well. A prelnary sulaton analyss, carred out n a few representatve scenaros, has shown that the perforance of ASLEEP s very prosng [4]. In ths paper we extend the analyss to ore general scenaros where sensor nodes are randoly deployed. In addton, we present a detaled analyss of the perforance durng steady state operatons. Our fndngs are also supported by soe experental results obtaned fro easureents carred out n a real testbed [3]. Both sulaton and experental results show that, thanks to ts flexblty, ASLEEP largely outperfors coonly used fxed duty-cyclng schees n ters of energy effcency, essage latency, and delvery rato as well. Hence, ASLEEP turns out to be a sgnfcant proveent n the context of ontorng, akng thus possble a long-ter deployent of WSNs. The reander of ths paper s organzed as follows. Secton II surveys the related work. Secton III ntroduces the reference syste odel and outlnes the an desgn prncples. Secton IV descrbes the ASLEEP protocol. Secton V presents the sulaton setup and the results. Secton VI descrbes a prototype pleentaton of ASLEEP n a real testbed and copares experental and sulaton results. Fnally, Secton VII concludes the paper. II. RELATED WORK A very large nuber of energy conservaton schees for WSNs have been proposed n the last years. The reader can refer to [5] for a detaled survey on the ost relevant proposals. In the followng we wll focus on duty-cyclng,.e., technques aed at nzng the energy consupton of a sensor node by adaptvely swtchng off the rado subsyste durng dle tes. Accordng to [], duty-cyclng can be acheved through two dfferent (and copleentary) approaches: topology control and power anageent. Topology control protocols [8], [9] explot nodes redundancy and adaptvely actvate the nu subset of nodes whch allow network connectvty. Nodes whch are not currently needed for connectvty can swtch off ther rado and save energy. Ths ncreases the network lfete by a factor (typcally n the order of 2-3) that depends on the degree of redundancy. The reader can refer to [2] and [4] for detaled surveys on topology control protocols. However, even nodes selected by the topology control protocol do not need to rean actve all the te. Instead, they can swtch off ther rado when there s no network actvty, thus alternatng between sleep and wakeup perods. Power anageent protocols are aed at coordnatng the sleep perods of neghborng nodes approprately, so as to allow councaton even n presence of a very low duty-cycle. As sensor nodes actvty s typcally very lted, power anageent protocols can reduce the energy consued by a node to soe percent (wth respect to the case wthout power anageent), thus ncreasng sgnfcantly the network lfete. In the followng we wll focus on power anageent protocols. Power anageent can be pleented ether at the MAC layer by ntegratng a duty-cyclng schee wthn the MAC protocol or as an ndependent sleep/wakeup protocol on top of the MAC layer (e.g., at the network or applcaton layer). The frst knd of approach allows to optze edu access functons based on the specfc sleep/wakeup pattern used for power anageent. On the other hand, general sleep/wakeup protocols pert a greater flexblty as they can be talored to the applcaton needs, and, n prncple, can be used wth any MAC protocol. The soluton proposed n ths paper belongs to the class of ndependent sleep/wakeup protocols. We detal the two classes of power anageent technques below. II.A MAC protocols wth a low duty-cycle Several coon MAC protocols for wreless sensor networks nclude a duty-cyclng schee for energy savng. For exaple, B-MAC (Berkeley MAC) [37] defnes a dutycycle through a channel saplng technque called Low Power Lstenng (LPL) and based on essages wth long preables. Nodes wake up perodcally (.e., every check nterval) to check the channel for actvty, and rean awake as long as the channel s beng used. The preable duraton of each B-MAC frae s at least equal to the duraton of the check nterval, so that each node can always detect an ongong transsson when t wakes up. The S-MAC (Sensor- MAC) protocol [49], nstead, defnes a dstrbuted schedule dssenaton schee n order to for vrtual clusters,.e. set of nodes whch agree on the sae sleep schedule. The channel access te s splt n a lsten perod (nodes exchange sync packets to coordnate ther sleep/wakeup perods and 2

3 specal control packets for collson avodance) and a data transfer perod. Nodes not concerned wth the councaton process can sleep untl the next lsten perod. The IEEE MAC protocol [7] supports a beaconenabled ode based on a superfrae structure. Each superfrae conssts of an actve perod and an nactve perod. In the actve perod sensor nodes councate wth the coordnator node they assocated wth. Durng the nactve perod nodes enter a low power ode to save energy. All the above energy conservaton schees are statc. An proveent over the aforeentoned solutons s represented by MAC protocols wth an adaptve duty-cycle schee. T-MAC (Teout MAC) [], provdes a teoutbased echans to talor the actvty perod of nodes to ther actual needs. Also a refneent of S-MAC proposed n [5] ncludes an adaptve lstenng echans whch can adapt the actvty of nodes to sudden changes n the network traffc. Although they present a basc for of adaptaton, these protocols suffer fro an addtonal latency n essage forwardng. Ths sleep latency s ntroduced because a forwardng node has to wat untl the next hop wakes up before transttng a essage, and t ncreases wth the nuber of hops. Ths s a proble coon to all protocols operatng at the MAC layer. The ost effectve way to overcoe ths ssue conssts n explotng upper layer nforaton as n DMAC [25], whch s an adaptve dutycycle MAC protocol optzed for sensor-network routng trees. DMAC explots the knowledge of the topology n order to stagger nodes schedules accordng to ther poston n the routng tree. However, to the best of our knowledge, t has not been pleented on real sensor platfors. Another approach for pleentng duty-cyclng at the MAC layer conssts n usng a TDMA (Te Dvson Multple Access) approach for channel access [4], [2], [3], [22], [39]. In such schees te s dvded n slots that are assgned to nodes accordng to a certan algorth. Slotted schees are nherently energy effcent as nodes rean actve only durng slots assgned to the. On the other hand, they have a nuber of drawbacks that lt ther usage n real WSNs [4]: () they lack flexblty n adaptng to topology changes caused by te-varyng channel condtons, physcal envronental changes, nodes that run out of energy, and so on; () they have lted scalablty, () they requre tght synchronzaton aong network nodes, whch ntroduces a overhead n ters of control-essage exchange and, thus, addtonal energy consupton, (v) fndng an nterferencefree schedule s a very hard task snce nterference ranges are typcally larger than transsson ranges,.e., any network nodes ay nterfere even f they are not n the transsson range of each other [2]. In concluson, duty-cycled MAC protocols allow the desgner to optze the channel access fro an energy conservaton perspectve. However, they lack flexblty as a specfc MAC protocol could not be always used n an actual sensor platfor. In addton, t ght be unable to explot nforaton ade avalable by the applcaton. On the other sde, general sleep/wakeup protocols are ore flexble as they can be used on top of dfferent MAC protocols. In addton, they can better explot applcaton-specfc nforaton. II.B General sleep/wakeup schees We can broadly classfy general sleep/wakeup schees nto three an categores: on deand, asynchronous and scheduled rendezvous schees. On-deand schees assue that destnaton nodes can be awakened soehow ust before recevng data. To ths end, two dfferent rado transcevers are typcally used [42], [48]. The frst rado (data rado) s used durng the regular packet exchange, whle the second one (wakeup rado) s a very low-power rado whch s used to awake a target node when needed. These schees can acheve a very hgh energy effcency and a very low latency. However, they cannot be always used n practce because coonly avalable sensor platfors only have one rado. In addton, the wakeup rado has typcally a transsson range sgnfcantly shorter than the data rado. A dfferent opton s usng an asynchronous schee [9], [35], [48], [5]. In ths case a node can ust wakeup whenever t wants and stll be able to councate wth ts neghbors. Although beng robust and easy to pleent, asynchronous schees generally present hgh latency n essage forwardng and have probles wth broadcast traffc. The last class of general sleep/wakeup protocols s represented by scheduled rendezvous schees, whch requre that nodes are synchronzed and all neghborng nodes wake up at the sae te. Our ASLEEP protocol belongs to the last category. By focusng on scheduled rendezvous schees, a possble approach conssts n establshng a coarse-graned TDMA schedule defned at the applcaton layer, and explotng an underlyng MAC protocol for actual data transfer. Ths approach s used by Flexble Power Schedulng (FPS) [5], [6], whch ncludes an on-deand reservaton echans capable to dynacally adapt to traffc deands. Snce slots are relatvely large a strct synchronzaton aong nodes s not requred. However, FPS borrows soe drawbacks [4] fro TDMA schees,.e. t has lted scalablty and flexblty n adaptng to traffc and topology changes. Most solutons followng a scheduled rendez-vous approach use plan duty-cycle based schees. For nstance, the wellknown TnyDB query processng syste [46] nclude a sleep/wakeup schee based on a fxed duty-cycle. All sensor nodes n the network wake up at the sae nstant and rean actve for a fxed te nterval. An proveent over ths sple approach s the staggered schee ncluded n TAG (Tny AGgregaton) [27], whch reles on a routng tree rooted at the snk node. In ths schee the actve tes of sensor nodes are staggered accordng to ther poston n the routng tree. Nodes located at dfferent levels of the routng tree wake up at dfferent, progressve tes, lke n a ppelne. Due to ts nce propertes ths schee has been consdered and/or analyzed n any subsequent papers ([7], [24], [25], [26], [3] aong others). In partcular, the authors of [2] analyze the perforance of the TAG staggered approach as well as several ts varants. Although provdng a basc for of adaptaton (wakeup tes are staggered to the network topology), ths schee s not 3

4 able to react to varyng operatng condtons as actve tes are fxed and equal for all nodes n the networks. Ths constrant splfes the coordnaton aong nodes, but results n low energy effcency and hgh essage latency. Lke TAG, our proposal leverage a staggered approach. However, n our proposal nodes actve perods are dynacally adapted to the observed network condtons and can be talored to the actual needs. By nzng the actve perod of each sngle node, our adaptve protocol (sgnfcantly) ncreases network lfete and reduces essage latency. III. NETWORK MODEL AND DESIGN PRINCIPLES In the followng we wll refer to a data collecton scenaro where data typcally flow fro source nodes to the snk, whle data fro the snk to the sources are uch less frequent. We wll assue that nodes are organzed to for a logcal routng tree (or data gatherng tree) rooted at the snk and used for data forwardng. Ths s very coon n practce as any popular routng protocols rely on a routng tree [8], [25], [27], [3], [43], [46]. The routng tree ay change over te due to lnk falures, node falures, or nodes runnng out of energy. Also, t ay be re-coputed perodcally to better share energy consupton aong nodes. However, as nodes are assued to be statc, we wll assue that the routng tree once establshed reans stable for a reasonable aount of te. Note that ASLEEP do not requre a specfc algorth to buld the routng tree. In the above network odel, nodes can acheve both low energy consupton and low latency n transferrng data to the snk f ther actve perods are staggered accordng to the poston along the routng tree. Nodes at dfferent tree levels wake up at progressve tes startng fro leaf nodes and proceedng upward to the root (.e., snk). Ths optzes the latency experenced by essages flowng fro source nodes to the snk [2]. In [2] t s shown that essages delvered to the snk experence a axu latency gven by the su of actve perods of all traversed nodes, whle essages flowng n the opposte drecton can traverse only one level per data reportng perod. However, to nze the energy consupton and essage latency n the above staggered schee, the followng desgn prncples should also be taken nto account. A sensor node should rean actve for the nu te requred for recevng data fro all ts chldren and forwardng the to ts parent. Accordng to [2], ths nzes both energy consupton and essage latency. Snce the optal actve perod depends on the network operatng condtons, t should be adusted dynacally. The optal actve perod should be chosen on an ndvdual bass. As sensor nodes located at dfferent locatons typcally handle dfferent aounts of traffc and experence dfferent network condtons, the optal actve perod should be talored to each ndvdual sensor node. The algorth for decdng the node s actve perod should be local and sple. Ths further reduces energy consupton at sensor nodes, because a global algorth would requre the exchange of nforaton aong nodes. In addton, a coplex algorth ght not be sutable for sensor nodes wth lted coputatonal capacty. Protocol operatons should ensure a consstent networkwde schedule when local condtons change. A varaton n the actve perod of a sngle sensor node should not coprose the correctness and energy effcency of the global schedule. As a consequence, a robust cooperaton echans s requred for nodes to anage the networkwde sleep schedule. Our adaptve sleep protocol descrbed below addresses all the above desgn prncples. In fact, t s based on a staggered schee, and allows each sngle sensor node to adust dynacally ts own actve perod based on local easureents of the current network actvty. Fnally, t ncludes a sleep coordnaton algorth to re-organze the global sleep schedule when a varaton n the talk nterval of a node occurs. IV. PROTOCOL DESCRIPTION In ths secton we present the ASLEEP protocol. After a general overvew, we wll descrbe the core coponents of the protocol,.e., the sleep predcton algorth each node uses for dynacally estatng ts expected actve perod, and the sleep coordnaton algorth used to propagate the new sleep schedule throughout the network. Fnally, we wll descrbe soe optzatons to prove the robustness and the effcency of the protocol. k IV.A TI Node status TI Awake Sleep CP Fgure. Sleep schedulng protocol paraeters. Protocol Overvew As entoned n Secton III, sensor nodes are assued to for a logcal routng tree (rooted at the snk) for data forwardng, whch s recoputed perodcally. Nodes clocks are assued to be synchronzed by soe te synchronzaton protocol (e.g. [36]). The councaton between a parent and ts chldren occurs n councaton perods (CPs) that repeat perodcally. Each councaton perod ncludes an actve nterval (AI) durng whch nodes councate by usng the underlyng MAC protocol, and a slence nterval (SI) durng whch nodes turn ther rado off to save energy. As shown n Fgure, actve ntervals are staggered so that nodes at lower levels n the routng tree wake up earler than ther ancestors. Each (nteredate) sensor node spans ts actvty over two adacent talk ntervals (TI), the frst one wth ts chldren and the other one wth ts 4

5 parent. Throughout, we wll refer to the talk nterval shared by a generc node and all ts chldren, durng the -th councaton perod CP, as TI. The duraton of the talk nterval to be shared wth chldren n the next councaton perod s dynacally estated by each parent node, accordng to the algorth descrbed n Secton IV.B. Although parent nodes can ndependently set ther talk nterval, a collectve effort s needed for the schedule of the whole network to rean consstent and energy effcent. Hence, as a result of a change n the talk nterval of a sngle parent node, the network-wde schedule needs to be rearranged. Ths s accoplshed by approprately shftng actve ntervals of a nuber of nodes, so as to ensure that () the actve ntervals of all nodes are properly staggered, and () the two talk ntervals of each node are contguous (see Secton IV.C). Two specal essages, drect beacons and reverse beacons, are used for propagatng schedule paraeters to downstrea and upstrea nodes, respectvely. Drect beacons are broadcast by every parent node to all ts chldren durng each councaton perod. Instead, reverse beacons are sent n the opposte drecton.e., fro a chld to ts parent at any te durng the talk nterval. As drect beacon essages are crtcal for correctness, ASLEEP also ncludes echanss to () ncrease the probablty of successful recepton durng drect beacon transssons, and () enforce a correct (even f non-optal) behavor of nodes n case they ss a drect beacon. These echanss wll be dscussed n Secton IV.D. We antcpate here that, to ncrease the probablty of successful recepton, drect beacons are transtted n a specfc te nterval reserved only for drect beacon transsson (Beacon Perod). IV.B Talk Interval Predcton In the ASLEEP protocol a sleep schedule s bascally defned by the councaton perod and the talk nterval of each ndvdual node. The length of the councaton perod s closely related to the specfc applcaton and thus, t s a global paraeter specfed by the snk when dstrbutng the query. For exaple, the user can query the axu value of teperature, to be collected every two nutes. In ths case the councaton perod s set to two nutes by each sensor node. A varaton n the councaton perod corresponds to a odfcaton of the query,.e. the new nterval for the perodc data acquston. Choosng an approprate talk nterval s soewhat ore nvolved. Ideally, each parent node should set the talk nterval wth ts chldren to the nu te needed to successfully receve all essages fro all chldren. However, ths te depends on a nuber of factors such as nuber of essages to be receved, underlyng MAC protocol, channel condtons, degree of contenton, and so on. Furtherore, the nuber of essages to be receved depends on the nuber of chldren, the essage generaton rate at source nodes, and the network topology. Obvously, the snk has only the talk nterval wth ts chldren, whle leaf nodes have only the talk nterval wth ther parent. 5 Fro the above dscusson t clearly eerges that coputng the deal talk nterval would requre the global knowledge of the network. Moreover, ths value should be contnuously updated as the network topology and operatng condtons change over te. Snce such an approach s not practcal, we propose here an adaptve technque that approxates ths deal schee. Our approach lets every parent node choose ts own talk nterval wth ts chldren. The decson nvolves only local nforaton and, thus, t does not requre to know the network topology. In prncple, any algorth can be used to estate the expected talk nterval n the next councaton perod. We used the sple algorth dscussed below. Each parent node easures and stores the followng quanttes. Message nter-recepton te ( ). Ths s the dfference between the te nstants at whch two consecutve essages are correctly receved. Nuber of receved essages ( n pkt ). The total nuber of essages correctly receved n a sngle councaton perod. The te expected to get all essages sent by chldren n the ax next councaton perod s then estated as, ax where and n are the average nter-recepton te and pkt the axu nuber of receved essages over the last L councaton perods (observaton wndow), respectvely. Usng n s a conservatve choce to nze the essage ax pkt loss probablty. The above te nterval should be ncreased approprately to allow the parent node to send a drect beacon at the end of talk nterval. Fnally, to reduce the nuber of possble values, the expected talk nterval s dscretzed nto a nuber of te slots, whose duraton s denoted by q. Hence, the expected talk nterval for the ()-th councaton perod can be expressed as TI ax = cel( n + BP) / q) q, where BP est denotes the Beacon Perod,.e., a te nterval reserved for beacon transsson only (see Secton IV.D.), and q denotes the te slot. The q value should be chosen as a trade-off between effcency and stablty. Fro one hand, a low q value allows a fne granularty n settng the talk nterval duraton, but ay ntroduce frequent changes n the sleep schedule. On the other hand, a large q value akes the schedule ore stable, but ay lead to talk ntervals larger than necessary, thus wastng energy. It ay be worthwhle notng that the expected talk nterval cannot be lower than one slot. Ths guarantees that any chld has always a chance to send essages to ts even after a phases durng whch t had no traffc to send. + Advertsng TI est to chldren as the next talk nterval ght lead to soe flappng of the protocol paraeters. To sooth the varaton of estates, the talk nterval for the next councaton perod, g s deterned as follows. If TI + TI g then TI s edately set to the est + estated value (.e., + TI = TI est ). If the predcted talk nterval s below the current value and the dfference s greater than, or equal to a guard threshold g down 2q (.e., pkt n pkt

6 TI TI est + g ) then the talk nterval s decreased by down ust one te slot ( TI + = TI q ). Fnally, f < TI TI est < g down, the talk nterval s not edately decreased. When the sae condton perssts for a nuber L down of councaton perods, then the talk nterval s reduced anyway. Accordng to the above rules, an ncrease n the talk nterval s anaged less conservatvely than a decrease. Ths s because an aggressve ncrease tends to nze the probablty that a node can ss essages fro ts chldren. IV.C Sleep Coordnaton As antcpated n Secton IV.A, the sleep coordnaton algorth s based on two specal essages (drect and reverse beacons), and the sleep schedule re-arrangeent after a talk nterval varaton s accoplshed by approprately shftng node s talk ntervals so as to ensure that the network-wde schedule reans consstent. Drect beacons are broadcast at each councaton perod by every parent node durng the Beacon Perod,.e. at the end of the talk nterval wth ts chldren. They nclude the schedule paraeters for the next councaton perod. Specfcally, the drect beacon sent by a node n the -th councaton perod contans: the length of the next councaton perod CP ; ts next wakeup te t ; the length of the next talk nterval to be shared wth ts chldren ( TI ). Conversely, reverse beacons are sent by chld nodes. They ay be sent at any te durng the talk nterval, and only nclude the aount of te the talk nterval of the parent node has to be shfted. As schedules are local, nodes only have to coordnate wth ther.e. they have to know the wakeup te of ther and use t as a bass for establshng schedules wth ther chldren. Let s dscuss now the operatons perfored by ASLEEP when the operatng condtons change. The nterested reader can refer to the Appendx A for a n-depth descrpton of the algorth and ts pseudo-code. In the followng, we wll descrbe the an operatons perfored durng talk nterval changes. There are two optons: a talk nterval ncrease or a talk nterval reducton. In both cases, ASLEEP enters a transent phase whch s requred to propagate the new schedulng paraeters and ensure that the new network-wde schedule s consstent and energy-effcent. Recall that a generc node has frst the talk nterval wth ts chldren, and then the talk nterval wth ts parent. As a consequence, a node advertses schedule paraeters to ts chldren before recevng the updated nforaton cong fro the parent. Hence, there ght be the case n whch the two talk ntervals (the one wth the chldren and the other wth the parent) are not adacent. In such a stuaton, the node nserts a pause perod, n whch t cannot councate wth any other node. Ths ensure that the new (transent) schedules are consstently 6 handled durng transent schedule propagaton. Note that nodes can go to sleep durng the pause perod (f t s long enough to ake t convenent to swtch off and on agan). In addton, pause perods are only used durng the transent phase, so that they are not present n steady state condtons. Further detals are gven n the followng dscusson. l l n+3 n+3 k n+2 k k n+ (level) n+2 n+ (level) n n Key Actve perod Sleep perod Pause perod Beacons CP CP (a) Talk nterval reducton CP CP CP 2 (b) Talk nterval ncrease Fgure 2. Talk nterval adaptaton exaples 2 3 CP 2 When a node shortens the talk nterval wth ts chldren, t also defers ts actvaton by a perod correspondng to the dfference between the prevous talk nterval and the new one. To better understand, let s consder the case llustrated n Fgure 2a. Let s suppose that durng the -th councaton perod a node at the (n+)-th level has decded to reduce ts forthcong talk ntervals wth ts chldren,.e., nodes at the (n+2)-th level (ncludng k). In the -th councaton perod node announces the next talknterval duraton to ts chldren by eans of the drect beacon (). The chldren receve the drect beacon and wat for the next councaton perod CP to nfor ther chldren.e., nodes at the (n+3)-th level of the new schedulng paraeters (2). Because of ths, nodes at the (n+2)-th level ntroduce a pause perod between ther talk nterval wth ther parent and the other wth ther chldren. Ths behavor ensures that nodes at the (n+3)-th level (e.g., node l) do not lose coordnaton wth ther because they have already sent the nforaton about ther wakeup tes. The above actons are repeated by nodes at the (n+3)-th level and ther descendants (f any) n the next councaton perods (3). Therefore, the pause perod shfts to lower levels one councaton perod at a te. Hence, a new steady state schedule s reached after a nuber of councaton perods equal to the depth of the subtree rooted at the node orgnatng the new paraeters. Note that only the descendants of the node whch reduces the talk nterval are affected by the transent phase through the pause perod. The other nodes ust operate wth ther own paraeters as usual. A slar approach s eployed also when a node ncreases the talk nterval wth ts chldren. In ths case, the node has to force ts ancestors to defer ther talk ntervals, n order to accoodate the addtonal te requred for councaton. To ths end, the node akes use of reverse beacons, whch are sent to ts parent and forwarded up to the tree untl the snk node s reached. Note that ths step s

7 requred to ensure the correctness of the protocol,.e. that the talk ntervals of nteredate nodes do not overlap. As above, the exaple depcted n Fgure 2b wll help us understandng. Let s suppose that node k at the (n+2)-th level of the tree decdes to ncrease the talk nterval wth ts chldren,.e., nodes at the (n+3)-th level (ncludng node l). Frst, node k advertses the new talk nterval to ts chldren through the drect beacon (). Second, n the sae councaton perod, the node sends the reverse beacon (2) to ts parent at the (n+)-th level, to force a talk nterval shft ahead n te. Node receves the reverse beacon, adusts the paraeters for the next councaton perod and advertses the, va the drect beacon (3), to ts chldren. Because all ancestors have to shft ther talk nterval, node also propagates the reverse beacon (4) up to ts parent at the n-th level. Note that n ths case the schedule propagaton pacts all nodes n the network. In fact, asde fro the ancestors of the node whch ncreases ts talk nterval, also other nodes can be nvolved n a transent phase whch ay requre the ntroducton of a pause perod. For nstance, consder a node at the (n+)-th level of the tree (llustrated n the second row of the schee n the fgure) whch s not a drect ancestor of the node orgnatng the new schedule. Its parent (.e. node ) wll shft ahead and advertse the new talk nterval nforaton durng the -th councaton perod. For reasons slar to the talk nterval reducton, a pause perod s ntroduced n the subtree rooted at nodes,.e. the nodes below the n-th level of the tree whch are not drect ancestors of node k (whch orgnated the new schedule). Assung that () nodes clocks are properly synchronzed, and () drect and reverse beacons never get lost, the followng propertes hold (the correspondng proofs are gven n Appendx B): Property (Schedule agreeent). Chld nodes wake up at the nstant, and for the duraton, enforced by ther even when talk ntervals change. Property 2 (Non overlappng schedules). For any couple of nodes and such that s a chld of, the talk ntervals TI and TI are not overlapped. Property 3 (Adacent schedules). In steady state condtons, the talk ntervals shared by any node wth ts chldren and ts respectvely, are contguous. The above propertes guarantee that, after a change has occurred n one or ore talk ntervals, the global sensor network s able to reach a new coordnated and energyeffcent schedule. In partcular, Property guarantees that actvty tes of a parent and ts chldren are coordnated even after a schedule varaton. Property 2 ensures that talk ntervals of dfferent parent nodes rean properly staggered. Fnally, Property 3 guarantees that, n steady state condtons, each sensor node wakes up and goes to sleep ust once per councaton perod. The above propertes hold under assuptons () and (). Clock synchronzaton s beyond the scope of ths paper and can be acheved though any avalable clock synchronzaton protocol, e.g. the protocol descrbed n [36]. Note that, ASLEEP operates on top of the MAC layer and use coarsegraned te paraeters, so that a tght synchronzaton aong nodes s not requred. Assupton () above s rather strong and unlkely to hold n practce snce beacons can get lost due to transsson errors and/or collsons. To overcoe ths proble we devsed a Beacon Protecton echans to ncrease the probablty of successful beacon transsson, and a Beacon Loss Copensaton echans to offset the negatve effects of beacon losses. We show below that, thanks to the latter echans, ASLEEP s able to antan a correct schedule, even n the presence of beacon losses, at the cost of a decreased perforance n ters of energy effcency and delvery rato. IV.D Schedule robustness The ths secton we descrbe the addtonal echanss devsed for provng the protocol robustness. ) Beacon Protecton Beacon essages are crtcal for the protocol correctness. When a node sses a drect beacon contanng the new paraeters, t cannot schedule ts actvty for the next councaton perod. In addton, untl re-acqurng the correct schedule nforaton, the node cannot send drect beacons to ts chldren. As a consequence, the loss of coordnaton propagates along the routng tree to the descendants of the node whch s out of coordnaton. Drect beacons ay get lost, for exaple, due to collsons wth other beacons or regular essages transtted by nterferng nodes. In addton, as drect beacons are broadcast essages, echanss typcally used for enhancng the councaton relablty (.e. retransssons) cannot be used. Reverse beacons ay be lost as well. However, as they are uncast essages, they are retranstted by the MAC protocol. To add robustness to the drect beacon transsson and prevent collsons, the last part of the talk nterval referred to as Beacon Perod s reserved for the drect beacon transsson only. Chld nodes ust refran fro ntatng regular essage transssons durng the Beacon Perod. In addton, the transsson of the drect beacon s ntated wth a rando backoff delay. Fnally, two copes of the drect beacon are transtted back to back. 2) Beacon Loss Copensaton The Beacon Protecton echanss ncreases the probablty that a drect beacon s successfully receved by ts chldren, but does not elnate the proble of beacon losses. Therefore, we have also devsed the followng copensaton echans to overcoe the negatve effects of a drect beacon loss. Snce talk ntervals typcally rean constant for a nuber of councaton perods, nodes use the current schedule paraeters also for the next councaton perod when they ss a drect beacon. The predcted value s used only for a sngle councaton perod, n whch nodes expect to correctly receve a fresh beacon fro ther parent. If 7

8 ths s not the case, the node reans awake untl t reacqures a fresh beacon. Obvously, ths heurstc produces a correct schedule f the parent node has not vared the talk nterval n the eante, whch s true n alost all cases. Otherwse, t produces a non-optal behavor of the node (and ts descendants as well) for a lted nuber of councaton perods. The actual effect of a wrong predcton s dfferent, dependng whether the talk nterval has been ncreased or decreased. If a parent has reduced ts talk nterval, the chld node ust wakes up earler than the correct nstant, thus wastng soe energy and ncreasng the essage loss probablty. However, t reans awake untl the end of the councaton perod and, very lkely, receves a fresh drect beacon. On the other hand, f the talk nterval has been ncreased, accordng to old schedule paraeters, the chld node wakes up at the rght te but would go to sleep earler than the correct nstant. However, snce t ssed the drect beacon n the prevous councaton perod, t doesn t go to sleep untl t receves a fresh drect beacon. Thus, t s very lkely that t receves the new drect beacon alost edately. If ths s not the case, t wll rean actve untl a new drect beacon s receved. Despte ts splcty, ths copensaton echans s able to ensure a correct schedule even n the presence of drect beacon losses at the cost of an ncreased energy consupton and essage loss. There s also the possblty of ncorrect schedules due to the loss of a reverse beacon. In ths case the chld node ay wake up after the talk nterval wth the parent has elapsed. Hence, t s forced to rean awake untl a new drect beacon s receved. Fortunately, such extree stuatons occur rarely (as reverse beacons are uncast essages, they are typcally retranstted by the underlyng MAC protocol up to a axu nuber of tes). Nevertheless, ASLEEP s able to recover fro ths stuaton as well, at the cost of hgher energy expendture and ncreased essage lost. V. SIMULATION ANALYSIS To evaluate the perforance of the ASLEEP protocol, we pleented t n the ns2 sulaton tool [32]. We splt the analyss n two dstnct parts. In the frst part we nvestgated how the protocol reacts to changes n the operatng condtons. To ths a, we vared dynacally the network topology/traffc pattern and analyzed the response of the protocol. In the second part we analyzed the perforance of ASLEEP n steady state condtons. Sulaton setup In both parts of our analyss we referred to a network scenaro consstng of 3-5 nodes randoly deployed over a 5x5 2 area, wth the snk placed at the center of the sensng area. Each node generates a fxed nuber of essages per councaton perod, ndependent of ts poston on the routng tree. Ths scenaro corresponds to a rando deployent of sensor nodes over a gven area for perodc reportng of sensed data, whch s a typcal case n envronental ontorng applcatons. 8 As the ASLEEP protocol reles on a routng tree, we pleented the followng sple routng tree foraton algorth n our sulator. The snk starts the tree-buldng phase settng tself as the root and broadcastng an HELLO essage to neghborng nodes. Upon recevng an HELLO essage, a node sets the source node as ts wats for a rando backoff te, and forwards the HELLO essage wth an updated hop count. Nodes recevng HELLO essages fro several nodes, choose the one wth the nu hop count as ther parent. TABLE. OPERATIONAL PARAMETERS Paraeter Value Councaton Perod (CP) 3s Message rate sg/cp Message sze 2 bytes Observaton wndow (L) CPs TI te slot (q) s Beacon Perod 6 s TI decrease te threshold (L down) 5 CPs TI ncrease threshold (g down) 2q (2 s) In all our experents we used the IEEE /Zgbee MAC protocol n non-beacon enabled ode. We used the 2.4 GHz physcal layer and enabled MAC layer acknowledgeents. We set both the axu nuber of backoffs and retransssons to 8, n order to ncrease the probablty of successful essage transsson. The rado propagaton odel was two-way ground; the transsson range was set to 5 (accordng to the settngs n [52]), whle the carrer sense range was set to 3 (accordng to the odel presented n [2]). We carred out a prelnary sulaton analyss to tune paraeters such as the length of the teslot (q), the observaton wndow sze ( L ), and the adaptaton thresholds (L down and g 2 ). Unless stated otherwse, we used the sulaton paraeters shown n Table, and dd not consder any for of data aggregaton at nteredate nodes (.e., nteredate nodes forward all essages cong fro descendants to ther parent). For each scenaro, we generated dfferent rando topologes and, for each topology, perfored a sulaton run consstng of councaton perods. The results shown below are averaged over the dfferent topologes. We also show the related standard devatons. V.A Analyss n dynac condtons To nvestgate the ASLEEP behavor n dynac condtons we consdered the followng two knds of varatons n the operatng condtons. Traffc pattern varaton. In ths set of experents sensor nodes start generatng one essage per councaton perod. Then, after soe te, they ncrease the essage rate to 3 essages per councaton perod, and fnally they swtch back to the orgnal value. Ths scenaro ay occur when sensors are requested to report an event wth better fdelty (.e., ncludng addtonal physcal quanttes or usng a larger nuber of bts per saple) for a lted te. Topology varaton. These experents start wth an ntal confguraton where only one half of the nodes deployed

9 Talk nterval of the snk node Adaptaton for traffc varaton Councaton perod # Talk nterval of the snk node Councaton perod # Talk nterval of the snk node (s) Adaptaton for topology varaton Councaton perod # Talk nterval of the snk node (s) Councaton perod # TABLE 2. TRANSIENT TIME FOR TRAFFIC VARIATION (MEAN AND STD DEV) Metrc Transent (CPs) Up transent (CPs) 4. (3.) Down transent (CPs) 5.8 (2.2) TABLE 3. TRANSIENT TIME FOR TOPOLOGY VARIATION (MEAN AND STD DEV) Metrc Transent (CPs) Up transent (CPs).3 (.5) Fgure 3. Talk nterval adaptaton for traffc varatons n the sensng area report data. After soe te, also the reanng nodes start reportng data. Ths scenaro ay occur when addtonal nodes are requred to report data so as to observe the sensed phenoenon wth ncreased spatal resoluton. We consdered the followng perforance etrcs: Talk nterval. Plottng the talk nterval duraton over te provdes a graphcal representaton of the protocol s ablty to adapt to changng operatng condtons. Transent te duraton, defned as the nuber of councaton perods fro when the varaton occurs to when the new talk nterval stablzes. We consdered a talk nterval as stable when t reans constant for ore than L councaton perods. Ths etrc gves a easure of how quckly the protocol adapts to the new operatng condtons. ) Results In the frst set of experents we vared the traffc pattern. All nodes started generatng essage per councaton perod. After the 3 th councaton perod, the rate ncreased to 3 essages and, fnally, after the 4 th councaton perod, t reverted back to the ntal value. Fgure 3 shows the talk nterval shared by the snk and ts chldren as a functon of te, referred to a specfc topology (the trend s slar for the other topologes as well). Although the snk node ay be always on (typcally t s not energy constraned), ts talk nterval contrbutes to defne the actvty te of ts chldren whch are the ost loaded nodes n the networks (the overall actvty te of an nteredate node conssts of the talk ntervals wth ts chldren and parent). The trend of the talk nterval shown n Fgure 3 deserves a detaled dscusson. Intally, there s a sharp decrease n the talk nterval. Ths s because the talk-nterval predcton algorth takes an observaton wndow L before provdng the frst estate. Durng ths prelnary phase, a duy talk nterval s used so that nodes can refran fro beng Fgure 4. Talk nterval adaptaton for the topology varaton 9 always-on. In our experents we used an ntal talk nterval value of 2 seconds. Once the frst predcton s avalable ( L = n our experents), the estated value s set and, as shown n Fgure 3, reans constant untl the essage generaton ncreases to 3 essages per councaton perod. Then, the talk nterval quckly adapts to new traffc condtons,.e. t ust takes two councaton perods to reach the stablty. The newly selected talk nterval reans stable agan for about one hundred councaton perods. Then, t drops to a value whch s one slot greater than that before the varaton n essage generaton rate. Fnally, after an addtonal nuber L down of councaton perods, t reverts back to the orgnal value. Ths addtonal te s due to the heurstc used for estatng the talk nterval, whch s affected by the adaptaton thresholds. Recall that, due to the heurstc we use (cfr. Table ), the talk nterval s decreased by one slot when the estated talk nterval s two slots lower than the prevous value. Table 2 shows the duraton of the up and down transent te orgnated by the traffc varaton, averaged over the dfferent topologes. We can see that the up transent spans over about 4 councaton perods. Ths s strctly related to data propagaton process n a staggered schee. To reach the snk, the new schedule paraeters have to clb up the tree one level at councaton perod (recall that drect beacons advertse the paraeters for the next councaton perod). Apart fro the te requred to trgger adaptaton at the dfferent levels, the results show that the protocol reacts quckly to ncreases n the essage rate. Obvously, the actual values strongly depend on the gven topology whch, n our experents, changes at every run because nodes are randoly re-deployed at the begnnng of a new run. For exaple, t s clear that the transent te s condtoned by the nuber of levels n the tree, whch n our sulaton runs vares between 3 and 4. Ths also explans the relatvely hgh devaton n the obtaned results. On the other hand, we can see that the down transent te s longer,.e. about 6 councaton perods. Ths s the ont effect of two factors. Frst, the syste needs L councaton perods to copletely forget the prevous

10 traffc condtons (recall that the talk nterval s estated based on statstcs accuulated over the last L councaton perods, and L = n our case). Second, the adaptaton heurstc we have adopted s qute conservatve n reducng talk ntervals, snce t also ncludes the addtonal L down threshold (L down=5 n our experents). Clearly, ths heurstc s a aor drvng factor of the perforance n ters of transent tes. In the second set of experents we nvestgated how the algorth reacts to varatons n the network topology. In these experents, ntally only one half of the deployed sensor nodes (.e., 5 nodes) s actve and send essages to the snk. The other 5 nodes becoe actve only startng fro the 5 th councaton perod. Fgure 4 shows the varaton over te n the talk nterval shared by the snk and ts chldren n a representatve sulaton run. Agan, the resultng behavor atches the expected varaton, although n ths case the varaton s lted. Nevertheless the fgure shows the adaptve echans s effectve n ths scenaro as well. We can see that the rasng transent te s lower than n the prevous experent (Table 3). Ths s because the ncrease n the resultng traffc condtons s lted, so that the addtonal essages are lkely to ft n the last (unused) part of the current talk nterval. As a consequence, the ncong essages alost edately trgger the adaptaton. V.B Analyss n statonary condtons In ths secton we assess the perforance of ASLEEP under statonary operatng condtons, and copare t aganst other (non-adaptve) slar schees,.e. pleented above the MAC layer. Specfcally, we consder three addtonal schees, whch are shortly descrbed below. Always-on. In ths schee there s no duty-cycle: nodes are always actve and forward essages as soon as they receve the. Obvously, ths approach s never used n practce and s consdered here only for coparson purposes. TAG-lke staggered schee. Sensor nodes uses a staggered schee for sleep/wakeup perod. The talk nterval s fxed and equal for all sensor nodes. It s set to the value of used, for exaple, n TAG [27] where the talk nterval s calculated as the councaton perod dvded by the depth of the routng tree. Ths s the sae rule as n TAG [27]. Therefore, throughout we wll refer to such a schee as TAG. Fxed staggered schee. In ths schee the talk nterval s fxed and equal for all nodes, lke n the TAG schee above. However, we used a dfferent rule to calculate the value of the talk nterval. Ideally, t should be set to the nu te needed by any parent node to correctly receve all essages cong fro ts chldren. In practce the deal value cannot be known n advance, so that ths schee s thus unfeasble. In our experents, we set the talk nterval by usng the value estated by our ASLEEP protocol. In detal, we frst ran an experent wth our protocol and easured the axu talk nterval n that confguraton. Then, we replaced our protocol wth the fxed staggered schee and set all talk ntervals to the axu value easured before. The ratonale s to nvestgate the perforance of a fxed staggered schee when usng the sae paraeters as ASLEEP. For brevty, n the followng ths schee wll be referred to as Fxed. In all staggered schees (TAG, Fxed and ASLEEP) essages are assued to be generated ust before the begnnng of the talk nterval. To ake the coparson far, especally n ters of essage latency, when usng the Always-on schee we assued that essages are generated at the sae te nstants as n the Fxed schee. To copare the perforance of our protocol wth those of the above schees we consdered the followng perforance ndces. Average Duty-cycle, defned as the average fracton of te a node at one hop fro the snk reans actve. Ths ndex gves a easure of the energy consued by -hop sensor nodes. Snce all traffc orgnated by source nodes ust pass through these nodes, they deterne the network lfete [23]. Delvery rato, defned as the rato between the nuber of essages successfully receved by the snk and the total nuber of essages generated by all sensor nodes. Average essage latency, defned as the average between the essage generaton te at the source node and the recepton te of the sae essage at the snk. ) Results We started consderng the basc scenaro whose paraeter settngs have been specfed at the begnnng of Secton V (bascally, 3 nodes, 2-byte essages and no data aggregaton). Then, we vared the node densty, appled dfferent data aggregaton schees, and nvestgated the nfluence of each paraeter on the perforance of the dfferent duty-cyclng schees. As a prelnary reark, we have to ephasze that perforance ndces depends on the specfc paraeter settngs. Therefore, ther absolute values are of relatve nterest for us. What s really portant s to copare the perforance of the dfferent sleep/wakeup schees under the sae operatng condtons.