Analysis and Design of a MAC Protocol for Wireless Sensor etworks with Periodic Monitoring Applications

Transcription

1 Analysis and Design of a MAC roocol for Wireless Sensor eworks wih eriodic Monioring Applicaions Miguel A. razo, Yi Qian, Keie u, and Domingo Rodríguez Deparmen of lecrical and Compuer ngineering Universiy of uero Rico a Mayagüez Mayagüez, R 68 {miguel.erazo, yqian, lukeie, domingo}@ece.uprm.edu hone: , Fax: Absrac In his paper, we presen a new medium access conrol proocol (MAC) for wireless sensor neworks wih periodic monioring applicaions. In paricular, we design a lisen-sleep ime schedule for sensor nodes, and propose a pah algorihm for roue selecion. We perform deailed analysis and simulaions for he proposed proocol. Compared wih some previous sensor nework MAC proocols, we show ha our new proocol can reduce energy consumpion and packe delay, and a he same ime, drasically reduce collisions. Keywords: - wireless sensor neworks, medium access conrol, energy efficiency, delay, collisions. This research is suppored by he Naional Science Foundaion (NSF) under he gran CIS-CNS W I. INTRODUCTION ireless sensor neworks consis of baery-operaed sensor devices wih compuing, daa processing, and communicaion componens. xensive research has been conduced in his area due o he wide range of applicaions of wireless sensor neworks. nergy conservaion is one of he mos criical issues since baeries are he only energy source o power he sensor nodes. ike in all shared-medium neworks, medium access conrol (MAC) proocols enable he successful operaion of he nework. The MAC proocol in a wireless sensor nework mus achieve wo goals. The firs is he creaion of he nework infrasrucure. Since housands of sensor nodes are densely scaered in a sensor field, he MAC scheme mus esablish communicaion links for daa ransfer. This forms he basic infrasrucure needed for wireless communicaion hop by hop and gives he sensor nework self-organizing abiliy. The second obecive is o fairly and efficienly share communicaion resources beween sensor nodes. Consrains on energy resources made researchers look for energy-efficien MAC proocols. Several MAC proocols for wireless sensor neworks wih an ulimae goal of increasing he nework lifeime by conserving energy have been proposed in he lieraure. Our goal in his paper is o develop a new MAC proocol for wireless sensor neworks deployed for periodic monioring, e.g., environmenal monioring. In our scheme, he ase Saion (S) sars and mainains synchronizaion. Afer nodes are synchronized, hey only wake up on specific imes when a sample of a variable is aken. The main conribuions of his paper are design and analysis of he new MAC proocol for he wireless sensor neworks. In paricular: Design of a saggered lisen-sleep ime schedule where nodes only urn on heir radios when a sample is aken wih he obecive of consuming he leas possible amoun of energy. ropose an algorihm o make sure ha only one specific roue (pah) is acive a a ime. Wih his, we inend o lower delay, collisions and energy consumed for he sensor nodes. Comparison of he proposed new MAC performance wih some of he previous MAC proocols in erms of energy consumpion, delay and packe loss in simulaions. We show ha our new proocol reduces delay and collisions dramaically. In he remainder of he paper, a brief survey of relaed works is presened in Secion II. The descripion of our proocol is in Secion III. erformance analysis and simulaion resuls are presened in Secion IV. Finally, he conclusions are discussed in Secion V. II. RATD WORK Many proocols have been proposed for wireless sensor neworks. Mos of hem aim o achieve low energy consumpion in ransmiing packes beween nodes. These proocols also have he goals of low delay and minimum packe loss. razo e al. [] proposed SA-MAC, a medium access conrol (MAC) proocol for wireless sensor neworks (WSN) specialized for environmenal monioring applicaions. SA-MAC focused on reducing energy consumpion in environmenal monioring applicaions. Specifically, i reduces he duy cycle (DC) of nodes, lowering drasically idle lisening. Ye e al. [] proposed S-MAC, a MAC proocol designed for WSN. S-MAC uses a few novel echniques o reduce energy consumpion and suppor selfconfiguraion. Firs, nodes form virual clusers based on common sleep schedules o reduce conrol overhead and enable raffic-adapive wake-up. Second, S-MAC uses in-

2 channel signaling o avoid overhearing unnecessary raffic. Finally, S-MAC applies message passing o reduce conenion laency. Ye e al. [3] inroduced SC-MAC, a proocol which uses Scheduled Channel olling (SC) o achieve more energy savings han hose of proocols ha use coordinaed ransmissions and lisen periods. The conribuions of SC-MAC are he ulra low duy cycles i achieves and is capaciy o adap o variable raffic loads. Dam e al. [4] proposed T-MAC, a conenion-based Medium Access Conrol proocol for wireless sensor neworks. u e al. [5] proposed D-MAC, a proocol whose obecive is o achieve very low laency. Mainwaring e al. [6] provided an in-deph sudy of applying wireless sensor neworks o real-world habia monioring. A se of sysem design requiremens are developed ha cover he hardware design of he nodes, he design of he sensor nework, and he capabiliies for remoe daa access and managemen. In his paper we propose a MAC proocol o achieve low energy consumpion for periodic monioring applicaions. Our proocol akes advanage of he fac ha he raffic paern is periodic o achieve low energy consumpion levels. Furhermore, our approach focuses on reducing packe delay and collisions. III. ROTOCO DSIGN OVRVIW A. Overview of roposed roocol The obecives of he design of our new MAC proocol are he following: firs, lower energy consumpion by reducing duy cycle; second, reduce delay and collisions; and finally make synchronizaion simpler and unique in he whole nework. For he applicaions, we assume ha i is desirable o sample physical variables periodically. In his way, we know in advance how many samples mus be aken in an inerval of ime and he specific insans when hey are aken. In our proocol, nodes periodically wake up when a sample is aken. In his way, nodes save a lo of energy in avoiding idle lisening beween wo consecuive samples. In a wide area large scale nework i is likely ha packes experience a huge delay, paricularly when nodes are far away from he closes sink or ase Saion (S). For online monioring, i is desirable he delay o be as low as possible. For ha purpose, we use a saggered wakeup schedule as proposed in [5]. In ha scheme, nodes on a pah wake up sequenially o forward a packe o nex hop, so sleep delay is reduced. Also, we did no use he RTS/CTS mechanism for ransmiing packes whose desinaion is he S. Tha reduces delay even more. If he densiy of nodes wihin a nework is high, collisions are very likely o occur. In order o reduce collisions, we propose an algorihm which compues ime schedules ha sensor nodes follow in a way ha collisions should no occur. Synchronizaion for our proocol has been designed o be simple and robus. We inend o lower he frequency of synchronizaion losses in he nework. Furhermore, he synchronizaion is unique for he whole nework because only he ase Saion sars and mainains synchronizaion while he oher nodes only broadcas synchronizaion packes generaed previously.. Time schedule We can idenify hree main communicaion paerns in sensor nework applicaions. The firs involves local daa exchange among nearby nodes. The second involves he dispach of conrol and synchronizaion packes from he sink o nodes. The hird and mos significan raffic paern in WSN is daa gahering from sensor nodes o sink. We have proposed delay and collision reducion echniques for he hird ype of raffic paern and creaed a separae acive ime slo for conrol packes. Our proocol uses a schedule in which nodes only wake up when a sample from environmen is aken. No periodic sleep/lisen schedule will be necessary as i was proposed in proocols like S-MAC [], T-MAC [4] or D-MAC [5]. Furhermore, i will no be necessary a shor wake-up one for senders o guaranee rendezvous as i was in SC- MAC [3] because nodes know in advance when hey have o urn on is radio o sample monioring variables. Time schedule of our proocol is shown in Figure. In our proposed ime schedule we sagger he schedule o lower he delay in our nework in similar way as proposed in [5]. Furhermore, we design our ime schedule in such a way ha all daa gahered from sensor nodes be delivered o he sink in us one acive period, i.e. a node would only wakeup once o receive and ransmi packes desined o he sink. Fig.. roposed ime schedule for wireless sensor nework Figure shows ha as a node approaches he sink, is receiving and ransmiing periods ge larger. This happens because he closer a node is o he sink, he more daa i receives from oher nodes and he more packes i has o relay. In our approach we do no consider aggregaion [7], which would reduce even more he receiving and ransmiing inervals. The ime schedule of a node in our proocol is shown in Figure. Wihin a period (T), a node has a slo o receive

3 packes ( ), anoher o ransmi packes ( ) and anoher o sleep ( S ). We denoe he duraion of an acive slo as which is he sum of and. TA I is of symbols used for schedule parameers Symbol Meaning, Duraion of ransmiing slo of node i wihin roue, Duraion of receiving slo of node i wihin roue Fig.. Time schedule parameers As saed above,, and S (schedule parameers) do no have o ge he same value for all nodes wihin a roue. Their values depend on he posiion of a node wihin a paricular roue. Figure 3 shows how we characerize a roue wihin a nework o deermine he value of he schedule parameers. S, R Duraion of sleeping slo of node i wihin roue Size of roue C. Roue pariion Time o Tx/Rx a bye Daa packe lengh In order o lower delay even more and reduce collisions, we propose a mechanism ha called roue pariion. Roue pariion is a mechanism horough which only one roue is acive a a ime. When only one roue is acive a a ime and using he saggered ime schedule described in, our proocol nullifies he hidden erminal problem wihou using he RTS/CTS packe exchange or he virual carrier sense []. Figure 4 shows an example of roue pariion. Fig. 3. Nodes are numbered according o is posiion from he sink Values of schedule parameers for a paricular node wihin a roue are given by he following expressions:,, S, = = = T, ( R i ) ( R i), () () (3) Table I summarizes erms used in above expressions. Fig. 4. Roue pariion mechanism Fig. 5. Time offse in ime schedule in a roue of size 3

4 In our proocol, he sink compues he ime schedule ha every node in he nework will follow. A node wihin he nework can be involved in more han one roue and consequenly will follow more han one schedule. Roue pariion has wo saes: iniializaion and mainenance. A he beginning of iniializaion sae, sensor nodes dispach roue adverisemen (RA) packes o he sink. As RA packes raverse he nework, each packe is added he address of each node i goes hrough. In his way, each RA originaed by a sensor node conains he complee roue from he node o he sink. Once he sink has received RA packes from all sensor nodes, i runs our proposed algorihm o compue ime schedules of all sensor nodes. Time schedules will be disseminaed hrough schedule disseminaion packes (SD). Upon receiving SD packes, nodes will sar o follow heir respecive ime schedule. As par of he mainenance sae, sensor nodes send RA packes and he sink broadcass SD packes periodically o find possible changes. Due o he saggered ime schedule ha we proposed in our proocol, nodes sar heir acive periods a differen imes depending on heir posiion wihin a roue. As shown in Figure, nodes ha are farher from he sink sar ransmiing earlier han hose closer o he sink. We denoe he offse in ime beween he sar of ransmission of furhes node from he sink in roue and he sar of ransmission of node i wihin he same roue as. φ Figure 5 depics he concep of offse wihin a roue. TA II Time schedule algorihm in sensor nodes TA III ah compuing algorihm in he sink

5 The sink, when broadcasing SD packes for a paricular roue, includes he sampling period (T), he ime when a node sars o follow he ime schedule of a paricular roue (TS), and he roue size node compues is own offse following expressions: R i( i ) φ = φ, ( i ) R R 3 R φ, =. Then, every sensor φ according o he (4) (5) Upon receiving SD packes, sensor nodes follow he algorihm proposed in Table II whereby ime schedule can be compued for each roue a sensor node belongs o. In Table II, r denoes a vecor in which every elemen r [i] is he address of he sensor node ha is i hops away from sink in roue. As saed in previous paragraphs, he sink receives RA packes from all sensor nodes wihin he nework. Consequenly, afer some ime inerval, he sink should have packes. Then, he sink builds up a marix of roues called R in which every column represens he pah from a sensor node o he sink. A paricular elemen of R, r is he address of a node ha is i- nodes away from he sink ha belongs o roue. The necessary assumpions for he algorihm o pariion roues, which will be run in he sink, are: Sensor nodes are fixed wihou mobiliy. Roue o he sink are durable and are esablished by a suiable rouing proocol. The sink is he only desinaion of daa packes. We denoe a pah as which is a se of all node addresses belonging o a column of R. In his way, = r i R, ( ). A pah k will be called a { } subpah of k if. In his case we say ha conains k and only should be broadcased in a SD packe. Table III shows he proposed algorihm o deec subpahs and eliminae hem for fuure broadcas in SD packes. Finally, he algorihm will oupu a marix R which will be used by he sink o broadcas all remaining useful roues in SD packes. D. nergy analysis We consider a nework of pahs which corresponds o he number of columns of marix R, which is he oupu of he algorihm described in Table III. We compue he energy consumed in he whole nework as he sum of he energy consumed in every pah. The expression for energy consumpion is: = ' R T i, = i= (6) In expression (6), is he energy consumed by node i in pah. The energy spen in each node is he energy consumed in ransmiing, receiving, and sleeping. ach erm can be expressed as he average power in ha sae muliplied by he ime he node is on ha sae. Table IV summarizes he erms we use. The expression for he energy consumed in he whole nework in a ime inerval T is: = = = Symbol x rx lisen sleep T daa T sync,, ( ( S S,, I,, i S,, ), i, S S, ) I TA IV is of symbols used in energy analysis Meaning ower in ransmiing ower in receiving ower in lisening ower in sleeping Time o Tx/Rx a bye Daa packe period Synchronizaion packe period r daa Daa packe rae (/T daa ) ID, r sync Synchronizaion packe rae (/T sync ) daa sync x rx cs sleep Daa packe lengh Synchronizaion packe lengh Average ime a node is ransmiing Average ime a node is receiving Average ime a node is on carrier sense Average ime a node is sleeping Replacing equaions () and () ino (7) we ge: = ( ( S S, ID, ( R i )) ( R i) (8) quaion (8) gives he energy consumpion in any sensor node. We also have he energy consumpion for he sink and for nodes ha are mos disan from he sink in every pah: = R (9) sin k ID, R = ID, () The sleeping ime ( S, ) in sensor nodes is no consan and depends on he posiion of he nodes wihin a pah and he number of pahs a sensor node belongs o wihin he nework. However, S is significanly less han or and consequenly we consider S, negligible for our (7)

6 purposes. Replacing equaions (8), (9) and () ino (6) we ge he following expression. T = = ' = ' = R i= R sin k ( ) R R ( ) R R I ( R ) ( )( ) R Taking average on boh sides we finally ge: [ T ] = ( R ( ) ( σ ( R R. The synchronizaion )) R R )( The sink is he only node ha can sar and mainain synchronizaion while he oher nodes only disseminae synchronizaion in a mulihop environmen. This ype of synchronizaion saves energy because every node only follows one global ime schedule. The sink dispaches synchronizaion packes a a synchronizaion rae r SYNC. To synchronize, sensor nodes do no follow he saggered ime schedule proposed o cope wih hird ype of communicaion raffic described briefly in subsecion. Insead, nodes se heir ime offse o and disseminae synchronizaion packes from he sink o he furhes node. Once sensor nodes have received he synchronizaion packe and have successfully relayed i o he nex node wihin a pah, hey go o sleep. Figure 6 shows he proposed ime schedule for synchronizaion. () ) () configuraions and oher 9 nodes are placed around he sink in a random locaion. We use AODV rouing proocol [9]. Figure 7 shows our simulaion configuraion. Fig.7. Simulaion configuraion Figures 8, 9 and show he resuls of simulaion for our proposed proocol in erms of energy consumpion, packe delay and packe loss rae. The ransmission inerval is he ime beween ransmissions of sensor nodes. nergy consumpion [J] Seconds nergy consumpion Transmission inerval [s] Fig. 8. nergy consumpion acke Delay 5 5 Transmission inerval [s] Fig. 9. acke delay Fig. 6. Synchronizaion ime schedule IV. XRIMNTA RSUTS A. Resuls of proposed proocol We have esed our proocol by simulaions using he nework simulaor ns- [8]. In he simulaion seup, we use nodes, where he sink is placed in he middle of he ercenage [%] acke loss rae Transmission inerval [s] Fig.. acke loss rae As expeced, he energy consumpion increases as he ransmission inerval decreases, as shown in Figure 8. Figure 9 shows ha he delay of daa packes in our proocol remains pracically consan for differen values of he ransmission inerval. Finally, Figure shows ha

7 a low percenage of daa packes are acually dropped for ransmission inervals less han 5 seconds. However, he packe loss rae is never more han %.. erformance comparison Figures, and 3 show he resuls of our proocol performance comparing wih S-MAC. lisening is reduced. Also, synchronizaion has been made unique in he whole nework in order o save energy by making nodes o follow only one source of synchronizaion. Simulaion resuls show ha our proocol achieves beer energy performance, packe delay and loss han hose of S-MAC. RFRNCS nergy consumpion [J] nergy consumpion 5 Transmission inerval [s] Fig.. nergy performance comparison acke Delay Our proocol SMAC- 5% [] M. razo, Y. Qian, SA-MAC: Simple nergy Aware MAC roocol for Wireless Sensor Neworks for nvironmenal Monioring, roceedings of ISWC 7, San Juan, R, February 5-7, 7. [] W. Ye, J. Heidemann, D. srin, Medium Access Conrol Wih Coordinaed Adapive Sleeping for Wireless Sensor Neworks, I/ACM Transacions on Neworking, Volume:, Issue: 3, ages:493-56, June 4. [3] W. Ye, F. Silva, J. Heidemann, Ulra-ow Duy Cycle MAC wih schedulled Channel olling, ACM SenSys 6, November, 6. Seconds ercenage [%] Transmission inerval [s] Fig.. acke delay comparison acke loss rae 5 5 Transmission inerval [s] Fig. 3. acke loss rae performance comparison Our proocol S-MAC 5% DC Our proocol S-MAC 5% DC The resuls show ha our proocol ouperforms S-MAC, in erms of energy consumpion. Furhermore, our proocol exhibis consan packe delay and ouperforms S-MAC. Finally, he packe loss rae of our proocol increases slighly as ransmission inerval decreases, while he packe loss rae of S-MAC increases dramaically. [4] T. van Dam and K. angendoen, An adapive energy-efficien mac proocol for wireless sensor neworks, roceedings of he Firs ACM Conference on mbedded Neworked Sensor Sysems, pages 7 8, os Angeles, California, USA, November 3. [5] G. u,. Krishnamachar C. S. Raghavendra, An Adapive nergy-fficien and ow-aency MAC for Daa Gahering in Wireless Sensor Neworks, roceedings of IDS 4, April 4. [6] A. Mainwaring, J. olasre, R. Szewczyk, D. Culler, J. Anderson, Wireless Sensor Neworks for Habia Monioring, Firs ACM Workshop on Wireless Sensor Neworks and Applicaions, Alana, GA, USA, Sepember 8,. [7] Z. and. Improving hroughpu in mulihop wireless neworks, I Transacions on Vehicular Technology, Vol.55, No.3, pp , May 6. [8] The ns- simulaor: hp:// [9] C.. erkins and. M. Royer, Ad hoc On-Demand Disance Vecor Rouing, roceedings of he nd I Workshop on Mobile Compuing Sysems and Applicaions, New Orleans, A, February 999, pp. 9-. V. CONCUSIONS In his paper we have proposed a new MAC proocol o reduce energy consumpion, delay and collisions for periodic monioring applicaions. Our proocol reduces drasically he energy consumpion by making nodes o wake up only when a sample is aken. In his way, idle