TRIDENT: In-field Connectivity Assessment for Wireless Sensor Networks

Transcription

1 TRIDENT: In-field Conneciviy Assessmen for Wireless Sensor Neworks Timofei Isomin 1, Ramona Marfievici 1, Amy L. Murphy 2, Gian Piero Picco 1 1 Universiy of Treno, Ialy firsname.lasname@unin.i 2 Bruno Kessler Foundaion, Treno, Ialy murphy@fbk.eu ABSTRACT Real-world deploymens of wireless sensor neworks (WSNs) are nooriously difficul o ge righ, parly due o he fac ha heir low-power wireless communicaion is grealy affeced by he characerisics of he arge environmen. An early in-field conneciviy assessmen of he laer provides fundamenal inpu o he WSN developmen. Unforunaely, sae-of-he-ar ools require a secondary neworking infrasrucure (e.g., USB cables) for gahering experimenal daa a luxury one can rarely afford in-field. We presen TRIDENT, a ool expressly designed o suppor infield conneciviy assessmen by relying only on he WSN nodes, wihou addiional infrasrucure. The ool is also designed o be easy o use, enabling domain expers (insead of WSN expers) o direcly configure conneciviy assessmen owards heir specific needs, wihou requiring coding. The ool is provided in wo varians, argeing TMoe Sky moes running TinyOS, and Waspmoes running he sandard ZigBee sack, covering popular plaforms in research and indusry, respecively. Caegories and Subjec Descripors C.4 [Performance of Sysems]: Measuremen echniques; C.2.1 [Nework Archiecure and Design]: Wireless communicaion Keywords , Link qualiy, Conneciviy assessmen 1. INTRODUCTION Wireless sensor neworks (WSNs) are finding heir way ino realworld applicaions, whose successful deploymens are increasingly repored. This accrued experience has also evidenced how he ask of deploying a WSN enails a number of challenges. One of he mos imporan concerns he low-power wireless communicaion exploied by WSNs. This ype of wireless communicaion has peculiar characerisics, ha have been sudied by many researchers; a summary is provided by a recen empirical sudy [11]. Moivaion. Communicaion in he 2.4 GHz band of IEEE , commonly employed by WSN devices, is also affeced by fac- Permission o make digial or hard copies of all or par of his work for personal or classroom use is graned wihou fee provided ha copies are no made or disribued for profi or commercial advanage and ha copies bear his noice and he full ciaion on he firs page. To copy oherwise, o republish, o pos on servers or o redisribue o liss, requires prior specific permission and/or a fee. ExremeCom 14, Augus 11-15, 214, Galapagos Islands, Ecuador. Copyrigh 214 ACM $15.. ors srongly dependen on he environmen where he WSN is deployed, and hard o capure in he conrolled seups ypically used in he sudies above. These facors include he shape of he deploymen sie [1], variaions in emperaure [3] and humidiy [14] and heir combinaion in oudoor [8, 15] or indusrial [4] scenarios. Figure 1 provides a concree idea of he exen o which communicaion can be affeced by hese facors, albei using a smallscale seup. A single link beween wo nodes communicaing a dbm and placed 4 m apar in an oudoor open field is observed over a 2-day period. Figure 1(a) shows he average emperaure inside he plasic boxes holding he moes; Figure 1(b) shows ha, for TMoe Sky devices, his emperaure is negaively correlaed wih he Received Signal Srengh Indicaor (RSSI ) and Packe Delivery Rae (PDR) of he link. A 4 C emperaure increase in he box (caused by a mere 11 C increase ouside of i) causes a 13 dbm decrease in RSSI. This is enough o urn a perfec link (PDR = 1%) ino a dead one (PDR = %). Assessing quaniaively he characerisics of low-power wireless links in-field, i.e., in he arge environmen where a WSN mus be deployed, is key for several, inerwined goals: supporing he WSN deploymen, by deermining where o place he moes o ensure communicaion among hem; informing he selecion (or design) of proocols, o ensure hey are well-suied o he arge environmen; deriving models, o push he envelope of wha can be prediced (or simulaed) beforehand. Temperaure (C) PDR (%) PDR (%) : : 4: 8: 12: 16: 2: : 4: 8: 12: 16: Time (a) Temperaure PDR RSSI : : 4: 8: 12: 16: 2: : 4: 8: 12: 16: Time (b) TMoe Sky PDR RSSI : : 4: 8: 12: 16: 2: : 4: 8: 12: 16: Time (c) Waspmoe. Figure 1: Effec of emperaure on RSSI and PDR. RSSI (dbm) RSSI (dbm)

2 Unforunaely, he ools supporing conneciviy assessmen (e.g., SWAT [13], SCALE [7], RadiaLE [2]) were conceived o sudy he properies of low-power wireless links in conrolled environmens, e.g., an office esbed. Therefore, hey require a secondary, wired neworking infrasrucure (e.g., USB cables) for gahering experimenal daa a luxury one can rarely afford in real-world seings. Conribuion. This paper presens TRIDENT [1], a ool expressly designed o simplify he chore of in-field conneciviy assessmen. TRIDENT relies only on he WSN nodes whose conneciviy needs o be ascerained, wihou any exernal infrasrucure. TRIDENT is useful o WSN researchers and praciioners, who may use i owards any of he hree aforemenioned goals. However, he ool is designed o be easy o use also for domain expers who, being he users, have precise knowledge of where he WSN nodes should be deployed from he applicaion sandpoin, bu ypically do no have a very deep knowledge abou he inner working of he WSN, and definiely do no ake par in programming i. The conneciviy experimens can be configured easily wihou requiring any coding, and he daa colleced wih a sraighforward procedure. Therefore, TRIDENT empowers domain expers wih he abiliy o assess he conneciviy of WSN nodes deployed in-field, wihou he need for a supporing WSN exper. The goals and requiremens we se for TRIDENT are described in Secion 2. TRIDENT covers he enire workflow concerned wih conneciviy assessmen experimens. Afer he WSN nodes are flashed wih he TRIDENT sofware, appropriae user inerfaces enable he operaor o (re)configure he experimen seings, discover nodes, and download he resuls. The laer are sored in a daabase, simplifying he sorage and analysis of he (ypically large volumes of) daa gahered. The main operaion of TRIDENT is described in Secion 3, along wih he various componens of he ool. We originally developed TRIDENT for he popular research-oriened plaform consiued by TMoe Sky moes running TinyOS. However, conneciviy assessmen is relevan also o indusry-oriened plaforms, for which we chose Waspmoe devices running he sandard ZigBee sack as a represenaive. Ineresingly, supporing he laer plaform is no simply a maer of poring he code from he former; he fac ha he ZigBee sack is closed, unlike he TinyOS one, forced us o find ways o reliably measure he main meric of PDR, which canno be derived direcly oherwise. Alhough boh plaforms are based on radio chips complian wih IEEE , heir behavior is very differen, as shown again in Figure 1. The same seup and observaions we described earlier were also performed for a link beween a pair of Waspmoe nodes, whose behavior is shown in Figure 1(c). The wo experimens were performed in exacly he same condiions, by placing he wo pairs of moes side-by-side on differen channels. The char shows ha, for Waspmoe, emperaure is also negaively correlaed wih RSSI, bu o a much lesser exen w.r.. TMoe Sky, allowing he PDR o remain a 1%. These sharp differences moivaed differen implemenaions of TRIDENT, discussed in Secion 4. Finally, Secion 5 ends he paper wih brief concluding remarks and an oulook on fuure work on TRIDENT. 2. REQUIREMENTS In his secion we describe he requiremens we se for ourselves in designing TRIDENT, grouped according o heir naure. 2.1 Type of Daa Colleced Merics. We wan o suppor in-field collecion of several merics ypically used o perform conneciviy assessmen. The key meric provided is he Packe Delivery Rae (PDR), i.e., he raio of packes received over hose sen. PDR provides a direc assessmen of he abiliy of a link o reliably communicae packes. A number of merics are exraced direcly from he radio chip: Received Signal Srengh (RSSI ), Link Qualiy Indicaor (LQI ), and RSSI noise floor, sampled afer sending or receiving a packe. These merics provide insighs abou physical layer parameers influencing PDR, and heir correlaion wih i. Moreover, noise floor is useful o indirecly deermine he presence of inerference. Finally, TRIDENT suppors he acquisiion, upon packe sending or receiving, of environmenal parameers (e.g., emperaure and humidiy) from on-board sensors. These are useful o deermine how he environmen affecs communicaion, as highlighed by he works menioned earlier and concreely shown in Figure 1. Aggregaed vs. per-packe samples. The reason for which conneciviy assessmen is performed deermines he naure of he daa gahered. If a long-erm observaion is necessary, he amoun of daa recorded can rapidly become prohibiive for a resource-consrained plaform like TMoe Sky. Therefore, he ool should suppor he abiliy o sore only an aggregae of he merics colleced, compued over a well-defined sequence of packe exchanges. On he oher hand, recording he individual merics (e.g., RSSI and noise floor) associaed wih each packe provides a fine-grained deail ha is necessary in some cases, e.g., when he goal is o ascerain he ime-varian characerisics of links wih he resoluion necessary o inform he design of nework proocols. The choice beween aggregaed or per-packe samples should herefore be lef o he user, balancing he goals of conneciviy assessmen agains he sorage limiaions of he plaform a hand. Single packes vs. burss. Anoher relaed dimension is he way he channel is observed. Conneciviy assessmen is ofen performed by sending probe packes wih an iner-message inerval (IMI) relaively high (e.g., seconds), represenaive of several WSN applicaions. Neverheless, some applicaions (e.g., recording daa from acceleromeers [6, 16]) require he ransmission of burss of packes. Moreover, a well-known propery of he wireless channel is ha he ransmission of packes sen wih a small-enough IMI [12] is more reliable. The ool should allow he user o choose wheher o use single packes or burss of packes as probes. 2.2 Type of Experimens Suppored Ineracive vs. bach. Conneciviy assessmen may be needed for reasons yielding differen requiremens, as described earlier. If he ulimae goal is o suppor a WSN deploymen by helping deermine a node placemen enabling communicaion, his is ofen achieved by performing ess of shor duraion (e.g., a few minues). These are useful o quickly evidence which nodes experience low PDR values; his informaion is used by he operaor o relocae nodes and re-assess conneciviy wih anoher shor es. To effecively suppor his process, he ool mus provide a way o quickly (e.g., graphically) represen he PDR associaed wih WSN links. On he oher hand, conneciviy assessmen is ofen performed also o characerize he arge environmen. This requires longerm observaions (e.g., days); he coninuous presence of an operaor would be impracical. The ool mus provide he opion o run auomaically a baery of ess, including differen seings, defined by he operaor bu execued wihou her involvemen. In our experience, he wo modes of operaion are ofen used in conjuncion. Indeed, before saring a long-erm bach experimen, a shor-erm ineracive one is performed, o make sure ha all nodes are funcioning properly, and ha he baseline conneciviy is appropriae o he purpose of he experimen. Mobile nodes. Mobile WSN applicaions, e.g., involving nodes placed on humans, animals, robos, or vehicles, are becoming in-

3 creasingly popular. Therefore, he ool should suppor experimens where some of he nodes are mobile, o assess he conneciviy beween hese and he fixed nodes. An ineresing possibiliy, parially explored in our previous work [5], is o use mobile nodes as a way o perform a preliminary exploraion of he deploymen area. As he mobile node moves across he field and exchanges messages wih fixed nodes, i samples he conneciviy of a high number of locaions, cumbersome o explore individually only wih fixed nodes. 2.3 Suppor o Operaors In-field, wireless ineracion wih nodes. In-field operaors mus inerac wih he nodes for various purposes. The primary reason is o rerieve he resuls of experimens, sored on he nodes aking par in hem. Anoher key operaion is he re-asking of he nodes wih a new se of experimens. The operaor may also need o inerac wih he nodes for he sake of monioring he correc execuion of he experimen, e.g., o rerieve saisics abou he experimens performed or he baery level. Oher useful operaions are he abiliy o pu seleced nodes (or he enire WSN) in sand-by when hey are no involved in an experimen, and wake hem up laer on. In principle, some of hese operaions can be performed by direcly accessing he node; for insance, daa can be downloaded via USB. However, while his operaion is rivial in a lab, i becomes cumbersome when nodes are deployed in-field in a harsh environmens, e.g., oudoor in winer, or in places ha are no easily accessible. Therefore, all of he ineracions wih he nodes should be performed over-he-air, by leveraging he wireless channel. Daa sorage and processing. Conneciviy assessmen experimens may generae a huge quaniy of daa. Handling hese as individual files becomes rapidly impracical. Furher, he raw daa gahered ofen needs o be processed in an auomaed way o simplify inerpreaion. The ool should herefore inegrae a daabase for soring experimenal daa, enabling srucured access and querying, and he definiion of sored procedures providing a layer of absracion in daa manipulaion and inerpreaion. 2.4 Non-funcional Requiremens No infrasrucure. As we already argued in he inroducion, his non-funcional requiremen is a defining one. In-field experimens canno afford he luxury of a secondary nework; he experimen execuion mus rely only on he WSN nodes under es. No coding required. Our desire o suppor domain expers implies ha using he ool should no require wriing even a single line of code. The configuraion of experimens should occur enirely via he user inerface; a mos, domain expers mus learn how o flash moes wih a pre-canned binary before going in-field. Ease of use. The logisics of in-field experimens makes hem effor-demanding and ime-consuming. The siuaion should no be exacerbaed by a complex or cumbersome ineracion wih he ool. A graphical user inerface, providing inuiive suppor for all he phases of he experimens, is herefore an obvious requiremen. Flexibiliy. The experimen seings, including number of nodes, heir naure and role in he experimen, power and channel seings, number of messages, iner-message inerval, number of es repeiions, ec., should be designed in a way ha allows users o combine hem freely, o explore differen porions of he parameer space. Decoupling from hardware plaform. The ool should work on sandard nodes wihou modificaions o hardware. Neverheless, as here are many WSN plaforms available, supporing a new one in he ool should be simplified by is sofware archiecure, by confining plaform-specific deails in well-idenified componens s a r Round 1 Round 2 IPI s a r Experimen burs Figure 2: Sample TRIDENT experimen showing wo rounds, saggered ransmissions using single-packe and burs probes, and per-round synchronizaion. 3. DESIGN Nex we describe he design of TRIDENT. We begin wih a descripion of he execuion of conneciviy assessmen experimens, hen provide an overview of he TRIDENT oolse. 3.1 Experimen Execuion Definiions. An enire TRIDENT experimenal campaign is defined as a sequence of rounds, as shown in Figure 2. Each round has is own configuraion parameers, deailed nex, including wheher i uses single-packe probes or burs probes, i.e., muliple packes ransmied in rapid sequence. The ime beween he beginning of wo consecuive probes from he same node is he iner-probe inerval (IPI). For burs probes, we also define he iner-message inerval (IMI) as he ime beween wo messages belonging o he same burs. Boh IPI and IMI are configurable on a per-round basis. Probing he links. Conneciviy is assessed by probing he communicaion link wih packe ransmissions and evaluaing he received packes and heir properies. Therefore, TRIDENT experimens mus define precisely when each node ransmis and lisens. All nodes behave he same: ransmi a probe, pause for he IPI duraion, repea his process a configurable number of imes. In beween ransmissions, nodes can be configured o lisen for packes from oher nodes. TRIDENT does no duy-cycle he radio during experimens, ensuring ha no packes are los due o he radio sae. Moreover, o avoid collisions among simulaneously ransmied packes, which can confound he link evaluaion, TRIDENT ensures ha only one sender ransmis a any given ime. This is achieved by having nodes begin heir probe sequence in a saggered way based on heir node idenifiers. Specifically, he ransmi ime for he i-h probe of node n is defined as n,i = sar + nt IPI + int IPI where sar is he sar ime of he round, T IPI is he value of he IPI, n is he node idenifier, and N is he overall number of nodes. n and N are seup saically during he experimen design phase. Saggering ransmissions by assigning each nodes is ransmi slo, requires he nodes o be ime-synchronized. In TRIDENT his is achieved a he beginning of each round, as shown in Figure 2. This synchronizaion allows he sysem o compensae for clock drif during a long running experimenal campaign. Maser node. Time synchronizaion is iniiaed by a special node, called he maser. The laer acs in general as a coordinaor owards he res of he WSN nodes, as well as he access poin enabling he operaor o change he configuraion of experimens. The maser has he same binary code of he oher nodes; is special role is deermined by is idenifier, n =. The parameers describing he round configuraion are also disseminaed by he maser node during synchronizaion a he beginning of each round. The opion o change parameers in each round allows he inerleaving of rounds wih differen power levels, or inerleaving single-packe and bursy rounds, as shown in Figure 2. IPI IMI

4 Table 1: Per-round configuraion parameers and heir example values from [8]. Parameer Example Parameer Example # of nodes (N) 16 # of probes per sender 115 lis of senders single-packe probe yes lis of liseners iner-probe inerval 16 s ransmi power 27( 1 dbm) iner-message inerval N/A probing channel 18 aggregae logging yes This choice has muliple consequences. Firs, only he maser node is aware of he experimenal campaign, and herefore is he only one affeced by changes o he laer. As he maser can receive an enire experimenal campaign configuraion over-he-air, physical access o nodes is no required o change or iniiae a campaign. Second, prior o saring a long-running campaign, he operaors can ineracively run a number of small experimens, each ime uploading he round descripion o he maser, insrucing he maser o iniiae he round, hen collecing he resuls. Afer analysis, anoher shor experimen can be carried ou, or he long-running experimen can be iniiaed. This is all possible because he nodes are experimen-agnosic and he maser can be conrolled over-he-air. Per-round configuraion parameers. Table 1 shows he perround configuraion parameers available in TRIDENT, communicaed by he maser before saring a round. We already menioned some of hem, e.g., he overall number of nodes, he role (sender or lisener), he ype of probe, he values of IPI and IMI. Addiional parameers include he radio channel, ransmission power, overall number of probes ransmied per node, and number of packes per burs. The logging mehod mus also be defined, choosing beween soring informaion abou every packe, or only he average over he enire round, based on he needs of he experimen and he available sorage. Finally, rounds and/or enire experimens can be repeaed a configurable number of imes, o increase saisical relevance. As for merics, no shown in he able, PDR, RSSI, and (if available) LQI are colleced by defaul. If he plaform allows, sender and receiver can acquire per-packe environmenal condiions such as noise floor, emperaure, and humidiy. These values are recorded according o he per-round logging policy. Ineracing wih he nodes under experimen. Changing he perround configuraion parameers is no he only opion o inerac in-field wih nodes. Table 2 describes he commands available o he operaor. As already menioned, he operaor can upload he descripion of an enire campaign configuraion on he maser, which hen uses i o orchesrae he various rounds wih he appropriae per-round configuraion. However, he operaor can also sar and sop he execuion of he experimen; hese commands are propagaed nework-wide, wih a mechanism similar o he one used o mark he sar of a single round, described in Secion 4. Table 2: In-field commands. Command Targe Descripion UPLOAD maser load a campaign configuraion START nework sar he execuion of an experimen STOP nework sop he execuion of an experimen POLL 1-hop query baery level SLEEP 1-hop place nodes in low-power lisening WAKE-UP 1-hop remove nodes from low-power lisening DOWNLOAD(n) node download logs from seleced node ERASE(n) node erase flash of seleced node SNIFF operaor oggle packe sniffing operaor's lapop experimen planner repors configuraion scrips in-field assisan daabase commands daa Figure 3: The TRIDENT oolse. WSN The maser node can insruc all nodes o auomaically ener a sleep sae upon he end of an enire experimenal campaign, allowing hem o save heir remaining baery power. In TinyOS, nodes in his sae use he defaul low-power lisening (LPL) MAC wih a long sleep inerval, currenly se o 1 s. By duy-cycling he radio, nodes save baery power, bu can be woken up laer, e.g., o iniiae over-he-air daa download or o sar a new experimenal campaign. Alernaely, he operaor can also pu o (or wake-up from) sleep a subse of he nodes, and query for heir baery level. Oher commands enable he operaor o download he experimen logs from a seleced node, and o erase is flash memory afer successful daa ransfer is verified. Finally, passive packe sniffing can be acivaed on he node managed by he operaor, enabling he laer o ascerain wheher all nodes properly sared he experimen. 3.2 Toolse Overview Figure 3 depics he main srucure of TRIDENT. WSN nodes, he subjec of he experimen, are programmed wih a plaformspecific moe-level runime ha is experimen-agnosic; is behavior is esablished by he operaor wihou requiring any coding. This configuraion is performed hrough he GUI of a dedicaed componen, he experimen planner, which resides on he operaor s lapop. The experimen planner essenially enables he operaor o quickly and easily define he various deails of he experimen, by properly seing he values for he parameers in Table 1. This sep does no need o be performed in-field, as he planner enables only he definiion and sorage of experimens. The acual upload of experimens o he maser, and from here o he res of he WSN, is insead suppored by he in-field assisan, which also enables he execuion of he oher commands in Table 2. Communicaion wih he WSN is enabled by a moe acing as a gaeway, conneced o he USB por of he operaor s lapop. The in-field assisan provides also a simple visualizaion of he conneciviy map buil from available colleced races. An example is shown in Figure 4, visualizing he qualiy of he inbound links for node according o a inuiive green/yellow/red color-coding, whose semanics in erms of PDR is configurable. This feaure Figure 4: The resuls view of he TRIDENT in-field assisan, showing one-hop inbound conneciviy and PDR for node.

5 Table 3: Plaform suppor in TRIDENT. hardware TMoe Sky Waspmoe + XBee sofware TinyOS Arduino PHY layer (2.4 GHz) (2.4 GHz) radio chip CC242 EM25 TX power dbm dbm merics RSSI, LQI, noise RSSI burs probes yes no sorage flash chip, 1 MB microsd, 2 GB aggregae logging per round, on moes on operaor s lapop sensors emperaure, humidiy is paricularly useful when TRIDENT is used for a shor-erm assessmen, as i quickly informs he operaor abou areas wih conneciviy problems, whose nodes can hen be re-arranged. A similar visualizaion is provided also for mobile nodes; once he in-field assisan is fed wih a sequence of locaions, i can replay he maps, showing o he operaor how conneciviy evolved due o mobiliy. Finally, he daabase and he associaed ploing scrips simplify he sorage of he colleced daa and is offline analysis. The daabase conains generic and cusomizable sored procedures for daa manipulaion. The se of pre-canned scrips allows he user o quickly plo rends derived from he daa colleced, e.g., currenly including nework-wide PDR, cumulaive disribuion funcions of he links w.r.. heir PDR, spaial and emporal variaions of he merics, correlaion of PDR wih RSSI and LQI. 4. PLATFORM-SPECIFIC DETAILS TRIDENT currenly suppors wo hardware plaforms: TMoe Sky and Waspmoe. The former direcly inegraes he CC242 radio chip, while he laer relies on an exension module for radio communicaions, in our case he XBee S2 inegraing he ZigBeecomplian EM25 sysem-on-chip. Boh ransceivers implemen he 2.4 GHz IEEE physical layer. The sofware archiecure of TRIDENT confines he differences mosly in he plaform-specific runime insalled on moes, alhough a few changes are needed also o pars of he in-field assisan handling communicaion wih he WSN and parsing he logs for visualizaion. We refer o hese plaform-specific porions of he sofware as he backend, and summarize he differences in Table 3. The Waspmoe varian provides less feaures han he TMoe Sky counerpar, as TinyOS allows low-level access o he radio chip while Waspmoe provides only he high-level inerface of he ZigBee applicaion suppor sublayer (APS). These rade-offs are discussed in he res of he secion, along wih oher implemenaion deails. Available merics. The wo plaforms provide differen merics. TinyOS records RSSI and LQI for each received message, while he XBee radio module repors only RSSI. Moreover, unlike Zig- Bee, he low-level API available o TinyOS applicaions allows requesing RSSI when he channel is idle o measure he noise floor. The emperaure and humidiy sensor of TMoe Sky provides imporan informaion for our sudies of he environmenal effecs on conneciviy. In principle, he same holds for Waspmoe, given he wide range of sensors available for his plaform. However, we have no ye implemened suppor for hem in TRIDENT, alhough his does no pose any paricular echnical problem. Experimen execuion. On boh plaforms he experimen configuraion is insalled in he non-volaile sorage of he maser node. The TMoe Sky backend suppors uploading he configuraion wirelessly or via USB, and sores i in a dedicaed flash pariion, while sender receiver IPI 1 2.5s APP 3 PHY PHY APP TX TX TX TX TX TX TX TX TX x x RX RX?? x RX? 1 2 R1 R2 R3 applicaion-level packe arrival imesamps Figure 5: ZigBee ransmis each broadcas packe hree imes. he Waspmoe backend relies on a configuraion file on he SD card. As described in Secion 3, he nework is ime-synchronized a he beginning of each round o avoid collisions among probes. The backends implemen differen echniques. In he case of TMoe Sky, disseminaion relies on a TDMA scheme where each node has is own ime slos o repropagae commands, in a way similar o he mechanism oulined for probe ransmission in Secion 3. The common ime reference needed for boh he TDMA disseminaion phase and o calculae sar is esablished wih TinyOS packe-level ime synchronizaion service [9], yielding sub-millisecond precision. As ZigBee does no provide such a synchronizaion service, we rely on he sandard muli-hop broadcas feaure, basically a nework flooding. However, based on he ZigBee Pro feaure se [17], broadcas packes are always sen 3 imes in a row, increasing reliabiliy a he expense of energy consumpion. These broadcas packes are separaed by a 5 ms inerval plus a random delay beween and 4 ms. Therefore, in he wors case where he sar synchronizaion message is received only upon hird aemp, he ime synchronizaion error goes slighly above 1 s per hop. To secure a collision-free ransmission schedule he IMI should be se long enough o compensae for hese synchronizaion errors and also for he ime drif of he nodes. On TMoe Sky he synchronizaion error is negligible; he ypical ime drif of 1 ppm resuls in wo nodes drifing apar by 36 ms in half an hour. Therefore he use of 25 ms ime slos during 3-minue rounds can be considered safe, couning also he ime needed for inernal processing of he received packes. Insead, on Waspmoe he second-per-hop error should be compensaed; we achieve his by using 3 s ime slos. Moreover, ransmission of probes as one-hop broadcas requires an addiional second, again due o he riple ransmission performed by he ZigBee nework layer. Therefore, i is impossible o send burss of packes wih Waspmoe; on TMoe Sky, burss are insead suppored wih a configurable IMI, se o 2 ms by defaul. Anoher implicaion of ZigBee compliance is ha nodes mus join he wireless PAN (personal area nework) before sending applicaion daa. A muli-hop ZigBee nework is buil around is coordinaor wih he sandard join procedure, including channel scanning and handshaking, one hop a a ime. This process requires up o minues, depending on he nework diameer. In case he channel is changed in beween rounds, his affecs he minimum inerval beween hem, as he nework opology mus be rebuil from scrach. Deermining he link-level PDR. In principle, he value of PDR can be obained sraighforwardly, since he number of ransmied and received packes is known for each link. However, Wasp- number of packes packe arrival inerval, ms Figure 6: Disribuion of packe arrival ime. 3

6 moe inroduce addiional complexiy due o he riple ransmission of broadcas packes mandaed by he ZigBee nework layer. As shown in Figure 5, since duplicaes are auomaically filered a he receiver, he link-level PDR canno be deermined by simply couning he delivered packes a he applicaion layer. Consider wo hypoheical experimen runs, one where each probe sen is always received upon firs aemp, and one where i is received always upon he hird aemp. The link-level PDR would be a meager 33% in he second case, ye he applicaion-level PDR (he only direcly measurable) would be 1% in boh cases, providing a false represenaion of he qualiy of he wireless link. Neverheless, we can infer he number of failed aemps by looking a he packe arrival ime, based on he fac ha he hree broadcas ransmissions in ZigBee are spaced relaively far apar (5 ms). I is herefore possible o deermine, upon receiving a broadcas packe, wheher his was he firs, second, or hird ransmission. We confirmed his experimenally: Figure 6 shows he disribuion of he packe arrival inerval (modulo he nominal IPI) measured a he applicaion level for an inermediae-qualiy link. For a perfec link, all packes would be received wih he same IPI, 15 s in his case; however, his is no he case when packes are los. Consider an applicaion-level packe i, received on firs aemp. If he previous packe, i 1, was received only on second or hird aemp, he IPI beween he wo packes is smaller han 15 s. A similar reasoning holds in case he nex packe i + 1 is no received upon firs aemp, yielding an IPI greaer han 15 s. Clearly, he hisograms o he lef and righ of he cenral one in Figure 6 can be generaed also by inermediae combinaions, e.g., if i is received upon second aemp and i + 1 upon hird, he IPI will be around 14.5 s. Packe loss can be inferred by comparing he applicaion-level packe arrival imesamps (e.g., R 1 and R 2 in Figure 5), provided here is a leas one packe in he received sequence known o have arrived upon firs aemp. This can be saed cerainly when a leas one pair of packes (no necessarily consecuive) has eiher he minimum or maximum possible recorded arrival inerval (modulo he IPI), i.e., placed lefmos or righmos on he hisogram of Figure 6. Indeed, his means ha he one of he packes was delivered on firs aemp and he oher on las, as in he case of R 1 and R 2. In principle, i may happen ha no such IPI is recorded during he whole es. In pracice, his is unlikely o happen for inermediaequaliy links. However, one can sill infer he characerisics of he link based on he applicaion-level PDR. If he laer is very good, one can assume ha he majoriy of he packes were received on he firs aemp and base he analysis on his fac. For very bad links, i may be impossible o measure he acual PDR precisely when here are jus few packes received from he whole sequence. Soring he experimen daa. Due o he sorage limiaions of TMoe Sky, per-message logging can replaced by soring per-round averages of he recorded values, compued on he nodes hemselves. Waspmoe does no have sric sorage capaciy limiaion; full logs are always sored and he log analysis performed on he operaor s lapop. However, we plan o implemen on-board log processing also on Waspmoe, o reduce he downloading ime of large logs. 5. CONCLUSIONS AND FUTURE WORK In his paper we presened TRIDENT, a ool for in-field conneciviy assessmen. Unlike similar ools in he lieraure, TRI- DENT does no require any communicaion infrasrucure besides he WSN nodes. Moreover, i is designed o be easy o use for domain expers, which can perform heir conneciviy experimens (e.g., o deermine a correc placemen of WSN nodes) wihou coding. TRIDENT suppors several configuraion parameers and merics, enabling he invesigaion of many aspecs of low-power communicaion. Moreover, he operaor s inerface provides a rich se of commands ha simplify in-field managemen of experimens. We coninuously modify TRIDENT based on he lessons we learn from in-field experience in our deploymens and experimenal campaigns. Plans for he near fuure include suppor for remoe conrol of experimens and in-nework collecion of heir resuls, herefore significanly reducing he need for operaors o be presen in-field. Acknowledgmens. The auhors wish o hank Maeo Chini and Maeo Cerioi for heir work on early versions of TRIDENT. The research we described is parly funded by he European Insiue of Innovaion and Technology (EIT) ICT Labs, under aciviy n From WSN Tesbeds o CPS Tesbeds and n I 3 C Inelligen Inegraed criical Infrasrucures for smarer fuure Ciies. 6. REFERENCES [1] hp://wirelessriden.sourceforge.ne. [2] N. Baccour, M. Ben Jamaa, D. do Rosario, A. Koubaa, H. 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