Implementation of a Timestamping Service for SunSPOT Sensors

Transcription

1 Available olie at Procedia Techology 7 ( 2013 ) 4 10 The 2013 Iberoamerica Coferece o Electroics Egieerig ad Computer Sciece Implemetatio of a Timestampig Service for SuSPOT Sesors Jua A García Reyes a, Roberto Solís Robles a *, Blaca E Solís Recédez a, José G Arceo Olague a a Uiversidad Autóoma de Zacatecas, Ave. Ramó López Velarde 801, Zacatecas, 98000, México Abstract Recet techological advaces have eabled the developmet of sesig platforms that iclude differet types of sesors, ad itegrate several compoets such as memory, processor, ad radio. Time sychroizatio is a critical service for a wide rage of applicatios i wireless sesor etworks. Through this service, the sesors i the etwork are able to coordiate ad carry out sesig processes i a way that permits the correct orderig of the global evets, ad save eergy. The protocols that provide better accuracy rely o a timestampig mechaism which allows them to elimiate several sources of error i the time sychroizatio process. However, i ewer platforms, such as SuSPOT, o timestampig mechaism is provided. I this paper, we preset a implemetatio of a microsecod accuracy timestampig service i the SuSPOT platform, which provides the foudatio for the implemetatio of accurate time sychroizatio protocols i a wireless SuSPOT sesor etwork The 2012 Authors. Published Published by Elsevier by Elsevier Ltd. Ltd. Selectio Ope access ad/or uder peer-review CC BY-NC-ND uder resposibility licese. of Global Sciece ad Selectio Techology ad peer-review Forum uder Pte resposibility Ltd of CIIECC 2013 Keywords: Wireless, Sesor, Network, Time Sychroizatio, Protocol 1. Itroductio Recet techological advaces have eabled the developmet of sesig platforms that iclude differet types of sesors, ad itegrate several compoets such as memory, processor, ad radio. A wireless sesor etwork cosists of two or more of these devices that commuicate wirelessly i short distaces usig geerally a low power cosumptio protocol. These sesor etworks are ideal for applicatios such as atural habitat ad ifrastructure moitorig, data acquisitio i iaccessible areas, etc. [1, 2] Time sychroizatio is a critical service for a wide rage of applicatios i wireless sesor etworks. Through this service, the sesors i the etwork are able to coordiate ad carry out sesig processes i a way that permits the correct orderig of the global evets, ad save eergy. Several algorithms have bee proposed to provide this time sychroizatio service [3, 4, 5, 6, 7, 8]. Although a few of them have bee implemeted i wireless sesors platforms such as MICAz or * Correspodig author. Tel.: Ext addresses: garciar.ja@gmail.com, rsolis@uaz.edu.mx, bsolre@yahoo.com.mx, arceoljg@yahoo.com.mx The Authors. Published by Elsevier Ltd. Ope access uder CC BY-NC-ND licese. Selectio ad peer-review uder resposibility of CIIECC 2013 doi: /j.protcy

2 Jua A. García Reyes et al. / Procedia Techology 7 ( 2013 ) TelosB [9], the eed for a better performace, ad the appearace of better hardware features such as faster processors or more available memory, make it essetial to use ewer devices such as the SuSPOT [10]. The SuSPOT platform is programmed usig Java, which meas, that as i all devices that use Java, it has a Java Virtual Machie (JVM) that i this case acts as operatig system. The SuSPOT platform icorporates the Java 2 Micro Editio (J2ME) ad the Coected Limited Device Cofiguratio (CLDC) 1.1. Several APIs are provided for the platform, which allow the easy developmet of applicatios to be ru i the SuSPOT. However, there is o API available to provide a timestampig service at the MAC (Medium Access Cotrol) level, which is ecessary for several of the time sychroizatio protocols that have bee proposed, such as FTSP [6] or DMTSP [8]. Give the eed we have to use a time sychroizatio protocol that requires the existece of this kid of timestampig mechaism, we decided to implemet the timestampig service. This paper presets the issues ecoutered i the developmet of a software module to provide a timestampig service with a microsecod precisio ad which is trasparet to the user. The module is tested usig the Floodig Time Sychroizatio Protocol [6], maybe the more widespread sychroizatio protocol give its capability to support etwork scalability ad chages i the etwork topology as well as its simplicity, low overhead, fault tolerace ad low average sychroizatio error. 2. Related Work As already metioed, several time sychroizatio protocols have bee proposed, but two of them stad out sice they provide a better accuracy that the rest. This is due to their use of a MAC layer timestampig mechaism. This timestampig mechaism allows a protocol to elimiate several sources of error i the time sychroizatio process, ad therefore provide better sychroizatio accuracy. The first protocol that uses a MAC timestampig mechaism is FTSP, Floodig Time Sychroizatio Protocol ad its details ca be foud i [6], where we ca also fid the results obtaied for its implemetatio i the MICA2, which is a obsolete sesor platform. The protocol was tested i a 60-ode multihop etwork, with a maximum of 6 hops betwee ay ode ad the root ode. The average sychroizatio error obtaied was 16 microsecods. The secod protocol that uses the MAC timestampig mechaism is DMTSP, Distributed Multihop Time Sychroizatio Protocol [8], which also works i a multihop eviromet, but does ot eed to establish a topology before the sychroizatio ca take place. It was also implemeted i the MICA2 platform, ad the average sychroizatio error was 20 microsecods for a 9-hop etwork. As metioed, both of these protocols sychroize a etwork of odes usig sychroizatio messages which are timestamped at trasmissio ad at receptio. Although these timestamps elimiate some sources of error, there is a eed to compesate for the drift i the odes clocks. This drift is caused because the quartz crystals that feed the timers geerate light frequecy chages which i the log term accumulate. Therefore, a liear regressio ca be performed to estimate the value of the global time. This liear regressio is performed i each ode to estimate its skew ad offset with respect to the global clock i a give time. It is worth metioig that ulike i other platforms where there is a wide rage of documetatio regardig the features of the operatig systems they use, such as TiyOS [11]; for the SuSPOT there is practically o documetatio available for several of the features, icludig the way the MAC layer is programmed. We could oly fid a icomplete report regardig the implemetatio of a microsecod clock [12] ad a couple of otes o the cofiguratio of the timers [13, 14], which we use as a departure poit for our implemetatio. 3. System Descriptio Time sychroizatio usig FTSP ivolves three mai stages, which are treated as idepedet software modules i this implemetatio, ad are later liked together to form a complete time sychroizatio system. The modular divisio simplifies its implemetatio give the features of the object orieted programmig laguage used (Java) ad the properties of the protocol. It also provides a easier way to measure the sychroizatio errors itroduced i each stage ad compare such errors with the oes obtaied i other implemetatios. Each module offers its services to other layers. The aforemetioed stages are: 1. Timestampig. This module iteracts with the lower protocol layers. It specifies the time sychroizatio packet ad offers a iterface to allow the other modules to sed ad receive sychroizatio messages. It also provides a MAC layer timestamp whe a packet is received or trasmitted, ad i the case of trasmissio, it puts the timestamp i the packet that is beig set.

3 6 Jua A. García Reyes et al. / Procedia Techology 7 ( 2013 ) Clock drift maagemet. This module maages the liear regressio table, formed by the sychroizatio poits (global time, local time) ad used to calculate the offset ad skew required to estimate the global time. 3. Multihop etwork sychroizatio. This module decides which ode will act as the root, what will be doe i case of ode failures ad aalyzes the data obtaied from the sychroizatio messages received, filterig every message that does ot meet the protocol criteria ad stores every valid message i the liear regressio table. Fially, each module offers its services at some stage of the sychroizatio process (e.g., receptio or trasmissio) ad otifies its state to other modules if ecessary. Fig. 1 shows how each module, from the receptio to the trasmissio of a message, takes part i the time sychroizatio process i a multihop etwork. Fig. 1. Model for the time sychroizatio process i a multihop etwork usig FTSP 4. Implemetatio Details 4.1. SuSPOT The SuSPOT devices developed by Oracle Labs [10] icorporate a hardware module with light, ad temperature sesors, a 3D accelerometer, a A/D coverter ad a data acquisitio board that allows the iput ad output of sigals from/to exteral devices. It has a 400MHz processor, with 1MB RAM, 8MB of flash memory ad a IEEE radio which trasmits i the 2.4GHz bad to provide wireless commuicatios with a 80 meter rage. A rechargeable 3.7 V, 770mAh lithium-io battery is used as eergy supply, which lasts up to 909 days i deep sleep state [15]. With the objective of obtaiig a microsecod precisio i the sychroizatio achieved, we require a microsecod precisio clock. However, the available API oly provides millisecod accuracy through the use of System.curretMillis(). Therefore we eed to cofigure a clock to achieve the microsecod precisio required. The hardware module i the SuSPOT icorporates two timers, each with three chaels that ca be cofigured programmatically to implemet a clock capable of updatig its local time with a microsecod precisio. I the ext subsectio we will discuss how this is doe. To be able to provide a MAC layer timestampig service, we ca wire the SFD pi from the CC2420 radio i the SuSPOT with the timer i the microprocessor ad capture the pulse geerated from the SFD pi [16] Logical Clock Cofiguratio The hardware module i the SuSPOT s microprocessor icorporates two AT91 timers. Each of these timers icludes three idetical chaels, each with a 16-bits couter. Two of these chaels are reserved for the system ad oly four are available for applicatio use. A chael ca operate i two differet modes: capture mode ad sigal geerator mode [13]. The basic idea, as show i Fig. 2, is to cofigure oe of these chaels, say Chael 0, as a pulse geerator at a 1 MHz frequecy, ad coect this chael to aother oe, say Chael 1, eabled i capture mode. This way, the geerator i Chael 0 will sed the couter i Chael 1 a pulse every microsecod, ad the Chael 1 couter will icremet the value i its register util it reaches its maximum value. Oce this maximum value is reached a 64-bit variable will be updated by addig to it the maximum value. By havig this 64-bit variable we ca keep a much larger clock cout that the oe kept with the 16-bit couter i the timer.

4 Jua A. García Reyes et al. / Procedia Techology 7 ( 2013 ) Fig. 2. Block diagram of the timer cofiguratio used to costruct a logical clock with microsecod precisio. With the help of the iformatio obtaied from [13] ad [14] we were able to cofigure the clock. First we describe the particular cofiguratio for chael 0 i our pulse geerator. This chael has three importat registers for our implemetatio: the first oe is the couter value (TC_CV) register which is a 16-bit register which icremets its value with each positive trasitio from the selected clock iput, ad the other two (TC_RA ad TC_RC) are used as compariso registers. I order to obtai a higher precisio we feed this chael with the MCK/2 sigal (Master Clock divided by 2) which is the fastest available speed i the processor board with a MHz frequecy. Usig this frequecy, the couter i the 16-bit register TC_CV icremets its value approximately every 15 aosecods. Whe the value of the couter reaches the value set i the 16-bit register TC_RA, the output i the multipurpose iput/output TIOA0 pi chages to iactive. Whe the value of the couter reaches the value set i the 16-bit register TC_RC, the output i the TIOA pi chages to active. This way, the geerated pulse will be equal to TC_RC -TC_RA. To cofigure the sigal geeratio to be active every 1 microsecod, we eed to set the value of the registers as follows: TC_RA =22 ad TC_RC=67. Chael 1 is cofigured i capture mode. This chael allows us to update the local time periodically. The logic i this timer is similar to the oe described for chael 0, the icomig sigal i TCLK1 that comes from the pulse geerator icremets the value of TC_CV. If we cofigure the iterrupt i a correct way, whe the couter reaches its maximum value, a TC_COVFS iterrupt is geerated, the global variable used to keep the time is updated ad TC_CV is reset to zero. Sice TC_CV is 16 bit log, its maximum value is 0xFFFF, ad therefore the value is added to the global variable. To get the local curret time, we access the TC_CV register ad add it to the value i the global variable. I our implemetatio, a iterface was added to the API so a applicatio ca make use of the method getcurrettime(), which performs the steps we just describe to obtai the curret local time Timestampig Accordig to the FTSP protocol descriptio, whe a sychroizatio message is set, the trasmittig ode timestamps the message beig set. Also, whe the receivig ode receives a message it geerates a receptio timestamp. This two timestamps form a sychroizatio data poit that allows the calculatio of a skew ad a offset with respect to the global time (which for the basic setup betwee oly two odes, would be the time at the seder). I the implemetatio of FTSP uder TiyOS i the Mica2 platform, the messages were timestamped multiple times to provide a better sychroizatio. Ufortuately, i the CC2420 radio trasceiver of the SuSPOT, we caot perform this multiple timestampig. This limits the precisio i our implemetatio, give that the use of multiple timestamps allows a 90% reductio i the sychroizatio error betwee two odes, accordig to the descriptio foud i [6].

5 8 Jua A. García Reyes et al. / Procedia Techology 7 ( 2013 ) 4 10 Fig. 3. State of the SFD pi i the CC2420 radio trasceiver at trasmissio ad receptio [16] I our implemetatio, the messages are timestamped oly oce, by detectig the Start of Frame Delimiter (SFD), which idicates the begiig of the trasmissio (or receptio) of a message, as show i Fig. 3. I trasmissio mode, oce the SFD is detected the trasmissio of the iformatio (packet) cotaied i the TXFIFO buffer foud i the CC2420 radio chip has begu, ad therefore we eed to obtai the local clock described i Sectio 4.2 to place it i the packet beig set as soo as possible. Curretly, the APIs icluded i the SuSPOT Developmet kit v6.0 allow us to register our ow protocols easily by usig the ILowPa iterface. Through this registratio, a user ca establish a 8-bit umber which acts as a idetifier for the protocol i the otificatios set to the upper layers whe packets are received. The structure ad format of the packet used i the time sychroizatio protocol is show i Fig 4. We ca observe that the timestamp is to be placed at the last possible positio so we have time to obtai the curret local clock ad place it i the packet before the packet is completely set. Fig. 4. Time sychroizatio packet format i the layers, based o [17] I receptio mode, the SFD is detected oce the Start of Frame Delimiter field has bee stored i the RXFIFO buffer i the CC2420 radio chip, ad the value of the local clock is obtaied as a result of this detectio Clock Drift Maagemet As i [8], we assume that the behavior of a clock through time follows the liear form: T i = t+o where T i is the local time, ad O are respectively the relative speed of the clock (skew) ad the offset with respect to some other ode, ad t is the real time. The liear regressio method is used so each ode i the etwork is able to estimate the global time (e.g., the time at some root ode). With two or more sychroizatio poits (each formed by the trasmissio ad receptio time of a give message), the global time (G) ca be estimated usig liear regressio i the followig way: G= L+O, where L is the local time ad both ad O are calculated as follows: i 1 i 1 RT i R i 2 i R T i i i 1 i 1 2 Ri i 1 O T T ad R are the average trasmissio ad receptio timestamps respectively, ad is the umber of sychroizatio poits cotaied i the liear regressio table. For our implemetatio, the clock drift maagemet software module maitais the liear regressio table with a maximum capacity of 15 sychroizatio poits but ad O are calculated oce five sychroizatio poits are available. Oce these calculatios are made, the software module otifies the module that updates the local time so it has the iformatio required to estimate the global time. I this way, ay class ca obtai the global time at ay time. R

6 Jua A. García Reyes et al. / Procedia Techology 7 ( 2013 ) Results Oce we fiished our implemetatio of the timestampig service, we implemeted FTSP for a two-ode sychroizatio havig oe of the odes sed a sychroizatio message every 15 secods. Oce the receivig ode has received five sychroizatio messages, as metioed before, it is able to estimate the skew ad offset with respect to the trasmittig ode usig liear regressio. For each message received we obtaied the curret local time at receptio ad based o the trasmissio timestamp icluded i each message ad the skew ad offset estimated, we observe a sychroizatio error of 45 microsecods. The sychroizatio error is larger tha the oe obtaied by the TiyOS implemetatio of FTSP [6] because we were ot able to timestamp a message multiple times at trasmissio ad receptio, ad because the Garbage Collectio service at the Java Virtual Machie (JVM) of the SuSPOT geerates delays for the iterrupt hadlers. Nevertheless, the sychroizatio is ow much fier that the oe provided by the APIs i the SuSPOT which is i the order of millisecods. Fig. 5. Average sychroizatio error obtaied whe sychroizatio messages are set every 15 secods. 6. Coclusios I this paper we have preseted a implemetatio of a timestampig service with microsecod precisio for the SuSPOT sesor platform, which is required for the use of time sychroizatio protocols such as FTSP or DMTSP. Although the sychroizatio error we have obtaied i this implemetatio is larger tha the oe obtaied with other platforms, it provides a much better precisio that the oe provided by the APIs icluded i the SuSPOT platform. The ext step is to try ad reduce the sychroizatio error by lookig ito ways to elimiate the egative impact the JVM s garbage collectio ad thread maagemet create i the iterrupt hadlers. Further experimetatio is also required to verify that the covergece ad fault tolerace properties that FTSP [6] exhibits i the previously existig implemetatios are also preset i our implemetatio whe ode failures are ijected. Refereces [1] Sudararama B, Buy U, Kshemkalyai AD. Clock sychroizatio for wireless sesor etworks: a survey. Ad Hoc Networks, 2005, Vol. 3, p [2] Va Voorst B. Time sychroizatio i wireless sesor etworks. 4th Twete Studet Coferece o IT Available olie: &rep=rep1&type=pdf [3] Elso J, Girod L, Estri D. Fie-graied etwork time sychroizatio usig referece broadcasts. Proceedigs of the 5th ACM Symposium o Operatig System Desig ad Implemetatio (OSDI-02), 2002, p [4] Sichitiu M, Veerarittipha C. Simple, accurate time sychroizatio for wireless sesor etworks. Proceedigs of the IEEE Wireless Commuicatios ad Networkig Coferece (WCNC), p.16. [5] Gaeriwal S, Kumar R, Srivastava MB. Timig-syc protocol for sesor etworks. Proceedigs of the first iteratioal coferece o Embedded etworked sesor systems (SeSys-03), 2003, p.138. [6] Maroti M, Simo G, Kusy B, Ledeczi A. The floodig time sychroizatio protocol. Proceedigs of the 2d iteratioal coferece o Embedded etworked sesor systems, 2004, p. 39. [7] Shahzad K, Ali A, Gohar ND, ETSP: A eergy-efficiet time sychroizatio protocol for wireless sesor etworks. Proceedigs of the 22 th Advaced Iformatio Networkig ad Applicatios - Workshop, p [8] Solis R, Borkar VS, Kumar PR. A New Distributed Time Sychroizatio Protocol for Multihop Wireless Networks. Proceedigs of the 45th IEEE Coferece o Decisio ad Cotrol, 2006, p [9] MEMSIC, Ic, Wireless Modules. Available olie: html. [10] Su Labs, Project SuSPOT, Available olie: [11] Levis P, Gay D. TiyOS Programmig. New York: Cambridge Uiversity Press, 2009.

7 10 Jua A. García Reyes et al. / Procedia Techology 7 ( 2013 ) 4 10 [12] Sycspot - Google Code, Time Sychroizatio o SuSPOT. Available olie: [13] Atmel. AT91SAM9G20 ARM Datasheet, Available olie: [14] Ro Goldma. Usig the AT91 Timer/Couter, 2010, A SuSPOT Applicatio Note. Available olie: /TimerCouterAppNote.pdf [15] Su Labs, SuSPOT Mai Board Techical Datasheet Rev 8.0, 2010, Available olie: html. [16] Chipco. CC2420 Datasheet, Available olie: [17] IEEE Computer Society. Wireless Medium Access Cotrol (MAC) ad Physical Layer (PHY) specificatios for Low Rate Wireless Persoal Area Networks (LR-WPANs), 2003, Available olie: