Calculation and Simulation of Transmission Reliability in Wireless Sensor Network Based on Network Coding

Transcription

1 Calculatio ad Simulatio of Trasmissio Reliability i Wireless Sesor Network Based o Network Codig Zhifu Lua Weifag Uiversity of Sciece ad Techology, Weifag, Chia @qq.com Abstract The wireless sesor etwork (WSN) has peetrated ito every corer i our lives, ragig from atioal defese, biological medicie, evirometal moitorig, to traffic maagemet. It is of great sigificace to study the reliability of data trasmissio, a key determiat of the results of moitorig evets. The etwork reliability lies i fault tolerace: whe some odes or liks i the etwork fail, the data ca be recovered at the sik ode by selectig the appropriate fiite domai space. I this paper, etwork codig is used to improve the reliability of WSN. Firstly, the author calculated the data trasmissio reliability ad eergy cosumptio of etwork codig i siglepath ad multi-path scearios. The, the eergy cosumptio of etwork codig was compared with that of the traditioal method. Fially, the reliabilities of the two differet methods were simulated o MATLAB at differet chael loss rates. The experimetal results show that the reliability of the etwork codig techique is better tha the traditioal oe at the expese of a small amout of eergy. Keywords Wireless Sesor Network (WSN), Network Codig, Reliability Itroductio The wireless sesor etwork (WSN) has peetrated ito every corer i our lives, ragig from atioal defece, biological medicie, evirometal moitorig, to traffic maagemet. The etwork is formed through self-orgaizatio of sesors deployed i multiple sesig areas. I the WSN, the sesors ca work cooperatively to sese, collect ad process specific iformatio i the moitorig area, layig the basis for real-time acquisitio, processig ad trasmissio of iformatio []. It is of great sigificace to study the reliability of data trasmissio, a key determiat of the results of moitorig evets. The reliability of data trasmissio is a itegral part of etwork reliability. The research o etwork reliability ca be traced back to the 960s. However, the early research maily focused o commuicatio etworks, power etworks ad computer etworks. Origiated from etwork reliability, the WSN reliability has ot bee uiformly defied yet. I Referece [2], the cocept is defied as the probability of ormal commuicatio betwee odes i the etwork uder the give costraits. Refer- 50

2 ece [3] gives the defiitio of the ability to deliver material, iformatio ad eergy uder the specified coditios ad withi a desired time frame. Referece [4] defies WSN reliability as the success rate of source-to-destiatio data trasmissio i the etwork. Over the years, various models have bee costructed to study the WSN reliability. For example, Referece [5] costructs a Markov chai-based reliability model i light of the state trasitio i both software ad hardware. The model is further improved i Referece [6] with a quatitative aalysis method. Refereces [7, 8] itroduce a reliability modellig method based o Architecture Aalysis & Desig Laguage (AADL), which quatifies the reliability followig the idea of the Geeralized Stochastic Petri Nets (GSPN) model. The WSN reliability is maily reflected by such three parameters as data acquisitio, data trasmissio ad etwork coverage [9]. Amog them, the data trasmissio plays the most fudametal role [0, ]. Traditioally, the reliability of data trasmissio i the WSN are improved by Automatic Repeat Request (ARQ), Forward rror Correctio (FC) ad multi-path trasmissio. The reliability of data acquisitio is ofte modelled as a overlay problem [2], while the etwork coverage is a reflector of the moitorig quality ad the reliability of the acquired data. By defiitio, etwork coverage refers to the ability to moitor each locatio i a area of iterest. The greater the etwork coverage, the greater the moitorig ability. The parameter is ofte measured by the K-coverage [3, 4], that is, the locatio or area of iterest is covered by at least K odes. To meet differet data reliability requiremets i the area of iterest, the K value ca be set accordig to the uused Quality of Service (QoS). Data acquisitio is oe of the may fuctios of the WSN. The successful operatio of the WSN also relies o the reliable trasmissio of the acquired data to the observer. However, the reliability of data trasmissio is traditioally explored based o the reliable routig mechaism, ad rarely from the perspective of etwork topology. The oe-sided focus o reliable routig mechaism is attributable to the fixed topology of traditioal etworks. As sesor odes grow deser i distributio ad greater i quatity, the etwork topology has become icreasigly flexible, makig it possible to study data trasmissio reliability based o etwork topology [5-7]. I light of the above, this paper first itroduces the compositio ad characteristics of the WSN, ad the calculates the reliability ad eergy cosumptio of the etwork uder sigle-path ad multi-path coditios. Next, the etwork codig techique was compared with a traditioal method i terms of the eergy cosumptio. Fially, the reliabilities of the two differet trasmissio plas at differet chael loss rates were obtaied by MATLAB simulatio. The experimetal results show that the etwork codig techique achieves better stability tha the traditioal mode at the expese of a small amout of eergy, ad that etwork codig is superior to the traditioal mode i multi-path routig. ijo Vol. 3, No. 2, 207 5

3 2 Compositio ad Characteristics of the WSN 2. Compositio of the WSN The WSN is a task-orieted wireless ad hoc etwork [8], that itegrates sesor techology, embedded techology, wireless commuicatio techology with distributed iformatio processig techology. The etwork moitors the target i real time with various micro sesors, processes the moitorig data i a embedded module, ad trasmits the data to the remote moitorig cetre via wireless or physical etwork. The WSN cosists of four parts: wireless sesor odes, sik ode, trasmissio etwork, ad remote moitorig cetre. Its structure is show i Figure.,$#.(#"/*!"#$%"&$#'()*#%*& %&+#,!$,,$"#( #*%-"&.!"#$%"&$#'(#"/*!"#$%"&$#'(+&*+ Fig.. The structure of WSN 2.2 Characteristics of the WSN The WSN, as the combiatio of various emergig techologies, carries may characteristics that are ot available i geeral etworks [9].. Self-orgaizatio: the WSN is a ad hoc etwork formed with sesor odes based o the distributed etwork protocol. Without relyig o ay fixed ifrastructure, the etwork automatically adjusts the etry, exit ad migratio of odes. I this way, a plurality of active odes ca quickly form a idepedet etwork. 2. Decetralizatio: all odes i the WSN have equal status ad form a peer-to-peer etwork. The odes ca joi or leave the etwork mometarily at ay time if ecessary. The status of some odes does ot affect the operatio of the etire etwork. 52

4 3. Dyamic topology: I the case of battery exhaustio or failure, the WSN odes may exit the etwork followig the pre-set procedures. Owig to the mobility of sesor odes, exteral wireless sesor odes ca be admitted to the etwork at ay time. 4. Multi-hop routig: The iterodal commuicatio distace is relatively short i the WSN. The odes ca oly commuicate directly with eighbourig odes. Distat commuicatio requires a itermediate ode for data trasmissio. The multi-hop routig i the WSN is accomplished by commo odes, each of which both receives ad forwards iformatio. The data is trasmitted from each sesor ode to the sik ode via multi-hop relay. 5. Redudat height: For the sake of system reliability ad fault tolerace, the same moitorig poit is covered by the moitorig rages of multiple sesor odes. 6. Limited hardware resources: Uder the costraits of price, volume ad power cosumptio, the odes have much smaller computig power, program space ad memory space tha ordiary computers. This excludes complex protocol hierarchy from the desig of ode operatig system. 7. Limited power supply: The sesor odes are powered by battery, which caot be charged or replaced i may applicatios. Whe the battery rus out, the odes will lose their fuctios. Thus, eergy-savig must be cosidered i the desig of the WSN. 8. Fault tolerace: The WSN boasts strog fault tolerace ad robustess i that the odes ca joi or exit the etwork at ay time, ad that the etwork operatio is immue from the fault of ay ode. 9. Broadcast commuicatio: the broadcast commuicatio mode is adopted istead of the poit-to-poit mode i traditioal etwork. 0. Data processig: The traditioal emphasis o coectivity is replaced by the focus o data. The WSN odes are required to possess such fuctios as data aggregatio, fusio, capturig ad compressio. 2.3 ergy cosumptio of sesor odes The highlightig of eergy-savig differetiates the WSN from traditioal wireless etworks. The eergy savig is o big deal i traditioal wireless etworks due to cotiuous eergy supplemetatio. By cotrast, the WSN faces serious eergy costrait because of the large umber of widely distributed sesor odes. Therefore, it is the primary goal of WSN desig to make efficiet use of ode eergy ad prolog the life of the etwork. There are three parts i the eergy cosumptio module of sesor odes, amely sesor module, processor module ad wireless commuicatio module. All these modules should be desiged for the purpose of eergy-savig. Hereito, the power cosumptio of the processor module depeds o four factors: operatig voltage, operatig clock, iteral logic complexity, ad maufacturig process, ad calls for the developmet of eergy-efficiet microprocessors ad microcotrollers. Thaks to the recet improvemets i low-power circuits ad system desig techiques, it is ow possible to develop ultra-low-eergy microprocessors ad micro-cotrollers. The ijo Vol. 3, No. 2,

5 progress of itegrated circuit has further lowered the eergy cosumptio of processors ad sesor odes, makig the wireless commuicatio module the largest cosumer (Figure 2). The wireless commuicatio module has four states, icludig sedig, receivig, idle ad sleepig. I the idle state, the module always moitors the use of wireless chael ad checks if data have bee set to itself. I the sleepig state, the commuicatio module is shut dow. The sedig state marks the greatest eergy cosumptio of the module, followed closely by the idle ad receivig states. By cotrast, the module has the miimal eergy cosumptio i the sleepig state. Hece, the etwork commuicatio ca be made more eergy-efficiet by reducig uecessary forwardig ad receivig, ad speedig up the activatio of the sleepig state. The efficiet use of eergy may help to maximize the lifetime of the WSN. Fig. 2. The eergy cosumptio module of sesor odes 3 Trasmissio Reliability of Network Codig I the WSN, reliable data trasmissio ca be achieved at elevated redudacy. For example, the ARQ trasfers data packets by retrasmissio, the FC adds redudat packets to the source ode, ad the multi-path method trasmits redudat packets via multiple paths. Moreover, the reliability of WSN data trasmissio is easily affected by the quality of iterodal commuicatio. Oe of the most commo ways to ehace data trasmissio reliability lies i the reductio of chael loss rate with redudat packets. The simplest topology of the WSN is the star etwork, ad the reliability model of the same etwork is differet uder differet requiremets (Figure 3). 54

6 !"#$%#&'() Fig. 3. Star etwork If etwork codig techique is used aloe, it is impossible to avoid the chael loss effect. After all, the effect is the same whether the sik ode loses oe packet oly or all packets at oce. Thus, the etwork codig techique should be combied with the redudat techology to improve data trasmissio reliability. The popular ways to icrease redudacy iclude multi-path trasmissio, retrasmissio codig of data packets, ad icreasig redudat packets. The FC is ecoded oly at the source ode ad decoded at the destiatio ode. I etwork codig, however, the itermediate odes ca be recoded. Compared with FC, the etwork codig features better flexibility ad fault tolerace. For example, assume that 3 raw data packets are trasmitted such that the last-hop ode of the sik ode receives the ecoded data packets x, x2, x3 ad x4, where x is liearly correlated with the ecodig vector of x2, ad other data packets are liearly idepedet. If the FC is adopted, the ode will forward the packets directly. Suppose the packet x3 is lost after trasmissio, the sik ode will ot be able to decode the data packets x, x2 ad x4 because the ecodig coefficiet matrix is ot full rak. If the etwork ecodig is employed, the ode will receive the 4 ew ecodigs by ecodig the data packets, ad the coefficiet matrix of ay three of these packets is full rak. ve if a packet is lost, the sik ode ca decode it smoothly. To disclose the advatage of etwork codig, the reliability of data trasmissio ad eergy cosumptio of etwork codig were simulated with differet paths. The ijo Vol. 3, No. 2,

7 reliability of data trasmissio is deoted as R ad defied as the probability that the sik ode will receive the origial data packet successfully after oe trasmissio. Defiitio : The eergy cosumptio refers to the eergy cosumed by a successfully trasmitted packet. It ca be expressed as: Total eergy cosumed = Number of packets received () Defiitio 2: The ratio of the eergy cosumptio i etwork codig to that i the traditioal mode ca be expressed as:! = t c (2) 3. Sigle-path trasmissio I a multi-hop WSN, the packets are trasmitted by sesor odes via multi-hop routig, the iterodal commuicatio quality is susceptible to chael effects, ad lik failures are idepedet of each other. I the case of lik failure, the sesor odes will ot fail. Let us deote the chael loss rate as e, ad the eergy cosumed by a ode to sed a packet as!. Through a time hoppig, the probability for a packet to reach p odes is: p = (! e) (3) Assumig that the source ode seds to successfully receive K packets is: K packets, the probability for the sik ode P = (! e) K (4) where the probability P is equivalet to the reliability R defied i the precedig sectio. The eergy cosumed to sed a packet to the sik ode via a time hoppig is calculated as follows. Sice the packet is lost after a certai hop, the total eergy cosumed to sed K packets ca be obtaied by iductio. " =!(" ) + # i t K e! ik( " e) e i= (5) The umber of packets received by the sik ode is expected to be Kp, whereby the eergy cosumptio is: "!( " ) + # i K e! ik( " e) e t t i= = = Kp K( " e) # ( i"" ) i= =! +! i " e e (6) 56

8 The eergy cosumptio ratio is: c ( + ) µ! = = + t i## [ " + " i( # e) e]( K + m)(- e) $ i= (7) By Formula (7), it is leared that ">. I this research, radom liear codig is used for the simple computatio of odes. 3.2 Multi-path trasmissio Suppose there exist L o-itersectig paths which are of equal legth, ad the source ode seds K packets simultaeously through multiple paths to the sik ode. The, the probability for ay packet to successfully reach the sik ode after oe trasmissio is: p =![!(! e) ] L (8) Thus, the probability for the sik ode to successfully receive K packets is Kp, that is, the reliability R is, P = p = {![!(! e) ] } k K L K (9) If K packets are set alog the L paths, the total eergy cosumptio is: ' = t L t (0) The eergy cosumed to sed the K packets alog the L paths is: t ' ' = t ( ) = t! (! ) = LK e LK e t () Whe the etwork codig is employed i multi-path trasmissio, the itermediate odes oly ecode without icreasig redudat packets. I this case, the total eergy cosumed to sed K packets is: ' = [!(" ) + # c! (" i" ) ] + [( " ) + 2] i= L K e ik e e L (2) The umber of packets received by the sik ode is expected to be: LK( e)! (3) Therefore, the eergy cosumptio is: c ' = c LK(! e) ' (4) ijo Vol. 3, No. 2,

9 The eergy cosumptio ratio! is: '! ' = = c c ' t = + ' t ' + [(! ) L+ 2] e 3000 [ + (! ) + (!! )(! ) LK e e e ] (5) 4 Simulatio xperimet ad Result Aalysis The model give i the previous sectio was simulated i the eviromet of MATLAB Sigle-path trasmissio Suppose that the path legth is 5, ad the source ode seds 3 packets to the sik ode such that the sik ode receives 5 ecoded data packets. The, the reliabilities of the etwork codig ad the traditioal method were simulated at differet chael loss rates. As show i Figure 4, the etwork codig has achieved better reliability tha the traditioal method. This paper oly cosiders the redudat coded packets at the source ode. The reliabilities of the two methods still decreased with the loss of chael. I a sigle-path, the etwork codig improved the reliability to a greater extet tha the traditioal method at high chael loss rate of redudat data ad with few etwork ecodig packets; however, the results were still ot desirable. To fruther improve the data trasmissio reliability of etwork codig, appropriate redudat data packets should be added to the odes. Figure 5 presets the eergy cosumptio ratio of the two differet trasmissio methods. It ca be see that " icreased with e. However, regardless of the size of the e, the eergy cosumed by the sigle-path trasmissio combied with etwork codig always exceeded the eergy cosumed by that combied with the traditioal method. This meas etwork codig ca improve the reliability of data trasmissio i the etwork at the expese of a small amout of eergy. 4.2 Multi-path trasmissio Similarly, assume that 5 packets are set from the source ode alog 3 paths, each of which is 5 i legth. The, the reliabilities of the etwork codig ad the traditioal method were simulated at differet chael loss rates. As ca be see from Figure 6, the etwork codig method outperformed the traditioal method at the same redudacy. With the icrease of the chael loss rate, both method trasmitted fewer redudat data, idicatig that the trasmissio improvemet failed to offset lik failure ad results i loger paths. Figure 7 illustrates the eergy cosumptio ratio betwee the two differet methods. It shows that etwork codig ca improve the reliability of data trasmissio i the WSN at the expese of a small amout of eergy. 58

10 5 Coclusios This paper calculates ad aalyses the trasmissio reliability ad eergy cosumptio of etwork codig i sigle-path ad multi-path scearios. The simulatio o MATLAB 7.0 idicates that the techique is more reliable tha the traditioal method at the expese of a small amout of eergy. I additio, the author also simulated the reliabilities ad the eergy cosumptio ratio of the etwork codig ad the traditioal method. The experimetal results verify that the etwork codig techique is superior to traditioal trasmissio. Fig. 4. The reliabilities of the two methods Fig. 5. The eergy cosumptio ratio of the two methods ijo Vol. 3, No. 2,

11 Fig. 6. The reliabilities of the two methods Fig. 7. The eergy cosumptio ratio of the two methods 6 Refereces [] Su L.M., Li J.Z. (2005). Wireless Sesor Networks, Beijig: Tsighua Press. [2] Meg H.J. (200). Research o Optimizatio of etwork reliability ad its applicatio, Shaghai: ast Chia Uiversity of Sciece ad Techology. [3] Li R.Y., Huag N., Che W.W. (2009). A Practical Approach for Network Applicatio Reliability Assessmet, k-sploatacja Iiezawodosc-Maiteace ad Reliability, 4, pp

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