Research o Fault Tolerace for the Statc Segmet of FlexRay Protocol Ru I a, Ye ZHU a, Zhyg WANG b a Embedded System & Networkg aboratory, Hua Uversty, Cha b the School of Computer, Natoal Uversty of Defese Techology, Cha ru@hueduc, zhukovyear@foxmalcom, zywag@udteducom Abstract FlexRay s the ext geerato commucato protocol automotve etwork But the developmet was hdered because most of the researches gored traset fault message trasmsso I ths work, a complete framework s preseted to creases the relablty by retrasmttg messages the statc segmet It cludes system model, mathematcal abstracto ad a heurstc algorthm Gve a specfcato of the system model, our work wll ot oly get the system satsfy the relablty goal but also optmze the badwdth utlzato rate Smulato results show that our approach performs better tha the exstg algorthm badwdth utlzato rate (-9%), relablty (+007%) ad rug tme (-372%) I our opo, ths algorthm s so geerc that t ca be wdely used other tme-trggered ad relablty crtcal applcatos eywords Fault tolerace, Badwdth utlzato rate, FlexRay, retrasmt message, statc segmet I INTRODUCTION I moder hgh-ed cars, lke the BMW X7 seres, t s commo to see about 70 electroc cotrol uts (ECUs) exchagg 2500 electrc sgals sde the cars [] These ECUs are dstrbuted varous parts of the car ad are hardwred wth dfferet actuators or sesors Wth the crease of the ECU umber, tradtoal pot-pot lks have bee replaced by bus etwork [3] FlexRay appeared as a fast, fault-tolerat ext geerato etwork protocols 2004 Although FlexRay have a great advatage compared to other bus etwork protocol, ts developmet was stll be hdered for ts lackage of a applcato layer scheme to prevet traset fault the process of message trasmt Ad t caot guaratee relablty [] Varous techques for etwork parameters optmzato, tme aalyzg framework ad task schedulg have bee studed the feld of the FlexRay research The work [3] descrbes a FlexRay etwork parameters optmzato method that focuses o the legth of commucato cycle wth mmum worst case respose tmes (WCRTs) Performace aalyss framework for a etwork of ECUs that commucate va a FlexRay bus was troduced [2] Referece [5] provdes solutos for dfferet task schedulg polces based o a mx teger lear programmg framework However, oly a few of them focus o relablty of FlexRay I 200, Taasa presets a framework of geeratg fault-tolerat message schedules by retrasmttg messages the statc segmet of FlexRay [] But ther algorthm caot geerate the optmal retrasmsso scheme savg the badwdth The ma cotrbuto of ths paper s a algorthm of fdg the retrasmsso scheme whch performs better tha the exstg works badwdth utlzato rate, relablty ad rug tme Frst, a system model ad mathematcal abstracto were made for the fault tolerace problem FlexRay The a heurstc algorthm H- whch ca be used to fd the optmal retrasmsso scheme savg the badwdth ad at the same tme satsfy the relablty goal was preseted At last, some smulato tests were doe Matlab200 The results show that our approach performs better tha the exstg works badwdth utlzato rate (-9%), relablty (+007%) ad rug tme (-372%) I our opo, ths algorthm s so geerc that t ca be wdely used other tme-trggered ad relablty crtcal applcatos The rest of the paper s orgazed as follows I Secto II, formal model was proposed to aalyze fault tolerace the statc segmet of FlexRay Secto III troduces mathematcal abstracto of the problem ad a ew heurstc retrasmsso algorthm I Secto IV, a smulato comparso test was desged for the algorthms The cocluso of ths paper was preseted Secto V II SYSTEM MODE AND EQUATIO A Meda Access Cotrol of the FlexRay Our system model s based o the commucato level of FlexRay As show Fgure, every commucato cycle cotas the statc segmet, the dyamc segmet, the symbol wdow ad the etwork dle tme (NIT) [4] Statc segmet are desged for tme trggered messages usg Tme Dvso Multple Access (TDMA) scheme Whle the dyamc segmet s usg m-slottg based scheme to trasmt evet trggered messages The symbol wdow s a commucato perod whch a symbol ca be trasmtted o the etwork The NIT s a commucato-free perod that cocludes each cycle [4]
Fgure The system model cludg some parameters metoed our work Ths work oly cocetrates o the statc segmet As show Fgure, the statc segmet s made up of cosecutve tme tervals called slots Each slot s allocated to a task ad ca be used to trasmt oe FlexRay message If the task has o message to sed, the slot goes empty [2] B System model The system model s dvded to two parts: the message model ad the bus model Message model s a message set whch cludes messages M { M, M 2, M } ad some parameters about the message ) Cycle tme: The cycle tme set T { T T T }, 2, Cycle tme T s the costat tme terval betwee message producg twce successo M 2) Error rate: The message error rate set P { P P P},, 2 Error rate P s the falure probablty of trasmttg message M oce 3) Number of retrasmsso tmes: The umber of retrasmsso tmes set {, 2, } s the umber of tmes that message M wll be retrasmtted cycle tme T The bus model s preseted as follows: ) Commucato cycle legth: FC FC s the legth of a commucato cycle 2) Number of slots: It stads for the totally umber of slots oe statc segmet 3) Relablty goal: ρ It meas how relable the system message trasmsso process eeds to be 4) Global probablty of success: GP GP s the probablty that all messages trasmt successfully test perod τ If GP > ρ, t s beleved that the system s relable 5) Test perod: τ τ s the statstcal rug tme of the system As show Fgure, there are 4 messages trasmttg T ms,2 ms,4 ms,8ms o the bus ad the cycle tme set s { } FC s 2ms ad s 5 C Equatos I order to fd the optmal retrasmsso scheme badwdth utlzato ad satsfy the relablty goal, three equatos eed to be preseted frst () ad (2) were preseted by Taasa [] s defed as the lower boud of whch satsfy GP > ρ I (3), the badwdth utlzato rate s defed as the rato of the umber of used slots to the umber of slots suppled τ, whch ca be used to show the stuato of the used slots τ τ + T GP ( P ) () T log ρ τ log P (2) used slots + FC Badwdth utlzato rate (3) all slots T I T, M wll trasmt oce ad retrasmt tmes So ( ) + τ T slots wll be used to trasmt or retrasmt
all of the messages τ The, there are τ FC commucato cycles supplyg τ FC slots τ Fally, (3) was proofed by gettg the umber of used slots τ dvded by the umber of slots suppled τ Taasa regarded the umber of used slot oe FC as ts optmal obectve [] But we beleve the badwdth utlzato rate s much more sutable tha t A aalyss about ths wll be preseted wth the help of a example Ths example has oly oe message M wth oe retrasmsso ad cycle tme T 0ms The FlexRay cycle s FC 5ms wth 5 slots the statc segmet The the umber of used slots oe FC ca be computed as 0, or 2 Ths s because there are two FC oe T tme The trasmsso ad retrasmsso of M may happe the frst FC or the secod oe As a result, the umber of used slots oe FC s ot a proper optmal obectve for ths problem because t oly evaluates the stuato of oe FC ad T may be loger tha FC O the other sde, the badwdth utlzato rate s much more sutable to be the optmal obectve tha the umber of used slots oe FC I the example above, the badwdth utlzato rate ca be computed as 20% accordg to (3) Obvously, t shows the stuato of used slots test tme τ stead of FC correctly I our opo, the optmzato of the badwdth utlzato rate wll reduce the complexty schedulablty aalyss Ad the parameter cofgurato process could also beeft from t mmzg worst case respose tme [3] I cocluso, the badwdth utlzato rate was regarded as the optmal obectve ths work III RETRAMISSION AGORITHMS A Mathematcal Abstracto Dfferet from the exstg works usg () drectly, t s foud that after takg a logarthm computato to both sde of GP > ρ ad some smple trasformatos, GP > ρ wll log( + P ) τ be trasformed to < whch ca be T log ρ used to measure the effect of creasg oe message s retrasmsso tmes So the left part of t was defed as a ew fucto f( ) whch ca be regarded as the cotrbuto of M retrasmttg tmes The less f( ), the more relable the trasmsso wll be log( + P ) τ f( k) (4) T log ρ The the relable costra GP > ρ was trasformed to f ( ) < If f ( ) < we ca beleve that the system s relable The the mathematcal abstracto of the problem was preseted as follows T, P, τ, ρ, FC, are all kow The retrasmsso tmes set should be computed to satsfy the relablty goal f ( ) < The optmal obectve s to mmze the badwdth utlzato rate + FC T I order to descrbe the algorthm smply, aother auxlary fucto g( ) was defed (5) It descrbes the dfferece of cotrbuto retrasmttg M from to + tmes g ( ) f ( ) f ( + ) (5) B H- algorthm s dea The basc dea of the H- algorthm s: each loop, H- creases the curretly most effectve message s retrasmsso tmes oce utl the relablty goal f ( ) < s satsfed The most effectve message s the message that has the maxmum rato of cotrbuto dfferece g( ) to badwdth utlzato rate crease For example, f H- retrasmts M oe more tme, t wll reduce f ( ) by g( ) ad badwdth utlzato rate wll crease FC T accordg to (3) The crease of the badwdth utlzato rate s cosdered as T because both of FC ad are costat So the most effectve message we select to crease ts s the message wth maxmum cotrbuto dfferece value g( ) T badwdth utlzato crease C H- algorthm descrpto I our algorthm below, s talzed to usg (2) (le 2,3) The each loop (le 7), H- chooses the message wth the maxmum g( ) T(le 9) ad creases ts retrasmsso tmes by oe (le ) to optmze the badwdth utlzato rate utl the relablty goal s satsfed (le 7) or the badwdth utlzato rate s larger tha 00% (le 8) Iput: Message set M, probablty of falure set P, cycle tme set T, relablty goal ρ, commucato cycle FC ad the umber of slots the statc segmet for {, 2, } 2 compute 3 4 compute f( ) 5 ed for 6 SumOfCotrbuto f( ) 7 whle ( SumOfCotrbuto > )
(a) (b) (c) Fgure 2 Smulato test result (a) Badwdth utlzato rate (b) Relablty (c) Rug tme + FC 8 f ( < ) the T 9 the message umber wth max g( ) T { g( ) T, g2( 2) T2, g( ) T} 0 SumOfCotrbuto g ( ) + 2 else retur badwdth overflow 3 ed f 4 ed whle 5 retur {, 2, } As oted before, the H- algorthm s a ew method of fdg the retrasmsso scheme whch performs better tha the exstg work badwdth utlzato rate, relablty ad rug tme IV SIMUATION TEST As preseted above, ths work s focusg o fdg the retrasmsso scheme whch ca satsfy the relablty goal ad optmze the badwdth utlzato rate So the smulato expermet cocetrates o three evaluato dcators: the badwdth utlzato rate, the relablty goal s acheved or ot ad the rug tme 6 cases were geerated durg the umber of messages from 5 to 20 ad for each case 00 examples were studed For radomess ad accuracy, each example wll ru 00 tmes ad all of the examples were geerated radomly by varyg some message parameters For example, T were selected from 5ms to 20ms, P were set 0% to 50% Whle the others are assged statcally as follows: FC 5ms, 50, ρ 99%, τ hour The etre smulato has bee mplemeted a Wdows XP mache rug a Itel(R) Petum(R) Dual E6700 32GHz processor wth MATAB 200a The average badwdth utlzato rate of each case was preseted Fgure 2(a) ad the badwdth overflowg tmes 00 examples were show Fgure 3 Compared to the exstg works, our H- reduces the average badwdth utlzato rate by 9% ad t also grows slower tha Taasa s badwdth overflowg tmes Because our algorthm s optmal obectve s the badwdth utlzato rate stead of the umber of used slots oe FC Fgure 2 (b) shows the average relablty of each case Ad the relablty suffcet tmes 6 00 examples were plotted Fgure 3 Both of them satsfy the relablty goal wthout ay relablty suffcet examples ad our algorthm s a lttle bt more relable (average +007%) tha the prevous ways At last, we focus o the least cocered evaluato dcator: rug tme because H- s a statc algorthm for the statc segmet After tme complexty aalyss, H- s less or equal to the exstg algorthm 2 ( O ( )) As show Fgure 2(c), the average rug tme s reduced by 372% our algorthm Fgure 3 Badwdth overflow ad relablty suffcet tmes 00 examples
V CONCUSIO I ths work, a complete framework s preseted to creases the relablty by retrasmttg messages the statc segmet It cludes system model, mathematcal abstracto ad a heurstc algorthm amed H- whch ams at fdg the optmal retrasmsso scheme savg the badwdth ad satsfyg the relablty goal The smulato results show that H- algorthm performs better tha the exstg works ot oly badwdth utlzato rate (-9%) but also relablty (+007%) ad rug tme (-372%) I our opo, ths algorthm s so geerc that t ca be wdely used other tme-trggered ad relablty crtcal applcatos REFERENCES [] BTaasa, U D Bordolo, P Eles, ad P Zebo, Schedulg for Fault-Tolerat Commucato o the Statc Segmet of FlexRay, Real-Tme Systems Symposum (RTSS), 200 IEEE 3st, 200, pp 385-394 [2] A Hagescu, U D Bordolo, S Chakraborty, P Sampath, P V V Gaesa, ad S Ramesh, Performace Aalyss of FlexRay-based ECU Networks, Desg Automato Coferece, 2007 DAC '07 44th ACM/IEEE, 2007, pp 284-289 [3] P Iseok ad S Myougho, FlexRay Network Parameter Optmzato Method for Automotve Applcatos, Idustral Electrocs, IEEE Trasactos o, vol 58, pp 449-459, 20 [4] The FlexRay Commucatos System Protocol Specfcato 2 Rev A [O-le] Avalable: wwwflexraycom [5] Z Habo, Z We, M D Natale, A Ghosal, P Gusto, ad A Sagova-Vcetell, Schedulg the FlexRay bus usg optmzato techques, Desg Automato Coferece, 2009 DAC '09 46th ACM/IEEE, 2009, pp 874-877 [6] Feg, et al, Research o FlexRay commucato system, Vehcle Power ad Propulso Coferece, 2008 VPPC '08 IEEE, 2008, pp -5 [7] lobedaz, et al, A recofgurato approach for fault-tolerat FlexRay etworks, Desg, Automato & Test Europe Coferece & Exhbto (DATE), 20, 20, pp -6 [8] H opetz, A tegrated archtecture for depedable embedded systems, Relable Dstrbuted Systems, 2004 Proceedgs of the 23rd IEEE Iteratoal Symposum o, 2004, pp 60-6 [9] M ukasewycz, et al, FlexRay schedule optmzato of the statc segmet, preseted at the Proceedgs of the 7th IEEE/ACM teratoal coferece o Hardware/software codesg ad system sythess, Greoble, Frace, 2009 [0] P Maro, et al, Electrocs Automotve Egeerg: A Top- Dow Approach for Implemetg Idustral Feldbus Techologes Cty Buses ad Coaches, Idustral Electrocs, IEEE Trasactos o, vol 56, pp 589-600, 2009