Bluetooth for Safety Systems

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1 ISSC 0, Triity College Dubli, Jue 3-4 Bluetooth for Safety Systems Pava Kumar Pedli, Michael Schwarz, Has Dieter Wacker & Josef Boercsoek Departmet of Computer Architecture ad System Programmig Uiversity of Kassel, Kassel Abstract I early every factory floor ad idustrial settig, commuicatio liks carry vital iformatio betwee machiery, cotrol, ad moitorig devices. Safety related or safety critical systems demad that commuicatio liks are safe, reliable ad the trasmitted iformatio is timely delivered without fault ad errors. Additioally, real-time performace, robustess, optimized performace, optimised throughput, latecy ad power cosumptio issues are to be cosidered. This paper presets a approach exploitig Bluetooth short-rage radio techology with a additioal safety layer to achieve safe commuicatio for safety systems. Additioally, this paper presets a mathematical descriptio to show that this approach meets the requiremets of safety systems by achievig Safety Itegrity Level 3 (SIL3) accordig to the stadard IEC Keywords Bluetooth, Bluetooth protocol stack, Safety systems ad Safety layer. I INTRODUCTION I recet years, wireless commuicatio has bee a active area of research. A large umber of govermet ad idustry iitiatives, health care & medical fields, research efforts ad stadard activities have aimed at eablig wireless ad mobile etworkig techologies []-[5]. There are diverse set of wireless access techologies available from satellite etworks, to wide area cellular systems, from wireless local loop ad PCs to wireless LANs ad WPAN s. Bluetooth a WPAN techology is aalysed i this paper for safety related systems. As stadard Bluetooth commuicatio is ot suitable, a safety layer is developed, so that Bluetooth techology ca be used for safety related systems. Bluetooth Special Iterest Group (SIG), which was formed i 998 by mobile telephoy ad computig leaders like Ericsso, IBM, Itel, Nokia, ad Toshiba are workig o Bluetooth techology ad believe that it ca revolutioise wireless coectivity for persoal ad busiess mobile devices, eablig seamless voice ad data commuicatio via short-rage radio liks. The techology allows users to coect a wide rage of devices easily ad quickly, without the eed for cables, expadig commuicatio capabilities for mobile computers, mobile phoes ad other mobile devices, both iside ad outside of the office [6]. Bluetooth is iitially desiged keepig i mid short-rage wireless data/voice trasmissios, ad-hoc etworks ad cable replacemet purposes. It also provides to some extet reliability for applicatios. The techology was ot desiged for safety related, safety critical ad real time idustrial applicatios, where the messages are ofte short but typically demadig more reliable, robust, secured, safe, time critical ad faster trasmissio. Much of the research work o Bluetooth techology is doe to improve throughput ad reliability of Bluetooth commuicatio, to provide secured commuicatio agaist security threats, to improve Bluetooth RF trasceiver desig for low power cosumptio, to aalyse Bluetooth performace, to provide faster data trasmissio ad to improve the pollig time [7]-[]. This paper ivestigates Bluetooth techology to achieve safe commuicatio for safety related systems. The implemeted safety methods by the stadard Bluetooth techology are aalysed ad as they are ot sufficiet to provide safe commuicatio, a separate safety layer is desiged for this purpose. The mathematical equatios for the calculatios of safety are also derived with the proposed GEC wireless chael model ad the ew approach for wireless data trasmissio to reduce the bit errors, probability of udetected error ad the bit erasure (loss of iformatio) preset i the wireless chael. The calculatios show that with Bluetooth

2 commuicatio SIL3 is achieved which is the most accepted value for safety systems. The structure of this paper is as follows: sectio gives a brief overview of the Bluetooth protocol stack with lower ad upper layers. Sectio 3 gives the values of Safety Itegrity Level (SIL) for safety systems. Sectio 4 explais the implemeted safety methods by the stadard Bluetooth techology ad basebad layer SIL calculatios performed for differet Bluetooth ACL (Asychroous Coectioless) packets. I sectio 5, the modellig of a wireless chael with ew approach is explaied, i sectio 6 the safety layer is desiged at the LCAP (Logical Lik Cotrol ad Adaptatio Protocol) layer of the Bluetooth protocol stack to achieve fail-safe commuicatio usig Bluetooth techology for safety related systems ad fially coclusios i the last sectio. II BLUETOOTH PROTOCOL STACK Bluetooth protocol stack determies how the commuicatio betwee the devices usig Bluetooth techology is possible ad it is the core of the Bluetooth specificatio, which defies how the techology works [6]. It cosists of lower layers ad upper layers with Host Cotroller Iterface (HCI) as a iterface betwee them. The lower layers of Bluetooth protocol stack are Radio layer, Basebad, ad Lik Maager called together as a Bluetooth cotroller ad the upper layers cosists of LCAP, RF Commuicatio (RFCOMM) ad Service Discovery Protocol (SDP) called together as Host System as show i Figure. these services basebad layer also provides other services like error correctio, data whiteig, hop selectio security ad some safety methods explaied i the followig sectios. LCAP provides coectio-orieted ad coectioless data services to upper layer protocols with protocol multiplexig capability, segmetatio ad reassembly operatio ad group abstractios. LCAP permits higher-level protocols ad applicatios to trasmit ad receive LCAP data packets up to 64 kilobytes i legth. LCAP supports oly ACL liks; therefore, it is defied oly for data traffic such as file, audio/video trasfer. For detailed descriptio of the fuctioalities of each layer, refer Bluetooth specificatio [6]. III SIL LEVELS FOR SAFETY SYSTEMS Safety related or safety critical systems are systems i which failures/errors leads to hazardous situatios like death, ijury or evirometal damage, examples are such as idustrial process cotrol systems, process shutdow systems, railway sigallig equipmet, automotive cotrols, medical treatmet equipmet, faulty electrical devices etc. For such systems, the stadard IEC 6508 ad IEC 65 have defied four differet SIL s with two differet modes, low mode ad high mode for measurig the risk reductio for SIS (Safety Istrumeted Systems) as show i Table [3]. SIL PFD (Λ) (Low Demad) PFH (Λ per hour) (High Demad) SIL 0 - to to 0-5 SIL 0-3 to to 0-6 SIL3 0-4 to to 0-7 SIL4 0-5 to to 0-8 Table : Safety Itegrity Levels. I Table, the Probability of Failure o Demad/ Probability of Failure per Hour (PFD/PFH) values are give by the symbol Λ (lambda) ad these values determie the SIL level. Figure : Bluetooth protocol stack. The radio layer describes the physical characteristics of the compoets of the Bluetooth s trasceiver. Basebad layer maages physical chaels ad physical liks, ad is resposible for properly formattig data for trasmissio to ad from the radio layer. The basebad layer also maages sychroous ad asychroous liks, hadles packets, does pagig, ad iquiry procedures for the available Bluetooth devices i the rage. Apart from IV BLUETOOTH PROTOCOL STACK IMPLEMENTATION FOR SAFETY The geeral Bluetooth Basic Rate (BR) mode basebad packet format is as show i Figure. It cosists of access code (68/7 bits), the header (54 bits), ad the payload (0-745 bits) [6]. The header cosists of 3-bit logical trasport address (LT_ADDR) for idetificatio of the seder ad receiver, 4-bit packet type (TYPE) for packet type idetificatio, -bit flow cotrol bit (FLOW) for flow cotrol over the ACL lik, -bit for ackowledgemet idicatio (ARQN), -bit sequece umber (SEQN) for sequetial umberig scheme ad a 8 bit header error check (HEC) for securig the header.

3 l l Pue ( ε,c ) = Al ε ( ε ) () or i terms of dual code as [7]: Figure : Bluetooth basebad layer packet format. With these fields i the header, some of the safety methods like idetificatio, flow cotrol, ackowledgemet ad sequetial umberig specified i the documet EN [4] are achieved, to avoid the data trasmissio errors ad erasures that occur due to the list of failures as listed i the same documet EN With the payload protected by a 6 bit CRC-CCITT data safety method is achieved. With stop-ad-wait ARQ (SW-ARQ) or a uumbered ARQ scheme cocept at the basebad layer, Bluetooth provides very poor throughput performace. At the LCAP layer there are cofigurable optios which ca be cofigured such as maximum trasmissio uit (MTU), flush timeout (FTO) optio, quality of service (QoS) ad retrasmissio ad flow cotrol optio. With retrasmissio ad flow cotrol optio cofigured, some bufferig ca be itroduced at the trasmitter ad the protocols like go-back-n (GBN-ARQ) ad selective-repeat ARQ (SR-ARQ) are implemeted, from which sequetial umberig is further improved. With QoS optio cofigured, flow cotrol method is further improved. With MTU ad FTO optios cofigured, safety cotrol methods timestamp ad timeout are achieved. For the 6-bit CRC-CCITT payload protectio scheme used at the basebad layer, graph is plotted as show i Figure 3 for differet Bluetooth ACL packets to determie the SIL level: For the calculatio of SIL level, the followig equatios are used [5]: Λ = 3600 P ue ( ε,c ) v 00 ( m ) () where Λ is the PFH value give by the equatio (), P ue is the probability of udetected error with ε as bit error ratio ad C as the liear code of CRC protectio scheme used. The umber of safety related messages per secod is give by v=r/ (R is the data rate, is packet legth), m represets the umber of commuicatig devices, take its value as for calculatios ad % itroduced to avoid the safety commuicatio errors. The P ue is give by the formulas [6]: r l Pue ( ε,c ) = Bl ( ε ) ( ε ) (3) 0 Here, A l is the weight distributio of liear code C, B l is the weight distributio of liear dual code C with umber of codeword s of Hammig weight l ad ε is the bit error ratio. For Hammig codes, the formulas to eumerate A l or B l are derived i referece [8], but o geeral formula is kow for shorteed Hammig codes. The weight distributio of the dual of a shorteed Hammig code ca be computed with a method called direct method as explaied i referece [9]. By computig weight distributios of dual codes at a give ε, P ue for shorteed Hammig codes ca be determied but the computig requires mathematical calculatios or a algorithm to be desiged [7]. Istead of computig A l or B l for Hammig or shorteed Hammig codes, the P ue calculatios are performed with upper boud equatio (4) give as [0]: 7 d d P ue( ε,c ) π ε + R (4) r d! Here, ( ) ( ε ), 3& eve R( ε ) ( ) (5) ( ε ), 4&odd where r is the umber of parity bits, d is the hammig distace ad the remaider term R is give by the equatio (5). Equatio (4) is valid for proper codes for which P ue (ε,) icreases mootoically with ε values [0, 0.5] at a payload legth. The graph BER vs. PFH is draw i Figure 3 at the basebad layer for differet Bluetooth ACL packets. PFH SIL3 -DM3:=60-3-DH5:=60 3-DM3:=5 4-3-DH5:=5 5-DH3:= DH5:= BER (Bit Error Ratio) Figure 3: Bluetooth basebad layer packets BER vs. PFH for =06 to 04 bits. 6

4 The graph is plotted for payload legth from =60 to 04 bits, as the 6-bit CRC-CCITT code is proper at these values. SIL levels are achieved with the required PFH values for differet Bluetooth ACL DM3 (Data Medium), DH3 (Data High) ad 3-DH5 packets as show i Figure 3, but to achieve these SIL levels a BER 0-5 or for SIL3 a BER of 0-6 is required, which is ot a practical value for ay wireless or Bluetooth commuicatio. From these results, it is show that the CRC- CCITT used for payload protectio is ot sufficiet to use Bluetooth commuicatio for safety systems. Therefore, the data safety method has to be further improved. I the safety cotrol methods listed i the documet EN [4] the method redudacy with cross compariso is also ot implemeted by the stadard Bluetooth techology, which is implemeted here with a ew approach. V WIRELESS CHANNEL MODEL As the wireless commuicatio has both errors ad erasures preset i the wireless chael, the chael is modelled as a Geeralised Erasure Chael (GEC) model, which takes ito accout both the errors ad erasures preset i the chael. As the bits are lost, it is very difficult to decide if a sigle bit received as 0 has bee trasmitted ucorrupted or result of the fadig of the chael, so a ew method with trasmissio of each iformatio bit twice with iversio is proposed i this paper. The GEC wireless chael model is as show i Figure 4. Figure 4: GEC wireless chael model. I this model both the effects of a Biary Symmetric Chael (BSC) chael ad Biary Erasure Chael (BEC) chael are take ito cosideratio. The graphical represetatio of this model with the iformatio bit trasmitted twice with iversio is as show i Figure 5. output symbol. To detect the bit erasures ad to reduce the errors preset i the wireless chael, the origial iformatio bits are trasmitted twice with iversio, i.e. 0 bit is trasmitted as {0,} ad trasmitted as {,0} ad the erasure is represeted by symbol e. At the receiver, the received dual bits are checked for ativalece; if it is the case the the origial iformatio bits are extracted ad accepted. With this method, the bit errors are reduced ad the bit erasures are detected cofidetly. The calculatio of trasitio probabilities η, θ ad ζ with this method, whe the iformatio bits are trasmitted through a sigle GEC wireless chael are give as i equatio (6) with ε as bit error ratio ad φ as bit erasure: η = ε ( ϕ) θ = ( ε) ( ϕ) ζ = ϕ + ε( ε) ( ϕ) (6) The probability of udetected error of the GEC chael is give by []: l l Pue( ζ,η,θ,c ) = Al θ (7) ad by substitutig equatio (6) i (7) P ue is give as: l ( l ) Pue (, ϕ,c ) = ( ϕ ) Al ε ( ε ) ε (8) VI BLUETOOTH SAFETY LAYER The ew method to reduce the bit errors ad to detect the bit erasures is implemeted as a separate safety layer at the LCAP layer of the Bluetooth protocol stack as show i the Figure 6. Figure 5: GEC wireless chael model graphical represetatio. I Figure 5, η represets probability of error symbol, θ as probability of o error symbol ad ζ as probability of erasure or loss of iformatio symbol. X i represets the iput symbol ad Y o represets the Figure 6: Bluetooth protocol stack with safety layer ad safe data trasmissio. The iformatio bits protected with the CRC3c (c-castagoli) scheme termed as code word is trasmitted with the ew method proposed i this paper. The data trasmitted from the LCAP layer to

5 upper layers or lower layers of Bluetooth with the implemeted safety layer, by reducig BER ad by detectig the erasures occurred i the data trasmissio via wireless chael is cosidered safe data. With the implemeted CRC3c scheme ad the ew method for trasmittig the bits twice with iversio, the graph for the PFH values to achieve SIL levels is plotted as show i Figure 7. PFH =843,d=6 -=9456,d=6 3-=504,d=6 4-=49,d=6 SIL PE (Erasure) Figure 7: Bluetooth LCAP PE vs. PFH with double data trasmissio ad iversio, CRC-3/6. For a BER of 0-3 (a practical value for the wireless commuicatio), payload legth of =4k bits with hammig distace d=6, PE (Probability of Erasure) of 0-6 a safety itegrity level of SIL3 is achieved which is the most acceptable value for safety systems. For the calculatios, P ue upper boud equatio (9) is derived ad used, where φ is the probability of bit erasure, ε the bit error ratio ad C () represets the code word beig trasmitted twice. ( ), Pue( ε,c ) Pue( ε C ) ( ϕ ) Here, R( ε ) 7 d d π ε + R( ε ) r d! ( ) ( ε ), 3& eve ( ) ( ε ), 4 & odd (9) (0) VII CONCLUSIONS Stadard Bluetooth techology does ot provide fail-safe commuicatio ad is ot suitable for safety related systems. The techology should be further developed for this purpose. With the implemeted safety layer protected by a CRC3c scheme ad double data trasmissio with iversio safety is achieved. With the ew method proposed, istead of retrasmissio of the same message agai, each bit of the message is trasmitted twice with iversio. This itroduces a larger redudacy, icreases the trasmissio badwidth requiremets or message delay or both but reduces the probability that all of the origial data will be iverted or erased ad the erasure goes udetected, providig a safe wireless commuicatio for safety related systems. REFERENCES [] N.P.Mahalik, Fieldbus Techology, Idustrial Network Stadards for Real-Time Distributed Cotrol, spriger editio, Berli, 003. [] J.Alae, M.Hietikko & T.Malm, Safety of Digital Commuicatio i Machies, VTT Research Notes 65, Espoo 004. [3] R.Zurawski, The Idustrial Commuicatio Techology Hadbook, CRC Press, Florida, 005. [4] D.Mioradi, E.Uhlema, S.Vitturi & A.Willig, Wireless Techologies i Factory ad Idustrial Automatio - Part I ; IEEE Trasactios o idustrial iformatics, May 007. [5] B.Yu, L.Yag & C.C.Chog, ECG Moitorig over Bluetooth: Data Compressio ad Trasmissio, IEEE/WCNC, 00, pp-5. [6] Bluetooth SIG, Bluetooth Specificatio Versio 4.0, SIG, vol 0, 009. [7] J.Liag, Y.Li & B.Yu, Performace Aalysis ad Reliability Improvemet of Bluetooth Broadcast Scheme, IEEE Pervasive Computig ad Applicatios, 006, pp [8] T.Margaret & M.K.Aguilar, A Ivestigatio of Bluetooth Security Threats, IEEE/ICISA, 0, pp -7. [9] L.B.Li, Z.Huag, Y.Jiao, X.Zheg, & S.Wag, Low power RF trasceiver desig for Bluetooth applicatios, IEEE/ICSICT, 008, pp [0] X.Tia & J.Lu Performace of Bluetooth FH commuicatio system based o geeral quadratic prime code, IEEE/ICNIDC, 00, pp [] J.C.Cha & P.W.Tse, A Novel, Fast, Reliable Data Trasmissio Algorithm for Wireless Machie Health Moitorig, IEEE Trasactios o Reliability, 009, pp [] D.Cotreras & M.Castro Adaptive Pollig Ehaces Quality ad Eergy Savig for Multimedia over Bluetooth, IEEE Commuicatios Letters, 0, pp [3] IEC 6508, IEC 6508: fuctioal safety of electrical/ electroic/ programmable Safetyrelated systems, IEC, 000. [4] EN 5059-, Safety-related commuicatio i ope trasmissio system, Europea committee for electro techical stadardizatio, part-, 00, pp 44. [5] J.Boercsoek, J.Hoelzel & H.D.Wacker, Probability of Udetected Error with Redudat Data Trasmissio o Biary Symmetric ad No- Symmetric Chaels without Memory, WSEAS Trasactios o Commuicatio, Joural vol. 6, issue, ISSN: 09-74, 007, pp [6] S.B.Wicker, Error Cotrol Systems for Digital Commuicatio ad Storage, Pretice-Hall, 995.

6 [7] J.K.Wolf & R.D.Blakeey II, A Exact Evaluatio of the probability of udetected error for certai shorteed biary CRC codes, IEEE coferece, Qualcomm Ic, Sa Diego, CA, 988. [8] W.W.Peterso & E.J.Weldo, Jr, Error- Correctig Codes, M.I.T. Press, d ed., Cambridge, MA, 97, pp [9] T.Fujiwara, T.Kasami, S.Kitai & S.Li, O the udetected error probability for shorteed hammig codes, IEEE Trasactio Commuicatio, Vol. COM 33, No. 6, Jue 985 [0] H.D.Wacker & J.Boercsoek, Biomial ad Mootoic Behaviour of the Probability of Udetected Error ad the -r -Boud, WSEAS, Volume 7, Issue 3, March 008. [] T.Baicheva, S.Doduekov & P.Kazakov, Udetected error probability performace of cyclic redudacy-check codes of 6-bit redudacy, IEEE Proceedigs Commuicatio, Vol. 47, No. 5, Oct 000. [] H.D.Wacker, J.Boercsoek & H.Hillmer, Redudat optical data trasmissio usig semicoductor lasers, IEEE/ACS, 008, pp

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