IEEE 802.11e in Industrial Environments a Quality of Service Survey

IEEE802.11e in Industrial Environments:a Quality of Service SurveyMARKUS J¨AGERMaster’s Degree ProjectStockholm,Sweden2005-12-05IR–SB–EX–0525AbstractThe communication between control units andfield devices in industrial envi-ronments is based on so-calledfieldbus technologies-networks optimized for the realtime transmission of highly sensitive data for control and monitoring purposes.As a strong need for wirelessfieldbus systems arose,the applicability of available wireless standards has been examined.Some specialized wireless systems are arising on the market but so far,no appropriate general solution based on a common standard could be found.In this project,the upcoming standard IEEE802.11e is analyzed and its appli-cability tested by means of simulations in the network simulator ns-2.Several traffic scenarios,typical for communications in the automation industry are considered.In order to enhance the achievable performance in an error-prone environment,different algorithms and retransmission procedures are proposed. The obtained results show that,even though IEEE802.11e is not optimized for industrial applications,it has a potential for realtime applications with some jitter and error margins.iAcknowledgementsFirst of all I would like to thank the Advanced Industrial Communication group at ABB Corporate Research in V¨a ster˚as/Sweden for giving me the opportunity to work on this interesting project as well as for the helpfulness of all group members.I am particularly thankful for the excellent support and dedication of my supervisor Jimmy Kjellsson.I would also like to thank Erik Larsson for being my adviser and examiner at KTH.Furthermore,I would like to thank my project partner Maxime Maury for the great teamwork,his enthusiasm and countless interesting discussions we had during the last six months.I am really grateful that I had him as my project partner.Thanks also to all other thesis students at ABB Corporate Research for the pleasant working atmosphere and the great time we had together.My family and friends deserve a particular acknowledgment-you are most important in my life and give me so much!With all my heart,I want to thank my parents for their never-ending support,love and for letting me know where I am at home.Furthermore,I want to explicitly mention a friendship I highly appreciate and do not want to miss:thank you,Pancho,for being such a good st but not least I am deeply grateful for the encouragement and love of a wonderful person.Annemarie,you really make me happy!iiiContents1Introduction11.1Project Definition (2)1.2Motivation (3)1.3Scope of the Thesis (4)2IEEE802.11e52.1General Description (5)2.1.1Reference Model (5)2.1.2Network Components and Architecture (6)2.1.3Services (7)2.2Physical Layer (7)2.2.1General Description (7)2.2.2OFDM Physical Layer Convergence Procedure (8)2.2.3OFDM Physical Medium Dependent (8)2.3Medium Access Control Layer (9)2.3.1MAC Layer Architecture (9)2.3.2Fundamental Concepts of the IEEE802.11e MAC (9)2.3.3Enhanced Distributed Channel Access (11)2.3.4HCF Controlled Channel Access (12)2.3.5Further Quality of Service Enhancements (14)3Industrial Communication Systems173.1Communication Hierarchy in the Automation Industry (17)3.2Fieldbus Systems (18)3.2.1General Description (19)3.2.2Communication Model (19)3.2.3Medium Access Methods (20)3.2.4Traffic Classification (20)3.2.5Quality of Service Requirements (21)3.3Wireless Fieldbus Systems (22)3.3.1Requirements and Challenges (22)3.3.2IEEE802.11e in Industrial Environments (23)3.3.3Related Work (24)4System and Scenarios274.1System Description (27)4.1.1Topology and Protocols (27)4.1.2Packet Error Measurements (28)vvi Contents4.1.3Error Model (29)4.1.4Performance Measuring (31)4.2Scenario Definition (32)4.2.1Scenario1 (32)4.2.2Scenario2 (33)4.2.3Scenario3 (33)5Simulation Environment355.1Realization Approach (35)5.2Network Simulator ns-2 (36)5.3Protocol Implementation (37)5.3.1Retransmission Procedures (37)5.3.2Enhanced Scheduler (40)5.4Simulator Configuration (41)5.4.1General Settings (41)5.4.2Retry Limit Definition (42)6Results456.1Scenario1 (45)6.1.1Performance Analysis of Cyclic Data (45)6.1.2Performance Analysis of Acyclic Data (52)6.2Scenario2 (55)6.3Scenario3 (57)6.3.1Performance Analysis of Cyclic Data (57)6.3.2Performance Analysis of Acyclic Data (58)7Conclusions and Future Work61 References63 A Parameter Settings67List of Tables2.1Modulation and coding for different data rates in IEEE802.11a.82.2Inter Frame Spaces in IEEE802.11e (11)2.3User Priority to Access Category mapping (11)3.1Typical control layer characteristics (18)3.2Comparison of algorithms (25)4.1Error model parameter settings (30)4.2Traffic types and characteristics in scenario1 (33)4.3Traffic types and characteristics in scenario2 (33)4.4Traffic types and characteristics in scenario3 (33)5.1EDCA parameter settings (41)6.1Average latency of acyclic data in scenario2 (56)6.2Average latency of acyclic data(scenario1vs.scenario3),5nodes59A.1Physical layer parameter settings (67)A.2Medium Access Control layer parameter settings (67)viiList of Figures2.1IEEE802.11reference model (6)2.2IEEE802.11e architecture (7)2.3OFDM PLCP framing (8)2.4MAC layer architecture (9)2.5Relationship between channel access mechanisms (10)2.6Internal contention in EDCA (12)2.7Service Interval structure according to the simple scheduler (13)2.8Example frame exchange sequences during HCCA TXOPs (14)2.9Direct Link Set-up procedure (16)3.1Hierarchical structure of industrial communication systems (18)3.2Traffic classification on thefield level (20)3.3Example of cyclic data traffic (21)4.1Packet error measurement environment (28)4.2Spectrum analysis during packet error measurements (29)4.3Gilbert channel model (29)5.1Overall simulation process (37)5.2Calculation of the TXOP duration (39)5.3Immediate retransmission procedure (39)5.4Late retransmission procedure (40)5.5Capacity and error analysis(mode:immediate) (42)5.6Capacity and error analysis(mode:late) (43)6.1Illustration of a typical cyclic data exchange sequence (46)6.2Scenario1:per-stream analysis of cyclic data,mode:immediate466.3Scenario1:per-stream analysis of cyclic data,mode:late (47)6.4Scenario1:performance analysis of cyclic data,mode:immediate486.5Scenario1:performance analysis of cyclic data,mode:late (49)6.6Scenario1:performance comparison of cyclic data (50)6.7Scenario1:average jitter of cyclic data (50)6.8Scenario1:maximum jitter of cyclic data (51)6.9Scenario1:average packet loss rate of cyclic data (52)6.10Scenario1:latency comparison of acyclic data (53)6.11Scenario1:latency analysis of acyclic data,PER=5% (54)6.12Scenario1:latency analysis of acyclic data,PER=20% (54)6.13Scenario2:latency comparison of acyclic data,PER=5% (55)6.14Scenario2:latency comparison of acyclic data,PER=20% (56)ixx List of Figures6.15Scenario3:cyclic offset over time illustration,mode:immediate.576.16Scenario3:cyclic offset over time illustration,mode:late (58)6.17Latency comparison of acyclic data in scenario1and3 (58)6.18Time of packet generation and packet transmission within a cycle59List of AbbreviationsAAC.........................Access Category ACK.......................Acknowledgment AIFS.......................Arbitration Inter Frame Space AIFSN.....................Arbitration Inter Frame Space Number AP.........................Access Point API........................Application Programming InterfaceBBPSK......................Binary Phase Shift Keying BSS........................Basic Service SetCCAP.......................Controlled Access Phase CDF.......................Cumulative Distribution Function CF.........................Contention Free CFP.......................Contention Free Period CP.........................Contention PeriodCSMA/CA.................Carrier Sense Multiple Access with Collision Avoidance CSMA/CD.................Carrier Sense Multiple Access with Collision Detection CTS.......................Clear to Send CW........................Contention WindowDDCF.......................Distributed Coordination Function DCS.......................Digital Communication System DFS.......................Dynamic Frequency Selection DIFS.......................DCF Inter Frame Space DL.........................Downlink DLS.......................Direct Link Set-up DS.........................Distribution System DSS........................Distribution System Service DSSS......................Direct Sequence Spread SpectrumEEDCA.....................Enhanced Distributed Channel Access EIFS.......................Extended Inter Frame Space ESS........................Extended Service Setxixii List of AbbreviationsFFHSS......................Frequency Hopping Spread Spectrum FIFO......................First In First OutHHC........................Hybrid Coordinator HCCA.....................HCF Controlled Channel Access HCF.......................Hybrid Coordination FunctionIIBSS.......................Independent Basic Service Set IEEE......................Institute of Electrical and Electronics Engineers IFS........................Inter Frame Space IP.........................Internet Protocol IR.........................Infrared ISM........................Industrial,Scientific and Medical bandKKbps.......................Kilobits per secondLLAN.......................Local Area Network st In First Out LLC.......................Logical Link ControlMMAC......................Medium Access Control Mbps......................Megabits per second MSDU.....................MAC Service Data UnitNwork Allocation Vector nQAP......................non-QoS Access Point nQSTA....................non-QoS StationOOFDM.....................Orthogonal Frequency Division Multiplexing OSI........................Open System Interconnection OTcl.......................Object Tool Command LanguagePPC.........................Point Coordinator PCF.......................Point Coordination Function PER.......................Packet Error Rate PHY.......................Physical layer PIFS.......................PCF Inter Frame Space PLC.......................Programmable Logic Controller PLCP......................Physical Layer Convergence Procedure PMD......................Physical Medium Dependent PPDU.....................PLCP Protocol Data Unit PSDU......................PLCP Service Data UnitList of Abbreviations xiii QQAM......................Quadrature Amplitude Modulation QAP.......................QoS Access Point QBSS......................QoS Basic Service Set QIBSS.....................QoS Independent Basic Service Set QoS........................Quality of Service QPSK.....................Quadrature Phase Shift Keying QSTA......................QoS StationRRL.........................Retry Limit RTS.......................Request to SendSSAP.......................Service Access Point SI..........................Service Intervall SS.........................Station Service SIFS.......................Short Inter Frame Space STA.......................StationTTcl.........................Tool Command Language TCP.......................Transmission Control Protocol TDMA.....................Time Division Multiple Access TPC.......................Transmission Power Control TS.........................Traffic Stream TSF.......................Timing Synchronization Function TSPEC....................Traffic Specification TXOP.....................Transmission OpportunityUer Datagram Protocol UL.........................Uplink er PriorityWWLAN.....................Wireless Local Area Network WM.......................Wireless MediumChapter1IntroductionIn industrial networks the communication between control units andfield de-vices,such as sensors and actuators,is very crucial and implies hard realtime and reliability constraints.Traditional LAN technologies used in home and of-fice environments are designed for completely different types of applications, and are in general not capable to cope with the stringent Quality of Service (QoS)requirements of industrial communications on the factory level.There-fore,industrial networks at this level are based on so-calledfieldbus technologies specifically developed as highly reliable realtime communication systems.The strict separation of the industrial and office world seems to become blurred nowadays as there are various initiatives going on to use Ethernet or IEEE 802.3[1]respectively as a basis forfieldbus technologies.In order to fulfill the hard performance criteria,it is though usually necessary to modify the Medium Access Control(MAC)layer.The driving reasons for the trend towards Eth-ernet are its wide acceptance resulting in lower and lower costs as well as the steadily increasing data rates.Furthermore,using a worldwide standard,which undergoes continuous improvements,reduces the manufacturer’s costs of en-hancing their own solutions.[2,3,4]Another trend is to realize wirelessfieldbuses,as wireless communication poses many advantages such as the reduction of cabling costs,increasedflexibility and less cable breaks.As the trend in industrial communications goes towards Eth-ernet,the Ethernet-based standard IEEE802.11[5],which is very popular nowa-days,would be the logical technology for the realization of wirelessfieldbuses. However,the plain IEEE802.11standard without doing any modifications to the MAC layer cannot be used for many industrial applications as QoS cannot be guaranteed.The upcoming IEEE802.11e standard(currently available as draft version D13.0[6])faces this problem and might therefore be a potential technology to be used in wirelessfieldbus communication.Extensive research has been done to analyze its performance for multimedia services[7,8,9,10,11] but its applicability for industrial applications has not yet been fully specified. The above-mentioned benefits of wireless communications in general as well as the features and expected acceptance of IEEE802.11e in the consumer market make this standard highly attractive even for industrial applications.[12,13]12Chapter1.Introduction 1.1Project DefinitionTo main objective of this project is to analyze the potential of using IEEE 802.11e as a basis forfieldbus communications in industrial networks with re-spect to QoS requirements.The main question to be posed is:To which extent is IEEE802.11e able to fulfill the stringent requirements of industrial commu-nications?The thesis will not refer to a specificfieldbus technology but define several general traffic scenarios typical for communications in the automation industry.The result sought is the performance behavior of data traffic with respect to latency,jitter as well as reliability in the different traffic scenarios. These criteria will be analyzed as a function of the node density as well as trans-mission errors.The thesis is based on[14]where the utilization of IEEE802.11e for a certain simple industrial application under consideration of different implementation strategies is analyzed.Thefinally proposed algorithm will be analyzed more extensively in this thesis.As an extension to[14],additional and more en-hanced traffic scenarios will be considered.Furthermore,the assumptions of error-free transmission will be relaxed in order to obtain more realistic results as wireless channels are in general very error-prone.These issues will be ana-lyzed mainly with consideration of timeliness and reliability,which are the most important criteria for industrial communications.[15]poses an extensive research of using IEEE802.11forfieldbus communica-tions.It is concluded that the plain standard cannot be applied and changes to the MAC layer to adapt the standard to industrial requirements are proposed. However,IEEE802.11e is not considered.In[16],the author considers an early draft version of IEEE802.11e and proposes a new MAC layer for a specific fieldbus technology.The goal of this project is in contrast to define if and to which extent IEEE802.11e is applicable without doing any modifications to the standard itself.The reason for this is that compliance to the standard is neces-sary in order to be able to use mass market products as well as to enjoy ongoing improvements and further standard developments.Furthermore,it has to be considered that,as IEEE802.11e is not a standard yet and different draft ver-sions contained partly significant changes,research based on older drafts might not be completely valid.The analysis of using IEEE802.11e in industrial applications is based on a the-oretical study as well as simulations using the network simulator ns-2[17].It will be analyzed how the test traffic scenarios can be mapped onto the IEEE 802.11e MAC layer and what has to be done to improve the system performance. The network simulator ns-2is an open-source project widely used within the IEEE research community.A basic implementation of the IEEE802.11e MAC protocol is available and can be used and extended in this project. Throughout the report it is assumed that the standard IEEE802.11a is used by all devices as the physical layer supporting a constant data rate of54Mbps1.All scenarios are composed of one control station(master)and N nodes(slaves),1The utilization of IEEE802.11a is motivated in Sec.4.1.1.1.2.Motivation3 whereby the maximum communication distance is10m and the stations are not moving.Different packet error rates are considered in this report.Further as-sumptions and a more detailed system description is given in Sec.4.1.The thesis focuses on the performance of the MAC layer IEEE802.11e.No other layers will be analyzed.Furthermore,radio resource management issues, such as frequency planning or channel allocation schemes,as well as security issues will not be considered in this thesis.1.2MotivationToday there are three main global standards that could be considered for the implementation of wirelessfieldbuses:Bluetooth2,ZigBee3as well as IEEE 802.11[18].However,none of these technologies was developed specifically for industrial applications and involve therefore partly significant limitations. Bluetooth supports only seven active nodes per master,the main shortcoming of Zigbee is its very low data rate of at most250Kbps and IEEE802.11cannot provide for low and deterministic communication delays.[12]Due to the limitations of the above-mentioned technologies,companies provid-ing industrial communication solutions are forced to develop their own protocols resulting in considerable developing costs.Additionally,the market acceptance of company-specific solutions can in general expected to be lower than for tech-nologies that are based on a global standard.Therefore,even if there are some individual solutions available,there is still the need for a global standard able to fulfill the requirements of industrialfield level communications.IEEE802.11e is a promising amendment to IEEE802.11as it is supposed to eliminate the lack of predictability of communication delays,which was a major problem in the legacy version.The performance and applicability of the up-coming standard IEEE802.11e in industrial applications is of high interest for the automation industry and will therefore be analyzed in this report.The main challenge in wirelessfieldbus communication is to ensure a reliable data transmission in a very harsh radio environment.Every station exchanges information typically with an interval of around10ms,whereby latency and jitter have to be kept as low as possible.In[18],it is recommended to avoid wireless communication in case latencies of less than10ms are essential,which is an absolute necessity infield level applications.If IEEE802.11e can be used for such realtime constraints has not yet been fully defined.In[14],it was shown that IEEE802.11e allows deterministic communication with very low latency assuming error free data transmission.The impact of interference and conse-quently packet losses on the system performance is scope of this report.It can be expected that erroneous communication will strongly influence the number of stations that can be supported as packets need to be retransmitted.Conse-quently,also the jitter will increase.234Chapter1.Introduction The high interest in the promising standard IEEE802.11e within the automation industry,its novelty as well as the challenges in wireless industrial communica-tions constitute the driving factors for the realization of this project.1.3Scope of the ThesisThe thesis project is part of a collaborative project with Maxime Maury per-formed at the Advanced Industrial Communications department at ABB Cor-porate Research in V¨a ster˚as,Sweden.The project adviser at ABB is Jimmy Kjellsson;adviser and examiner at KTH is Erik Larsson.This thesis is organized as follows:First,a theoretical background about the relevant aspects of the standard IEEE802.11,its amendment IEEE802.11e and the physical layer IEEE802.11a is provided.Subsequently,an introduction to industrial communication systems with focus on thefield level is given,whereby communication models,medium access methods,traffic classification as well as QoS requirements are considered.It is also dealt with wirelessfieldbus systems potentially using IEEE802.11e.After the description of the system and scenar-ios,the simulation environment is closely examined in Ch.5.The results are presented and analyzed in Ch.6.The thesis concludes by stating the benefits and limitations of using IEEE802.11e in industrial environments.Furthermore, future work on this topic is suggested.Chapter2IEEE802.11eThis chapter provides the required background regarding IEEE802.11e and in this context also of IEEE802.11a as well as of IEEE802.11,which is often re-ferred to as the legacy version.First,a general description is given by discussing the reference model,the network components,architecture and services.After that,it is dealt with IEEE802.11a serving as the physical layer andfinally,the MAC layer IEEE802.11e is thoroughly described.2.1General DescriptionIEEE802.11e[6]is a MAC layer enhancement of the IEEE standard802.11[5] supporting LAN applications with Quality of Service(QoS)requirements.IEEE 802.11e is currently available as a draft version defined by the802.11e task group. As the standard is predicted to be published in late autumn20054,the current draft(D13.0)is not expected to undergo significant changes.2.1.1Reference ModelThe IEEE802.11protocol set represents the MAC layer and Physical layer (PHY)of the OSI reference model.The common IEEE802.2layer,amongst others also used for the lower-layer LAN technologies IEEE802.3(Ethernet)and IEEE802.5(Token Ring),applies as the Logical Link Control layer(LLC).As this layer is not specifically designed for wireless applications,the MAC handles the management functions related to the requirements in wireless communica-tions.Four different physical layers are defined within the IEEE802.11protocol suite: The transmission can be either based on Infrared(IR),Frequency Hopping Spread Spectrum(FHSS),Direct Sequence Spread Spectrum(DSSS)or Or-thogonal Frequency Division Multiplexing(OFDM).IR,FHSS and DSSS are specified as the PHY in the legacy IEEE802.11standard.Additionally,an enhancement of DSSS is given in IEEE802.11b[19]defining a high-rate DSSS with a rate of up to11Mbps.IEEE802.11g[20],an enhancement compatible to IEEE802.11b,uses both DSSS and OFDM providing data rates of up to 4/groups/802/11/56Chapter 2.IEEE 802.11e 54Mbps.Both IEEE 802.11b and IEEE 802.11g operate in the 2.4GHz fre-quency band.The IEEE 802.11a amendment to the original standard operates in contrast in the 5Ghz band and uses only OFDM.It also allows rates of up to 54Mbps.5Some vendors provide a data rate enhancement by using two neigh-boring channels simultaneously.This doubles the data rate and allows up to 108Mbps for IEEE 802.11a and IEEE 802.11g.The PHY is divided into two sublayers,the Physical Layer Convergence Proce-dure (PLCP)and the Physical Medium Dependent (PMD).The PLCP defines the mapping of the IEEE 802.11MAC frames into the framing format used in the PHY,while the PMD describes how this user data is transmitted over the wireless medium (WM).The interfaces between the sublayer entities,called Service Access Point (SAP),are the MAC-SAP between the MAC and LLC sub-layer entity,PHY-SAP between the MAC and PHY and the PMD-SAP between PLCP and PMD.The protocol stack is illustrated in Fig.2.1.IR FHSS DSSS OFDM 802.2 Logical Link Control LayerOSI Layer 2(Data Link Control)OSI Layer 1(Physical)802.11 Medium Access Control Layer PLCPPMDMACLLC MAC-SAP PHY-SAPPMD-SAPFigure 2.1:IEEE 802.11reference model2.1.2Network Components and ArchitectureThe network components and architecture defined for IEEE 802.11and its amendment IEEE 802.11e are very similar except of a slightly different nomen-clature in a way that the QoS support of devices is always explicitly mentioned in the latter standard.For example,an Access Point (AP)is referred to as AP in legacy version,whereas it is either a QAP if it is supporting QoS or a nQAP if it is not supporting QoS in IEEE 802.11e.Devices having enabled QoS and others without QoS support might coexist in an IEEE 802.11e network.How-ever,in the following only pure QoS networks are considered.The simplest type of an IEEE 802.11e LAN is the QoS Independent Basic Service Set (QIBSS)consisting of at least two QoS stations (QSTA)communicating directly without any QoS Access Point (QAP)(see lower part of Fig.2.2).The QIBSS is often referred to as ad hoc network.A second type of BSS is the infrastructure based QBSS.In such a system,a QAP provides access to a so-called Distribution System (DS)interconnecting different QBSSs.The medium5IEEE802.11a,IEEE 802.11b and IEEE 802.11g are the currently specified physical layer standards.Further enhancements,e.g.IEEE 802.11n,are under development.2.2.Physical Layer 7of the DS is not specified in the standard and consequently can either be wireless or wired.Such large area networks consisting of different QBSSs are called Extended Service Set (ESS).Finally,IEEE 802.11networks can be connected to other IEEE 802LANs via a so-called portal,which is a logical point at which MAC Service Data Units (MSDUs)from a non 802.11LAN enter the DS or vice versa.The overall architecture is illustrated in Fig.2.2.QSTAQSTAQSTA QIBSS QSTA QSTA QSTAQAPQBSS QSTAQSTA QSTA QAPQBSS802.x LAN Portal ESS STA STA STA IBSSSTA STA STABSS STA STASTA AP BSS 802.x LAN Portal ESSDSDSAP Figure 2.2:IEEE 802.11e architecture2.1.3Services Two categories of services used by the MAC sublayer are specified in IEEE 802.11:the Station Service (SS)and Distribution System Service (DSS).The setof services in a BSS and QBSS belonging to the SS are MSDU delivery,privacy and (de-)authentication.In IEEE 802.11e higher-layer timer synchronizationand QoS traffic scheduling are additionally defined.The DSSs are (dis-/re-)association,distribution,integration as well as QoS traffic scheduling.62.2Physical Layer IEEE 802.11e can be based on any physical layer specified for IEEE 802.11.Due to the issues outlined in Sec.4.1.1,IEEE 802.11a [21]is in the followingconsidered as the physical layer.72.2.1General DescriptionIEEE 802.11a operates in the 5GHz frequency band and uses 52-subcarrier OFDM with a maximum data rate of 54Mbps.Twelve non-overlapping channels with a bandwidth of 20MHz are reserved whereby each channel is composed of 52subcarriers.48of these carriers are used for data and four are pilot subcarriers.6Forfurther details on services refer to [5]and [6].7For further details about the physical layer refer to [21].A summary is given in [22].。