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Poovappa (Dinu) Subbaiah Real-Time Industrial Ethernet: Mini Literature Survey 1/9

Real-Time Industrial Ethernet: Mini Literature Survey

Poovappa Subbaiah (Dinu)

Introduction:Fast real time data exchange between controllers and sensors/actuators are normallyperformed in the Factory Communication networks. Vendor specific fieldbus solutionsdominate the Industrial network market. While these fieldbus solutions have a muchsmaller market and therefore remain high cost, Ethernet solutions are extremely costefficient. This along with the higher data rates offered by Ethernet has promptednumerous studies on its use in factory networks.

An end to end communication, integrating factory data with enterprise application isseeing new trends in the automation world today. High speed, low cost and dominancein the office networks is making Ethernet the prime communication protocol forautomation networking.

While Ethernet offers numerous advantages over fieldbus, some problems associated toits non deterministic behavior prevent its deployment on a wide scale. Standards suchas IEEE 802.1p and Fieldbus HSE have been introduced to address this issue. Althoughit is impossible to determine the arrival time of queued messages in Ethernet networks, aswitched architecture helps in eliminating the non determinism due to CSMA/CD byseparating collision domains. A high overhead is also a feature of the Ethernet whensmall factory data packets need to be communicated. While several such factors havebeen the deterrent for the use of Ethernet for low level real-time factory datacommunication, a plethora of studies and information suggest different approaches toachieve real time traffic on Ethernet networks.

This document is a survey of literature on Ethernet for real-time communications inIndustries. The list of research papers that are studied in this survey are given in thereference section.

1. On the use of Ethernet at low level of factorycommunication systems [5]

Introduction: The paper discusses the possibility of using Ethernet for low level factorycommunications. The study proposes UDP based protocols that compare with Fieldbus

protocols represented by Profibus DP and WorldFIP. The Master-Slave protocol and theProducer Consumer protocols are analyzed with Ethernet and the correspondingfieldbus implementations. The theoretical analysis results for fieldbus are derived fromother works on the subject, and the comparison with the Ethernet is presented in thepaper.

Description: The author emphasizes the changes brought into the Ethernet standardsthat enable Ethernet to satisfy real-time requirements. The main obstacle for real-timeuse of Ethernet is the non determinism caused by the possible collisions between

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frames deriving from multiple simultaneous accesses of stations to networks. Theauthor writes “ This [collisions] problem is often overrated because suitable network configurations eliminated collisions ”. The “suitable network configurations” include aswitching architecture that eliminates collisions, prioritizations standard 802.1p, etc. Theabsence of “ a standard and a well settled protocol, which allows for the implementation of typical functions foreseen at device level ” is seen as a critical issue.

The paper deals with Ethernet’s role in the device level communications within the threetier factory communication systems architecture. The device level communicationrequirements pose unique challenges in terms of strict timelines. The main functions of afieldbus operating at device level are the cyclical data exchange and the handling ofasynchronous urgent traffic. This is realized by techniques such as Master-Slave andProducer-Consumer. The same techniques are implemented on Ethernet, with the IPprotocol layer is interfaced to MAC through network adapter module. UDP is adoptedsince the packet size for factory data is small and overhead needs to be minimized.

It is important to note that the paper dealswith the device level communications asidentified in the following model of factorycommunication systems. The figure showsthe three tiers of factory communicationsystems. Ethernet is already being used forthe the plant and cell level communications.

The figure is taken from Ref.[5].

The essence of the Master-Slave protocol Ethernet implementation foresees thepresence of just one station which can arbitrarily access the network, while others canonly respond to specific interrogations. In other words, the Master sends a request frameto a slave to poll and gets the answer with the response frame. The Producer-Consumer

on the other hand requires each station to be supplied with a list of the variables it canproduce. The producers of the variable, upon receiving a request, send a packet with thedata, which can be picked up by the consumers.

The cycle time for Ethernet Master-Slave, neglecting the acyclic activity is expressed as:

TMSc = N

i=1 Tslv (i) + 2NT if Where N is number of slaves, Tif is inter packet time and Tslv is the time employed to poll the i’th slave

For the Producer-Consumer, based on a few assumptions the cycle time is given as:

TPCc = Tout + Tin + Tift where Tout is the time necessary to transmit all the output variables, Tin is the time

necessary to read all input variables and Tift is the total inter packet time.

The performances of the two real time protocols analyzed for Ethernet is compared withthose of two fieldbuses Profibus DP and WorldFIP. The Profibus DP performance isanalyzed in Ref.[6] and the results are substituted here for comparison. Similarly, for theProducer-Consumer, the performance is evaluated based on comparison with theanalysis in Ref.[7].

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Conclusions: The Master-Slave protocol, based on 100BASE-T Ethernet is seen toperform significantly better than Profibus DP. The comparison is done based on thecycle time achieved. Similar comparison between Producer-Consumer on 100BASE-TEthernet is seen to perform better than WorldFIP. However, the data coding efficiency issomewhat lower with Ethernet.

Although the theoretical comparison shows that Ethernet performs better, theassumption of no collisions upon network access by Ethernet devices needs to besatisfied. Also, the implementation of the protocol in the factory data acquisition anddistribution devices may not be trivial.

2. The Road to an End-to-End Deterministic Ethernet [2]

Introduction: The authors of this paper see the need for introducing a prioritizationmechanism into the protocol stacks and give arguments for applying these mechanismswithin automation networks for deterministic Ethernet behavior. The argument that thelatency behavior within the end nodes causes severe jitter is presented and studied inthe paper. The paper also presents the results of experiments using priority queuing,showing very promising results.

Description: The problems of fieldbus such as cost and bandwidth, is seen as amotivation for Ethernet’s candidature for automation networks. However, Ethernetcomes with a different set of problems in relation to factory data communications.Ethernet’s democratic nature where all nodes have an equal chance of accessing thenetwork, unpredictability and a fairly large overhead pose a problem in itsimplementation in factory communication networks.

While most of these problems are being studied and solved, the issue of end to endpredictability requires the end nodes to also be precise. While there are studiesconfirming that most of the end to end latency is a result of delay at the end nodes, theauthors believe that a study of QoS for an embedded environment and an evaluation ofperformance are necessary and have not been done before.

Switching is seen as a way to separate collision domains but the queues associated withthe switches introduce non deterministic delays. In the case of broadcasts, the Ethernetswitch has to route this data to every drop link in the system resulting in packets of samepriority landing in the same queue. Also, within each node there is usually a singlehardware queue associated with the Ethernet controller which also contributes to delayin certain circumstances.

The IEEE 802.1p alleviates the switch queue problem. A simple protocol associated withthe VLAN identifier in the TCI (Tag Control Info) allows for the removal of allunnecessary drop link traffic in an automation network based on “publish and subscribe”.Network data transfer time only forms a small part of the delay and to improve thelatency performance the concept of priority must be extended to the protocol layers inboth the sending and receiving end. This is accomplished, in the paper, by matchingprocess priority to packet priority and running the associated task when the packet

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arrives. Some issues that could emerge with this implementation are also discussed inthe paper.

The figure (taken from the paper)shows an example of a stackimplementation that would curemost of the queue problems of thestandard implementation. Thesolution consists of introducing athin layer between the Ethernetcontroller driver and the protocolmultiplexer.

Overall, an implementationwherein multiple priorities based processes or reentrant code (that can immediatelyexecute the receiving of the higher priority packets) is suggested for the receiving end. Amultiple priority based transmission queues seem to address the issue of buffer delaysfor the sent packets. Finally, a measurement setup with an application sending a time-stamped packet to the slave node and receiving it back was implemented and used formeasurement. Tests were conducted based on various criteria.

Conclusion: The paper addresses the issue of achieving application-to-applicationpredictable transfer time on Ethernet. The introduction of IEEE 802.1p standard hasallowed latency to be calculable in switched Ethernet infrastructure. Introducing aprotocol stack QoS mechanism and testing the end to end performance based onseveral criteria shows very positive results in terms of predictability and control of an endto end communication. Taking the priority criteria for transmission and reception ofpackets is shown to reduce the latency for real time traffic and also introducedeterminism.

3. Genetic Algorithms for Industrial Ethernet Network Design[4]Introduction: Ethernet is intrinsically non deterministic, while industrial communicationnetworks must provide bound end to end delay. The use of optimization and designtechniques for network control can result in better QoS. A genetic algorithm, foroptimization and network design is proposed and investigated in this paper. Theproposal includes finding the best distribution of industrial devices on a switchedEthernet architecture.

Description: The design of efficient topologies and the distribution of devices ondifferent switches are done through graph partitioning techniques. While the graphpartitioning is a NP-Complete problem, several heuristics exist that simplify the problem.This paper is a proposal and evaluation of such a heuristic method, the GeneticAlgorithms.

While a bad management of network cabling can create bottlenecks, adhering to someoptimization techniques, fault tolerance and real time scheduling enables the Ethernet tobe used in safety critical applications with hard real time constraints. The graphpartitioning techniques are used to find a good solution that groups the devices on theswitch according to their co-operation, and minimizes the inter switches exchanges.

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The weighted graph partitioning problem is stated as “ Given a graph with vertex and edge weights, partition the vertices into disjoint subsets such that sum of the vertex weights of each subset is close to the average sum, and the total cost of the cut edges (edges that connect vertices in distinct sub domains) is minimized ”. In other words, if weknow the amount of data flow between devices, the optimized network topology wouldkeep the devices that communicate with each other under one distinct switch such thatthe data bandwidth under each switch is about the same and the data between switchesis minimized. The optimization objective is to minimize cross traffic flows betweenfederative switch and maximize traffic on the second level switches.The figures taken from the paper illustrates the objectives clearly;

The genetic algorithm is a Meta heuristic inspired by the genetic processes of biologicalorganisms. A typical genetic algorithm can be briefly illustrated as follows.

1. Create initial population.2. Choose parent1 and parent2 from population3. compute offspring = crossover(parent1, parent2); mutation(offspring);4. if ok replace(population, offspring)

The metrics used to determine the quality of the solution are the sum of all edges whichconnect a vertex in one partition to a vertex in another partition and the balance betweengroups (grouping efficacy).

Conclusion: To demonstrate the algorithm, a traffic matrix consisting of device ids’ andtheir communication with other devices is taken. A ‘1’ shows that a particular devicecommunicates with the other device while the rest have a null. The java implementationof the algorithm is shown to generate a better or equal solution than the RSB algorithm.The evaluation is done by using the survey of [8] and [9] and the results demonstrate theimportance of having an algorithm to design the network topology. The objective is to beable to modify topology by using different kinds of switches until the required QoS isachieved.

4. Fuzzy Traffic Smoothing: an Approach for Real-TimeCommunication over Ethernet Networks[3]

Introduction: Soft real time applications in industrial data networks do not requiredeterminism. This is the premise with which the paper proposes an approach ofdelivering a statistical bound packet delivery. A solution for realizing a real time statisticalchannel on a shared Ethernet, called Traffic Smoothing was introduced in [10]. The ideais to provide statistical guarantees on timely delivery of Ethernet packets by keeping the

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total arrival rate of new packets generated by nodes below a given network-wide input.This approach is extended to provide dynamic traffic smoothing in several studies and inthis paper, these dynamic techniques are improved.

Description: In this paper, improvements to the dynamic traffic smoothing techniquesare introduced in two respects. The overall throughput and the number of collisionsobserved over an interval are used as input parameters for the smoothing. This results ina more complete indication of the actual network workload. Also a fuzzy controllerdynamically gauges the smoothing action according to the actual workload.

“Traffic smoothing” deals with the timely delivery of packets in statistical terms. In otherwords the probability P that a packet is lost during its transmission or misses its deadlineis less than a certain loss tolerance. Architecturally the traffic smoother is insertedbetween the TCP/IP and the Ethernet layers. The smoothing occurs based on the creditbucket depth and refresh period parameters. Every refresh period credits not exceedingthe credit bucket depth are replenished and the packet is sent to the Ethernet layer onlyif credits are available. The real time traffic is unaffected by the scheme and theparameters can be tweaked to produce desirable results.

In dynamic traffic smoothing these criteria is distributed among stations that really needto transmit. A fuzzy controller is also introduced which, according to the total throughputand the number of collisions locally detected, applies rules to choose the mostappropriate station input limit on a case by case basis. Three variables, throughput,collisions and refresh period form nine combinations with three possible values of high,medium and low. The fuzzy controller interpolates according to fuzzy arithmetic togenerate the output.

A Linux test bed connected with 10BASE-T Ethernet and a collision diameter of 10meter was used on 11 workstations. One of these was a monitoring workstation, whosetask was to configure the system parameters, activate and deactivate Real-Time/Non-

Real-Time messages etc. The test results were compared with the HIMD (HarmonicIncrease Multiplicative decrease on collision, traffic smoothing technique) approach testresults.Conclusion: While several tests were run, the most interesting were the ones where theroundtrip time was measured in the scenario with both RT and NRT traffic. The fuzzyapproach shows a much lesser delay of the RT traffic even when there are bursts ofNRT traffic, when compared to the HIMD approach. This improvement in performance isalso proved by the comparison of the total network throughput, where the throughput vsworkload graph is more regular and higher throughput is achieved.

5. Deterministic Real-Time Communication with SwitchedEthernet[1]

Introduction: Switch technology divides collision domains into simple point to pointconnections ensuring collisions no longer occur. This paper evaluates the real timecharacteristics by looking at distributions and upper bounds on transaction times. Bothsimulation methods and analytical method with network calculus is used to evaluate theresults. The conditions under which the network can be called deterministic areevaluated.

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Description: A line topology with a deviceassigned to every switch, creating a “fieldbuscompliant” cabling concept is considered for theanalysis. Several types of messages are taken intoaccount such as MT1, cyclic send and receive ofprocess data, MT2, Alarms etc., MT3, Monitoringand control, MT4, acyclic transmission like filetransfer. Since the data usually flows to a sink, thesink link is considered for bottleneck analysis.

Network Calculus is widely applicable for the assessment of the real time performance ofcommunication networks. While traditional queuing theory deals with probabilitydistributions, network calculus involves bounding constraints on packet arrival andservice. These constraints allow bounds on packet delays to be derived from which realtime network behavior can be quantified. It is shown that the end to end delay of thehighest order packets is bounded by D H = DH

out + (b + rD Hout) d rsp + Nd rsp + (N + 1)v

Where v is the maximum service time of lower priority packet, N the number of stations,d rsp the service time of the response and D H

out the delay of the highest priority packet.

With the Real-Time evaluation, for the line topology, it is shown that an upperdeterministic limit can be kept at 19ms for all message types for a load of p = 0.8, wherep is the ratio of the arrival rate to the link capacity. While the corresponding higherextreme values for the line and the star topologies are found to be near equivalent, themedian varies significantly. All these results are for the FCFS scheduling. For the PQscheduling with the two topologies under similar conditions, the extreme values are seento be much lower. The preferential treatment results in a very high upper limit for theMT4 but this is not important as the ones of higher priority outperform the same ones inFCFS.

Conclusion: With the upper bounds in transaction time seen in the simulations, we

know that these upper bounds can occur. However, higher values than these upperbounds could occur if the conditions are changed. Taking an analytical approach to thesame conditions as for the simulation shows that the values are 2 to 40 times higher.The values in the analytical approach are for the worst case source behavior, while withthe simulation it was random bad cases. The question if switched Ethernet is real timeor not still remains, but as shown in this paper, for a typical application with 50 devicesmaximum transaction times on the MAC level within milliseconds could be guaranteedfor real time services. It was also shown that using traffic class with user priorities leadsto a good isolation of time critical data.

Summary: The papers selected for the survey cover various aspects of real time behavior in

Ethernet for Factory data communications. While all the papers seem to acknowledgethat the priority standard IEEE 802.1p and the switched architecture address this issue,it is obviously not enough. Approaches based on Network design, algorithms, protocolsand traffic management are studied extensively in the search for a deterministic Ethernetindustrial data communication networks. Each of these approaches is briefly looked intoin this survey.

Each study presents a different approach to the problem. The paper “The Road to an End-to-End Deterministic Ethernet” Ref.[2] looks into the queuing delays and jitter in

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transmission and reception to address determinism. This proves to be useful but notcomplete since among the assumptions is that the network is designed optimally. This isalso looked into in the Ref.[3] where a solution for realizing a real time statistical channelon a shared Ethernet, called Traffic Smoothing was introduced and evaluated. Thepremise is that soft real time applications will only require statistical guarantees; howeverthe results of the evaluation are encouraging.

The Ref.[5] takes the opinion that the collision problem with Ethernet is overrated andpresents a “fieldbus like” implementation with Ethernet. Comparison is done analyticallyand the Ethernet is shown to perform better. However, the implementation seems to benon trivial and the assumption of no collisions must be realized. The study in Ref.[1] evaluates the real time characteristics by looking at distributions and upper bounds ontransaction times. Both simulation and analytical methods are demonstrated and theworst case with the analytical method is suggested as the upper bound, and thereforedeterministic.

The architecture design and optimization techniques for network control can result inbetter QoS. This NP complete problem is approached through the genetic algorithm thatis shown to result in a highly optimized design in Ref.[4] . The objective is to be able tomodify topology by using different kinds of switches until the required QoS is achieved.

Overall, some very interesting approaches to determinism are presented in the paperssurveyed. Although determinism, in strictly analytical terms is not achieved all the papersshow that Ethernet has come far on the road to a predictable end to end industrialautomation system.

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Reference:

[1] Jurgen Jasperneite, Peter Neumann, Michael Theis and Kym Watson. “DeterministicReal-Time Communication with Switched Ethernet”. 4 th IEEE International Workshopon Factory Communication Systems, Vasteras, Sweden, August 28-30, 2002.

[2] Tor Skeie, Svein Johannessen and Oyvind Holmeide. “The Road to an End-to-EndDeterministic Ethernet”. 4 th IEEE International Workshop on Factory CommunicationSystems, Vasteras, Sweden, August 28-30, 2002.

[3] A. Carpenzano, R. Caponetto, L. LoBello and O.Mirabella. “Fuzzy Traffic Smoothing:an Approach for Real-Time Communication over Ethernet Networks”. 4 th IEEEInternational Workshop on Factory Communication Systems, Vasteras, Sweden,August 28-30, 2002.

[4] Nicolas Krommenacker, Eric Rondeau and Thierry Divoux. “Genetic Algorithms forIndustrial Ethernet Network Design”. 4 th IEEE International Workshop on FactoryCommunication Systems, Vasteras, Sweden, August 28-30, 2002.

[5] S. Vitturi. “On the use of Ethernet at low level of factory communication systems”.Computer Standards & Interfaces 23 (2001) 267 – 277.

[6] S. Vitturi. “The effects of acyclic traffic on Profibus DP networks, LADSEB-CNRinternational report, October 2000.

[7] G. Cena, L. Durante, A. Valenzano. “Standard fieldbus networks for industrial applications”.Computer Standard and Interfaces 17 (2) (1995) January.

[8] E.W.Kamen, P.Torab, K.Cooper, G.Custodi, “Design and analysis of packet switchednetworks in control systems”, IEEE conference on decision and control, Phoenix, AZ,December 1999.

[9] P.Torab, E.W.Kamen, “Load analysis of packet switched networks in control systems”, 25 th annual conference of the IEEE industrial Electronics Society, IECON’99, USA, 1999, pp.1222-1227.

[10] S. Kweon, K.G.Shin, Q. Zheng, “Statistical Real-Time Communication over Ethernet formanufacturing automation systems”, Proc. of the 5 th IEEE Real-Time Technology andApplication Symposium, Vancouver, Canada, June 1999.

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