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Trends in Embedded Communication Systems : Traffic Shaping on CAN and introduction of FlexRay Nicolas Navet ESIEE – 13/06/2008
© 2008 RealTime-at-Work - 2 In-vehicle networking : will CAN be able to keep up the pace? Typically max. bus load is set to 35% Not enough wrt to short/medium term bandwidth needs … Solution 1: multiple CAN networks … but gateways induce heavy overhead Solution 2: switch to FlexRay … expensive for bandwidth alone Solution 3: optimize the scheduling of CAN frame.. Offsets provide a solution to make CAN predictable at higher network load (≥60%)
© 2008 RealTime-at-Work - 3 PART 1: Scheduling messages with offsets on CAN
© 2008 RealTime-at-Work - 4 Scheduling frames with offsets ?! Periods 20 ms 15 ms 10 ms , ,5 0 Periods 20 ms 15 ms 10 ms Principle: desynchronize transmissions to avoid load peaks Algorithms to decide offsets are based on arithmetical properties of the periods and size of the frame
© 2008 RealTime-at-Work - 5 System model (1/2) ECU Frame Transmission request task Frame response time Performance metric: worst-case response time CAN Higher prio. frames frame
© 2008 RealTime-at-Work - 6 System model (2/2) The offset of a message stream is the time at which the transmission request of the first frame is issued The offset of a message stream is the time at which the transmission request of the first frame is issued Complexity: best choosing the offsets is exponential in the task periods → approximate solutions Complexity: best choosing the offsets is exponential in the task periods → approximate solutions Middleware task imposes a certain granularity Middleware task imposes a certain granularity Without ECU synchronisation, offsets are local to ECUs Without ECU synchronisation, offsets are local to ECUs
© 2008 RealTime-at-Work - 7 But task scheduling has to be adapted… ECU Frame Transmission request task Frame response time In addition, avoiding consecutive frame constructions on an ECU allows to reduce latency CAN Higher prio. frames frame
© 2008 RealTime-at-Work - 8 Time Frame Simple offsets Algorithm (1/3) Ideas: assign offsets in the order of the transmission frequencies release of the first frame is as far as possible from adjacent frames identify “least loaded interval” Ex: f 1 =(T 1 =10), f 2 =(T 2 =20), f 3 (T 3 =20) f 1,1 f 1,2 f 2,1 f 3,1
© 2008 RealTime-at-Work - 9 Offsets Algorithm applied on a typical body network 21 ms 65 ms
© 2008 RealTime-at-Work - 10 Offsets Algorithm (3/3) Low complexity and efficient as is but further improvements possible: Low complexity and efficient as is but further improvements possible: add frame(s) / ECU(s) to an existing design add frame(s) / ECU(s) to an existing design user defined criteria : optimize last 10 frames, a specific frame, user defined criteria : optimize last 10 frames, a specific frame, take into account priorities take into account priorities optimization algorithms: tabu search, hill climbing, genetic algorithms optimization algorithms: tabu search, hill climbing, genetic algorithms …
© 2008 RealTime-at-Work - 11 Efficiency of offsets : some insight (1/2) Almost a straight line, suggests that our algorithm is near-optimal Work = time to transmit the CAN frames sent by the stations
© 2008 RealTime-at-Work - 12 Efficiency of offsets : some insight (2/2) A larger workload waiting for transmission implies larger response times for the low priority frames..
© 2008 RealTime-at-Work - 13 Computing worst-case response times with offsets
© 2008 RealTime-at-Work - 14 Computing frame worst-case response time with offsets CAN Controller buffer Tx CAN Bus AUTOSAR COM Frame-packing task 5ms Waiting queue: -FIFO -Highest Priority First (HPF - Autosar) -Carmaker specific Requirements : - handle 100+ frames - very fast execution times - ≠ waiting queue policy at the microcontroller level - limited number of transmission buffers
© 2008 RealTime-at-Work - 15 WCRT : State of the art Scientific literature: Scientific literature: Complexity is exponential Complexity is exponential No schedulability analysis with offsets in the distributed non-preemptive case No schedulability analysis with offsets in the distributed non-preemptive case Offsets in the preemptive case : not suited for > tasks Offsets in the preemptive case : not suited for > tasks WCRT without offsets: infinite number of Tx buffers and no queue at the microcontroller level WCRT without offsets: infinite number of Tx buffers and no queue at the microcontroller level RTaW software: NETCAR-Analyzer RTaW software: NETCAR-Analyzer
© 2008 RealTime-at-Work - 16 NETCAR-Analyzer : developed at INRIA, then RealTime-at-Work
© 2008 RealTime-at-Work - 17 NETCAR-Analyzer : an overview Worst-case response times/jitters on CAN with and without offsets for the frames Proven near-optimal offsets assignment algorithms with user-defined performance criteria: e.g. optimize the worst-case response times for a specific subset of tasks, for instance, the 10 lowest priority frames Exhibit the situations leading to the worst-case: results can be checked by simulations/testing Enable dimensioning transmission/reception buffers at the ECU and communication controller level Handle both FIFO and prioritized waiting queues at the ECU level Fast multi-core implementation: typically, an exact response time computation requires less than 30 seconds for 100 frames on a dual-core system Gateway support: forwarding of signals/frames from one network to another, In use since December 2006 at PSA Peugeot Citroën
© 2008 RealTime-at-Work - 18 Performance evaluation : Experimental Setup WCRT of the frames wrt random offsets and lower bound WCRT reduction ratio for chassis and body networks Load increase : add new ECUs / add more traffic
© 2008 RealTime-at-Work - 19 Experimental Setup Body and chassis networks Set of frames generated with NETCARBENCH (GPL- licensed – With / without load concentration: one ECU generates 30% of the load
© 2008 RealTime-at-Work - 20 Offsets in practice : large response time improvements (1/2) ms
© 2008 RealTime-at-Work - 21 WCRT Reduction Ratio Body Networks Chassis Networks Results are even better with loaded stations
© 2008 RealTime-at-Work - 22 Offsets allow higher network loads Typically: WCRT at 60% with offsets WCRT at 30% without offsets Typically: WCRT at 60% with offsets WCRT at 30% without offsets
© 2008 RealTime-at-Work - 23 Partial offset usage ms
© 2008 RealTime-at-Work - 24 Conclusions on offsets Offsets provide an cost-effective short-term solution to postpone multiple CANs and FlexRay Tradeoff between Event and Time Triggered Further large improvements are possible by synchronizing the ECUs … ET CAN CAN with offsets TT-CAN + Complexity + Determinism
© 2008 RealTime-at-Work - 25 PART 2: Protocol basics and some configuration problems
© 2008 RealTime-at-Work - 26 Protocol basics MAC de type TDMAMAC F-TDMA Typically ST segment: 3 ms and DYN: 2ms Frames: up to 254 bytes, size is fixed in the static segment (BMW:16bytes) Data rate: between 500kbit/s and 10Mbit/s 64 ≠ communication schedules max. (but a slot always belongs to the same station)
© 2008 RealTime-at-Work - 27 FlexRay configuration Extremely complex problem: Mixed of TT and ET scheduling Tightly linked with task scheduling Large number of parameters (>70) AUTOSAR constraints (OS, COM, etc) … Design objectives should be first clearly identified: Minimum bandwidth to use cheap components (2.5 Mbit/s, 5MBit/s ?) Enable incremental design ? Carry-over of ECUs ? No chance to solve the pb optimally – too many free variables, sub-problems alone are NP-hard
© 2008 RealTime-at-Work - 28 What is needed first : identify the intended use of FlexRay Use of FlexRay at BMW – from 
© 2008 RealTime-at-Work - 29 Requirements on FlexRay Performance requirements: both run-time (response times, jitters, etc.) and End-of-Line/Garage flash update Incrementality requirements: additional functions or ECUs Dependability requirements: fail-silence, babbling idiot, deadline failure probability under EMI, … Platform requirements: platform wide frames (e.g., NM), carry-over of ECUs, able to deal with com. controller + CPU + Autosar stack performances, …
© 2008 RealTime-at-Work - 30 Tasks run either synchronously or asynchronously wrt the communication cycle
© 2008 RealTime-at-Work - 31 Application software : synchronous or asynchronous wrt FlexRay time? Crucial question - 3 cases: all applicative modules are synchronized with FlexRay global time all applicative modules are running asynchronously combination of synchronized and asynchronous modules Case 3 is likely since there are modules that cannot be synchronized (engine controller?) that might be reused from previous designs Case 3 requires both Static Cyclic Scheduling (SCS) and Fixed Priority Scheduling (FPS)
© 2008 RealTime-at-Work - 32 Case 1 : synchronous case (1/2) Best possible results with regard to signal latency and dependability Picture from 
© 2008 RealTime-at-Work - 33 Case 1 : synchronous case (2/2) But strong constraints: But strong constraints: require Static Cyclic Scheduling require Static Cyclic Scheduling impose to re-design existing functions / design according to the bus configuration impose to re-design existing functions / design according to the bus configuration task periods constrained by FlexRay/Autosar rules (cycle repetition, e.g. 10, 20, 40, 80ms with a cycle=5ms) task periods constrained by FlexRay/Autosar rules (cycle repetition, e.g. 10, 20, 40, 80ms with a cycle=5ms) might require to artificially increase the frequency of some tasks → CPU load length of the communication cycle is crucial length of the communication cycle is crucial
© 2008 RealTime-at-Work - 34 Case 2 : asynchronous case Signals produced at variable time points in the round – depends on the scheduling of tasks Signals produced at variable time points in the round – depends on the scheduling of tasks Static segmentDynamic segment Signal Release interval Signal freshness constraint imposes a range of slots for transmission and a min. frequency (every x cycles) Typical case with legacy software
© 2008 RealTime-at-Work - 35 FlexRay configuration: sub-problems (1/4) 1.Network configuration: 1. Topology 2. Redundant transmission support (bmw=no) 3. Data rate (bmw=10Mbit/s) 4. Bus-guardian : local, star coupler, none (bmw=none) Contraints : dependability objective, traffic growth forecast, costs, interoperability with other FlexRay networks (same data rate might be a plus), etc..
© 2008 RealTime-at-Work - 36 FlexRay configuration: sub-problems (2/4) 2.Communication cycle: 1. Global size (e.g., bmw=5ms) 2. Length of the ST segment (e.g., bmw=3ms) 3. Length of the dynamic segment (e.g., bmw=2ms) Constraints : task periods, minimal sampling rate (e.g., 2.5ms), efficient use of bandwidth (small cycles unused slots), carry-over of ECUs, …
© 2008 RealTime-at-Work - 37 FlexRay configuration: sub-problems (3/4) 3.Configuration of the ST and DYN segments 1. Number of ST slots 2. Size of the ST slots (=size of the frame) 3. Number DYN slots Contraints : incremental design (how many ST + DYN slots left unused?), efficient bandwidth usage (e.g., one large slot for an ECU or 2 small?), carry-over of ECUs, …
© 2008 RealTime-at-Work - 38 FlexRay configuration: sub-problems (4/4) 4.Frame packing for the ST and DYN Slots 5.Allocation of the slots to the ECUs 1. assign ST slots to ECUs – construct the 64 schedule 2. assign DYN slots to ECUs – consider slot-multiplexing 6.Assign a local priority to each frame for the transmission on the DYN slots own by a station 6.Assign a local priority to each frame for the transmission on the DYN slots own by a station Algorithm support : point 5.1 to be solved in link with task scheduling points can be handled jointly
© 2008 RealTime-at-Work - 39 Configuring FlexRay The good news : The good news : many design choices are imposed by strategic decision many design choices are imposed by strategic decision Most configuration sub-problems have been addressed independently (see references) Most configuration sub-problems have been addressed independently (see references) The bad news : The bad news : some non-trivial adaptations/improvements needed to existing work some non-trivial adaptations/improvements needed to existing work problem gets more complex if resource optimization is a constraint (≠current BMW network) problem gets more complex if resource optimization is a constraint (≠current BMW network) optimal solution is out of reach anyway optimal solution is out of reach anyway
© 2008 RealTime-at-Work - 40 Questions, feedback? please contact me at
© 2008 RealTime-at-Work - 41 References
© 2008 RealTime-at-Work - 42 References – Offsets on CAN  M. Grenier, L. Havet, N. Navet, "Pushing the limits of CAN - Scheduling frames with offsets provides a major performance boost", Proc. of the 4th European Congress Embedded Real Time Software (ERTS 2008), Toulouse, France, January 29 - February 1,  C. Braun, L. Havet, N. Navet, "NETCARBENCH: a benchmark for techniques and tools used in the design of automotive communication systems", Proc of the 7th IFAC International Conference on Fieldbuses & Networks in Industrial & Embedded Systems (FeT 2007), Toulouse, France, November 7-9,  M. Grenier, J. Goossens, and N. Navet, “Near-optimal fixed priority preemptive scheduling of offset free systems”. In Proc. of the 14th International Conference on Network and Systems (RTNS’2006), Poitiers, France, May ,  R.I. Davis, A. Burns, R.J. Bril, and J.J. Lukkien, “Controller Area Network (CAN) schedulability analysis: Refuted, revisited and revised”, Real-Time Systems, 35(3):239–272, April  J. Goossens, “Scheduling of offset free systems”, Real-Time Systems, 24(2):239–258, March  K. Tindell and A. Burns, “Guaranteed message latencies for distributed safety- critical hard real-time control networks”, in 1st International CAN Conference Proceedings, Germany, September 1994, 1994.
© 2008 RealTime-at-Work - 43 References – FlexRay (1/2) FLEXRAY – protocol and usage by carmarkers  B. Schätz, C. Kühnel, M. Gonschorek, “The FlexRay Protocol”, to appear in the Automotive embedded Handbook, N. Navet, F. Simonot-Lion editors, CRC Press/Taylor and Francis,  Vector Informatik GmbH, interview of Mr. Peteratzinger (BMW), Mr. Steiner (BMW), “Use of XCP on FlexRay at BMW”, published in “Collection of professional articles”, 09/2006. Available at worldwide.com/articles  A. Schedl, “Goals and Architecture of FlexRay at BMW”, slides presented at the Vector FlexRay Symposium, March  J. Broy (Porsche A.G.), K.D. Müller-Glaser, “The impact of time-triggered communication in automotive embedded systems”, IEEE SIES’2007, July FRAME PACKING  R. Saket, N. Navet, "Frame Packing Algorithms for Automotive Applications", Journal of Embedded Computing, vol. 2, n° 1, pp93-102, DEPENDABILITY  B. Gaujal, N. Navet, "Maximizing the Robustness of TDMA Networks with Applications to TTP/C", Real-Time Systems, Kluwer Academic Publishers, vol 31, n°1-3, pp5-31, December 2005.
© 2008 RealTime-at-Work - 44 References – FlexRay (2/2) CONFIGURATION OF THE STATIC SEGMENT  S. Ding, N. Murakami, H. Tomiyama, H. Takada, “A GA-based scheduling method for FlexRay systems”, EMSOFT,  A. Hamann, R. Ernst, “TDMA Time Slot and Turn Optimization with Evolutionary Search Techniques”, Proceedings of the Design, Automation and Test in Europe Conference, Volume 1, p312–317,  E. Wandeler, L. Thiele, “Optimal TDMA time slot and cycle length allocation for hard real-time systems”, Proceedings of the 2006 conference on Asia South Pacific design automation.  M. Grenier, L. Havet, N. Navet, “Configuring the communication on FlexRay: the case of the static segment”, Proceedings of ERTS’2008. CONFIGURATION OF THE DYNAMIC SEGMENT  T. Pop, P. Pop, P. Eles, Z. Peng, A. Andrei, “Timing Analysis of the FlexRay Communication Protocol”, ECRTS  T. Pop, P. Pop, P. Eles, Z. Peng, “Bus Access Optimisation for FlexRay-based Distributed Embedded Systems”, DATE INTERFERENCE OF SCS TASKS ON FPS TASKS  T. Pop, P. Pop, P. Eles, Z. Peng, “Optimization of Hierarchically Scheduled Heterogeneous Embedded Systems”, RTCSA’2005.
Pushing the limits of CAN - Scheduling frames with offsets provides a major performance boost Nicolas NAVET INRIA / RealTime-at-Work
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