Time triggered arch.
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  • 1. Time-Triggered Architectures
  • 2. The Time-Triggered Architecture: What Is It?-The Time-Triggered Architecture (TTA) is a platform for safety-critical embedded systems E.g., aircraft and engine flight control-Functionally, it is a TDMA (time-triggered) serial bus-“Bus" understates its criticality and sophistication *It is the safety-critical core of the systems built above it
  • 3. • The components of a TTA will communicate using a time-triggered protocol. – Hardware support needed for running the protocol.
  • 4. TTA: Where Did It Come From?-Developed by the group of Hermann Kopetz, TU Vienna-Commercialized by TTTech-TTA is unique in being developed for mass- market for automobile applications (Audi, PSA etc.) but also used for aircraft applications (Honeywell)
  • 5. • Application domains: – Automotive electronics – Fly-by-wire cockpits – Railway signaling systems• Reason: time-deterministic executions.
  • 6. The Main Idea• Time-triggered – Every speaker is assigned a predetermined time slot. – After one round, the speaker gets a slot again. – Also, a topic-schedule has been worked out in advance. • Top1, Top2, Top4 in the first round. • Top1, Top3 and Top5 in the second round • Top2, Top4 and Top5 in the third round. – Ensure no one breaks the rules!
  • 7. Basic Characteristics of TTA• Exists in both bus and star topologies (logically still a bus)• All functionality implemented in the distributed interfaces (called TTP/C controllers)• And in the hub of the star topology (a modified controller)• Creates a synchronous, TDMA ring on a broadcast bus
  • 8. Time-Triggered Architecture
  • 9. Time-Triggered Architecture • Basic unit: NODE • Node:  A processor with memory  I-O subsystem  Operating system  Application software  Time-triggered communication controller
  • 10. Time-Triggered Architecture • Communication (TTA Protocol)  Nodes connect to each other via two independent channels.  The communication subsystem executes a periodic Time Division Multiple Access (TDMA) schedule.  Read a data frame + state information from CNI (Communication Node Interface) at predetermined fetch instant and deliver to the CNIs of all receiving nodes at predetermined delivery instants.
  • 11. Time-Triggered Architecture • Communication  All the TTPs in a cluster know this schedule.  All nodes of a cluster have the “same” notion of global time. (achieved by synchronizing local time)  fault-tolerant clock synchronization.  TTA BUS topology.
  • 12. System Overview• Replicated communication channels• The channel is a broadcast bus• Access is by TDMA driven by progression of global time• Local nodes time synchronized by TTP• Communication by rapid and periodic message exchanges
  • 13. Features of the TTP• Fault-tolerance• Only data signals (and no control signals) cross interfaces.• Integrates numerous services – Predictable message transmission – Message acknowledgement in group communication – Clock synchronization – Membership
  • 14. TTP Design Rationale• Sparse time base – Messages are sent only at statically designated intervals – Inflexible compared to Event-triggered (ET) model, but easier to test• Use of a priori knowledge – All nodes are aware of when each node is scheduled to transmit• Broadcast – Correctness of transmitted message can be concluded as soon as one receiver acknowledges message delivery (broadcast medium)
  • 15. Protocol Highlights• Bus access – A FTU will have one or two time slots depending on class of fault-tolerance – Number of slots in a TDMA round given to an FTU may also be different• Membership Service – If a message from a sending node does not occur in designated interval, its membership is set to 0 in other nodes – Membership checked before transmission. A node is alive if • Its internal error detection mechanism has not indicated error • At least one of its transmitted frames has been correctly acknowledged.
  • 16. Protocol Highlights• Temporary blackout handling – Correlated failure of a number of nodes – Identified by sudden drop in membership – Nodes send I-messages and perform local emergency control – After membership has stabilized, mode changed to global emergency service
  • 17. Protocol HighlightsTemporal encapsulation of nodes – Communication bandwidth assigned statically – Time base is sparse- every input can be observed and reproduced exactly• Testability – Easy to test the implementation in comparison to ET – Easy to simulate –finite number of execution scenarios • Uncontrolled interactions between nodes are prevented • Determinism: can replicate states of nodes
  • 18. Strengths• Can provide fault-tolerant real-time performance• Practical (MARS platform), efficient, and scalable – Can be implemented using available hardware, signalling mechanisms – Low overhead – High data rates, used in both twisted fiber and optical channels• Reusability, composability, and testability
  • 19. Weaknesses• The schedule is fixed so there is no bandwidth allocated for alarms and other spontaneous messages• All fault-tolerance mechanism is implemented at system level, this means that very little “freedom” is left for application specific implementations• Addition of nodes affects the existing system (although not the application)
  • 20. • Time-Triggered architectures and protocols will become important.• Seemingly simple – But quite sophisticated• for time-deterministic, robust distributed systems.