Time triggered arch.
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The Time-Triggered Architecture (TTA) is a platform for safety-critical embedded systems like aircraft flight control that uses a time-triggered protocol over redundant communication channels. Nodes are precisely scheduled to transmit and receive messages to ensure deterministic and fault-tolerant execution. While providing real-time performance guarantees, the rigid schedule allows easy testing but limits flexibility for spontaneous messages.
Time triggered arch.
- 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
- 10. Time-Triggered Architecture • Basic unit: NODE • Node: A processor with memory I-O subsystem Operating system Application software Time-triggered communication controller
- 11. 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.
- 12. 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.
- 13. 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
- 14. 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
- 15. 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)
- 16. 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.
- 17. 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
- 18. Protocol Highlights Temporal 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
- 19. 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
- 20. 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)
- 21. • Time-Triggered architectures and protocols will become important. • Seemingly simple – But quite sophisticated • for time-deterministic, robust distributed systems.