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Time-Triggered Architecture

Time-Triggered Architecture . A summary Tim Arrowsmith 2/6/2006. TTA- Introduction. Infrastructure/guidelines for partitioning large applications into nearly autonomous subsystems. Also control the complexity of the evolving system.

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Time-Triggered Architecture

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  1. Time-Triggered Architecture A summary Tim Arrowsmith 2/6/2006

  2. TTA- Introduction • Infrastructure/guidelines for partitioning large applications into nearly autonomous subsystems. • Also control the complexity of the evolving system. • Decomposes a large embedded application into clusters and nodes • Provides a FT global time base of known precision at each node • Taking advantage of the global time to simplify communications and ensure timeliness of real-time applications

  3. TTA – Architecture Model • Broken into 6 sections: • Model of Time • Time and State • RT Entities and RT Images • State Information vs. Event Information • Structure of the TTA • Interconnection Topology

  4. Model of Time • Real time progresses as an infinite set of instants • A happening that occurs at an instant is called an Event • Ordering example: • Node j increments clock • Event e occurs • Node k increments clock

  5. Model of Time – cont. • TTA introduces a sparse time base • Time is partitioned into alternating durations of activity and silence • External representation of time modelled according to the GPS time representation • Time-stamp is an eight-byte integer

  6. Time and State • Sparse-time provides a system-wide notion of time • “Interval of silence” on the sparse time base forms a system-wide consistent dividing line between the past and future and the interval when the state of the distributed system is defined

  7. RT Entities and RT Images • Dynamics of a real-time application are modeled by a set of relevant state variables, the RT-Entities that change their state as time progresses • State Variable • TT-model • A RT Image is a temporally accurate picture of a RT entity at instant t

  8. State Information vs. Event Information • State Attribute – and property of a RT entity tha tis observed by a node of the distributed RT at a particular instant. • State Information – corresponding information • State Observation – records the state of a state variable at particular instant • Event – sudden change of state of an RT entity that occurs at and instant • Event information – information that describes an event, difference between the state before and the state after the event

  9. Structure of the TTA • Basic building block of the TTA is a node

  10. Interconnection Topology • TTA – bus configuration • At every physical node there are three subsystems: the node and two guardians

  11. Interconnection Topology • TTA – star configuration • In cluster of n node n+2 packages are needed (as opposed to 3n with bus)

  12. Design Principles • Discusses principles that guided TTA design • Divided into 6 sections: • Consistent Distributed Computing Base • Unification of Interfaces • Composability • Scalability • Transparent Implementation of FT • Openness

  13. Consistent Distributed Computing Base • TTA exploits the short error detection latency of a TT protocol to perfome immediate error detection and distributed agreement membership

  14. Unification of Interfaces • The time-triggered transport protocol carries autonomously – driven by TT schedule – messages from the sender’s CNI to the receiver’s CNI

  15. Unification of Interfaces – cont. • An interface that prevents propagation of control errors by design is called a temporal firewall • There are three types of interfaces of a node: • Real-time service (RS) • Diagnostic and Maintenance (DM) • Configuration Planning (CP)

  16. Composability • Must distinguish between architeture design and node design • Stability-of-prior service principle ensure that the validated service of a node is not refuted by the integration of a node into a system

  17. Composability – cont. • Constructive integration principle requires that if n nodes are already integrated then the integration of the n+1 node must not disturb the correct operation of the n already integrated nodes

  18. Composability – cont. • Replica Determinate if all members of this set have the same externally visible state, and produce the same output messages at points in time that are at most an interval of d time units apart • ‘d’ is the time it takes to replace a missing message from redundant replicas

  19. Scalability • TTA is designed for very large distributed real-time applications • Horizontal layering (abstraction) • Vertical layering (partitioning)

  20. Transparent Implementation of FT • In TTA the FT mechanisms are implemented in a dedicated FT layer • The FT CNI is identical in structure and timing to the basic non-FT CNI

  21. Openness • “ Provided that the CORBA security clearance is passed, it is thus possible to investigate remotely (via the Internet) the internals of every TTA node while the system is delivering its real-time service.”

  22. Communication • Divided into 4 sections: • The TTP/C Protocol • The TTP/A Protocol • Event Message Channels • Performance Limits

  23. TTP/C Protocol • Fault-tolerant time-triggered protocol that provides: • Autonomous FT message transport with know delay and bounded jitter between CNI (via TDMA) • FT clock synchronization, without relying on a central time server • Membership service to inform every node about the “health-state” of every other node • Clique avoidance

  24. TTP/A Protocol • Time-triggered fieldbus protocol of TTA. • Connect low-cost smart transducers to a node of the TTA. • Interface file system (IFS) holds real-time data, calibration data, diagnostic data, and configuration data. • Information between the IFS of the smart transducer and the CNI of the TTA node is exchanged by TTP/A. • TTP/A supports a “plug-and-play” mode.

  25. Event Message Channels • Event message channels constructed on top of basic TT communications • Bytes designated a priori • Two message queues provided at CNIs: • Sender queue at sender’s CNI • Receiver queue at receiver’s CNI • Filter service and garbage collection service

  26. Performance Limits • Must maintain a 5µs inter-frame gap • Testing currently being perfomed on 1GBit/s systems using COTS

  27. Fault Tolerance • Fault Hypothesis – it is assumed that a chip is a single fault-containment region. • Fault-Tolerant Units – CNI implements replica determinism, it is up to host software to ensure replica determinism within the complete node. Also supports self-checking pairs. • Never-Give-UP Strategy – highly application specific. • Redundant Transducers – uses an agreement protocol.

  28. TTA Design Methodology • Architecture Design – application decomposed into clusters and nodes. • Node Design – application software for host computers developed. Testing from the bottom-up. • Validation – designed to reduce the validation effort. • Design Tools – supported by a comprehensive set of integrated design tools of TTTech AG

  29. Conclusion • Guiding principle: take maximum advantage of the availability of global time. • TTA currently occupies a niche position. • The designers hope to broaden as mainstream application designers start to utilize time instead of attempting to dismiss it.

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