TimeTriggered Architecture

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

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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

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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

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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

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Structure of the TTA

• Basic building block of the TTA is a node

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Interconnection Topology

• TTA bus configuration
• At every physical node there are three
  subsystems the node and two guardians

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Interconnection Topology

• TTA star configuration
• In cluster of n node n2 packages are needed (as
  opposed to 3n with bus)

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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

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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 senders CNI to the receivers CNI

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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)

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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

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Composability cont.

• Constructive integration principle requires that
  if n nodes are already integrated then the
  integration of the n1 node must not disturb the
  correct operation of the n already integrated
  nodes

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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)

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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

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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

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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

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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.

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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 senders CNI
• Receiver queue at receivers CNI
• Filter service and garbage collection service

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Performance Limits

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

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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.

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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

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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.