Image Processing Reference
In-Depth Information
14
Time-Triggered Communication
. Time-andEvent-TriggeredCommunication .........
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. Time-Triggered Communication in the
AutomotiveDomain .................................
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. DependabilityConcepts..............................
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Fault, Error, and Failure
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Fault Containment
. Fundamental Services of a Time-Triggered
CommunicationProtocol ............................
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Clock Synchronization
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Periodic Exchange of State
Messages
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Fault Isolation Mechanisms
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Diagnostic Services
. Time-TriggeredCommunicationProtocols...........
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TTP/C
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TTP/A
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TTCAN
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TT Ethernet
References .................................................
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Roman Obermaisser
Vienna University of Technology
14.1 Time- and Event-Triggered Communication
It has been recognized that communication protocols fall into two general categories with corres-
ponding strengths and deficiencies: event- and time-triggered control. Event-triggered protocols
(e.g., TCP/IP, CAN, Ethernet, ARINC) offer flexibility and resource efficiency. Time-triggered
protocols (e.g., TTP/C, FlexRay) excel with respect to predictability, composability, error detection,
and error containment.
In event-triggered architectures, the system activities, such as sending a message or starting com-
putational activities, are triggered by the occurrence of events in the environment or the computer
system. In time-triggered architectures (TTA), activities are triggered by the progression of global
time. he major contrast between event- and time-triggered approaches lies in the location of control.
Time-triggered systems exhibit autonomous control and interact with the environment according
to an internal predefined schedule, whereas event-triggered systems are under the control of the
environment and must respond to stimuli as they occur.
The time-triggered approach is generally preferred for safety-critical systems [Kop,Rus]. For
example, in the automotive industry a TTA will provide the ability to handle the communication
needs of by-wire cars [Bre]. In addition to hard real-time performance, TTAs help in managing the
complexity of fault-tolerance and corresponding formal dependability models, as required for the
establishment of ultrahigh reliability (failure rates in the order of
−
failures/h). he predetermined
points in time of the periodic message transmissions allow error detection and establishing of mem-
bership information. Redundancy can be established transparently to applications, i.e., without any
modification of the function and timing of application systems. A time-triggered system also sup-
ports replica determinism [Pol], which is essential for establishing fault-tolerance through active
redundancy.
