1 The Time-Triggered Model of Computation Lior Zimet

2 Outline Introduction Temporal accuracy of information Layering versus Partitioning The TT model of computation The client-server model versus the TT model Conclusions

3 Introduction Hard real-time systems cannot tolerate violation of computational deadline More difficult to understand and design and therefore need a reduced representation of the system model Client-server models are used for distributed systems, but they have no notion of time. TT-model is based on the time of the real-time information TT- model objective is to provide the high level description of large real-time system, focusing on temporal data accuracy

4 Temporal accuracy of information Real-time data losses validity as time progress Real-time Entity – a significant set of state values of a controlled object that were captured in time. Observation – captures the information from the RT entity Observation = Real-time Image – current picture of an RT entity Temporal Accuracy – relationship between the RT image and RT entity history

5 Temporal accuracy of information RT image lag behind RT entity because of the transmission time from the observing node to the receiving node The delay between the observation of entity and the use of image causes an error: State estimation – estimates the point in time where the value of RT image is causing an action in the environment State estimation can be used to bring the RT image into better agreement with RT entity

6 Layering versus partitioning Two kinds of system structuring to reduce complexity: 1.Successive hierarchically-ordered layers 2.Partitioning of a large system into nearly independent subsystems with their own resources and interfaces Two steps to structure a system: 1.Partition into nearly decomposable subsystems of high inner connectivity and low external connectivity 2.Each subsystem internally can be structured according to the layering technique

7 Layering versus partitioning Functional partitioning of the hardware can lead to number of advantages: 1.Reduced need for resource multiplexing 2.Easier to observe resource failure and errors 3.Implementation complexity of a partition can be hidden behind it ’ s interface specification. 4.Reusability of partitions

8 The TT model of computation The TT model is based on partitioning of a the large distributed system to autonomous subsystems with four basic blocks: – Interface –Communication system –Host computer –Transducer Air traffic control

9 The TT model of computation - Interface Contains shared memory element between two subsystems Temporal firewall describes the properties of the I/F: –Phase-insensitive –Phase-sensitive

10 The TT model of computation - Interface Stable properties: –Names and syntactic of data items –Point in time when data items are accessed –Temporal accuracy of data items The producer is obligated to provide temporal accurate RT images The consumer is obligated to sample the information in a sampling rate that will ensure temporal accuracy

11 The TT model of computation - continue The communication system connects interfaces within a priori known deterministic time bounds The host computer reads and writes the data from the interfaces in a known a priori time instants The Transducer models the input/output of the system. It makes the translation from the environment to RT image and vice versa

12 Client-Server versus TT model CharacteristicClient-ServerTT-Model FocusProcess-basedState-based StructuringHorizontal-layerVertical-partition Driven byEvent messagesSampling of states Locus of ControlGlobal system wide events Local within subsystem Temporal concernTimely request-response transactions Temporal accuracy of interface data Temporal composability Not supportedSupported

13 Client-server versus TT model ArchitectureHard real-time application Soft real-time application Time-Triggered Architecture Adequate Too expensive, because there is no need to provide all the resources required to handle all specified load and fault scenarios. Client-Server Architecture Too expensive, because the fear that the computer system might miss its deadline at the critical instant leads to the installation of hardware overcapacity with a low resource utilization. Adequate

14 Conclusions The TT model specifies interface and communication of subsystems of a large distributed hard real-time system Separates cleanly the design of the interaction among components from the design of the components themselves. Supports constructive implementation of large real-time systems Requires the availability of the timed triggered communication implementation.