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Distributed real-time systems

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  1. Abstract
  2. Meaning of real time system
  3. Structure of distributed real-time systems
  4. Time Division Multiple Access - TDMA
  5. Real-Time, Dynamic and Static scheduling
  6. Conclusion
  7. References

Real-time systems form one category of specialized distributed systems named ?Distributed Real-time systems?. Real-time programs(and systems) interact with the external world in a way that involves time. when a stimulus appears ,the system must respond to it in a certain way and before a certain dead line. Many Real-time applications and systems are highly structured much more so than general purpose distributed systems. Distributed Real time systems are needed in large complex applications in which the CPU may have to deal with multiple event streams and when it will become more difficult for one machine to meat all the dead lines and real time constraints. Unlike other Distributed systems ,the Real time Distributed systems can often be structured as a collection of computers connected by a network, some computers are Connected to external devices that produce or accept data or expect to be controlled in real time, and computers have sensors for receiving signals from devices or actuators for sending signals to them.

[...] Suppose that a periodic real-time distributed system has m tasks and N processors to run them on. Let Ci be the CPU time needed by task and let Pi be its period, that is, the time between consecutive interrupts. To be feasible, the utilization of the system, µ, must be related to N by the equation m µ = ? ( ci pi ) N i For example, if a task is started every 20 msec and runs for 10 msec each time, it uses up 0.5 CPUs. [...]

[...] Real-Time Communication Communication in real-time distributed systems is different from communication in other distributed systems. While high performance is always welcome, predictability and determinism are the real keys to success. In this section we will look at some real-time communication issues, for both LANs and WANs. Achieving predictability in a distributed system means that communication between processors must also be predictable. LAN protocols that are inherently stochastic, such as Ethernet, are unacceptable because they do not provide a known upper bound on transmission time. [...]

[...] Real-time distributed systems operating over wide-area networks have the same need for predictability as those confined to a room or building. The communication in these systems is invariably connection oriented. Often, there is the ability to establish real-time connections between distant machines. When such a connection is established, the quality of service is negotiated in advance between the network users and the network provider. This quality may involve a guaranteed maximum delay, maximum jitter (variance of packet delivery times), minimum bandwidth, and other parameters. [...]

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