Simulation concepts and architectures

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Simulation concepts and architectures
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Simulation Basics

• System a collecting of entities that act and
  interact together toward the accomplishment of
  some logical end.
• State the collection of variables necessary to
  describe a system at a particular time, relative
  to the objectives of a study
• Model a description about how a system behaves,
  that consist of a collection of assumptions.

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Figure 1. Ways to study a system
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• Experiment with the actual system
• vs. Experiment with a Model of the system
• Costly, disruptively.
• System could be only a conceptual matter.
• Validity the accuracy of the model.
• Physical model
• vs. Mathematical model
• Cockpits, miniatures, etc. are iconic. Sometimes
  useful in operations research like scale models.
• Analytical solution
• vs. Simulation
• v d/t
• Large nonsparse matrix inversion, manufacturing
  system, the Matrix

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Simulation models

• Static vs. Dynamic simulation models
• Whether time plays an role in the model
• Deterministic vs. stochastic simulation models
• Whether random elements are taken into account
• One of the main disadvantage of simulation
• Continuous vs. discrete simulation models
• Whether state changes happen instantaneously in
  countable time points.

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Time advance mechanisms for discrete-event models

• Next-event time advance
• .while ( !done)
• System-gtnext_Event()
• Simulation_clock-gtupdate()
• ..
• Fixed-increment time advance
• .while (time lt end_time)
• System-gtupdate_states()
• time
• .

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Distributed simulation

• Why distributed simulation?
• Ever-complex system and simulation models require
  more computing power
• Advance in hardware and software technologies
• Lower cost of hardware
• Network computing capabilities

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• Users dont know where the resources are located
• Resources can be moved without changing their
  names
• Users dont know the number of existing instances
  of the object
• Many users can share the resources automatically
• Some processes may execute in parallel on the
  network.

• Distributed systems the middle tier contains
  computers without shared memories. Acts as one
  single machine to users.
• Dialogue between computers is make possible by a
  unique inter-process communication mechanism

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Distributed simulation architecture

• Basic elements
• An object interface language describe
  interface.
• Distributed applications require more abstract
  level of communications than ordinary ones.
• Provide interoperability between distributed
  objects
• An object manager
• Passing references to requesting clients
• Instantiating objects and marshalling(coordinating
  ) object requests between different machines.
• Objects therefore become indifferent to invokers.
• A naming service
• The mechanism by which a server informs clients
  about objects available for access.
• Making the communications between objects can be
  delayed until runtime.

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High level architecture
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Common Object Request Broker Architecture
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J2EEs RMI

• Remote Method Invocation/Internet Inter-ORB
  Protocol (RMI/IIOP) Protocol that enables Remote
  Method Invocation (RMI)
• Programmers to combine the benefits of using the
  RMI APIs and robust
• CORBA IIOP communications protocol to communicate
  with
• CORBA-compliant clients that have been developed
  using any language
• compliant with CORBA.

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Comparison

• Both CORBA and HLA are concerned with legacy
  applications, possibly in different languages.
  C, Ada, Java, etc.
• RMI keeps to single language, now has common
  interface with CORBA
• HLA is more simulation specific, whereas CORBA
  and RMI are designed for general applications.
• HLA s notion of transfer of object ownership is
  unique .
• RMI uses TCP/IP, CORBA uses IIOP, HLA leaves the
  choice to RTI developers/vendors (which is
  considered a disadvantage to interoperability).
• RMI has unique security Manager.