Time Management in Hierarchical Federation Communities

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Time Management in Hierarchical Federation
Communities

• Anthony Cramp (DSTO, University of Adelaide)
• Michael Oudshoorn (University of Adelaide)
• John Best (DSTO)

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Introduction

• Systems of Systems Simulation
• Hierarchical Federation Communities
• Time Management
• In the HLA
• In Hierarchical Federation Communities
• Extensions
• Conclusion

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Systems of Systems Simulation

• Integrating models of subsystems to simulate a
  complete system
• In a distributed environment the subsystem models
  can form the distributed components
• In HLA speak
• Federates implement the subsystem models
• The complete system is simulated in the federation

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Virtual Maritime System (VMS)

• Goal is to provide a virtual representation of a
  maritime platform
• Provides representation via the interoperation of
  models of the platforms constituent systems
• Systems modelled include
• Sensor systems
• Weapon systems
• Navigation systems
• Command and control systems

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Problem

• Problem occurs when trying to model multiple
  complete systems in a single federation
• Data intrinsic to a single system is exposed to
  the subsystems of other systems modelled in the
  federation
• Federates modelling the subcomponents of systems
  must audit received data to ensure it was
  generated by a federate modelling a subcomponent
  of the same system

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Problem
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Solutions

• Local Data Filtering
• Assign an identifier to each system
• Data generated is tagged with this identifier
• Approach used by the VMS
• Data Distribution Management (DDM)
• System identifier used to define a region
• Intra-system data is communicated via this region

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Federation Communities

• Simulate each system in its own federation
• Data internal to a system remains in the
  federation
• Enable inter-system communication via
  inter-federation communication
• Federations may form subcomponents of higher
  order systems
• Group to form a federation community of the
  higher order system
• Continue in a hierarchic manner

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Hierarchical Federation Community

• Provides data encapsulation at multiple levels
• Beneficial in a wide area network setting as data
  intrinsic to a system can be contained to a local
  area network
• Requires the definition of a new communication
  framework

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Distributed Federate Proxy (DFP)

• Communication via the HLA defined services

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Time Management

• Simulation time an abstraction of physical time
• Time management refers to controls put in place
  to allow advance of time and causal ordering of
  time stamped messages
• Relatively easy in monolithic simulations
• Not so easy in distributed simulation

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Time Management in HLA

• Provides various time management strategies
• Conservative time management
• Federates request a time advance of the RTI
• RTI grants a time advance to a federate when
  causal delivery of time stamped messages can be
  guaranteed

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Time Management in HFC

• Provision of service for causal delivery of time
  stamped messages between federates in a
  hierarchical federation community
• How the service is implemented is dependant on
  the architecture of the hierarchical federation
  community
• The Distributed Federate Proxy architecture
  utilises the existing HLA time management
  services
• Components generally time regulating and time
  constrained

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Time Management in DFP - DFPC

• Deliver time stamped messages from RTI to SimNode
  and vice versa
• LBTS(DFPC)RTI Time(DFPC)
  Lookahead(DFPC)SimNode
• LBTS(DFPC)SimNode Time(DFPC)
  Lookahead(DFPC)RTI

SimNode
DFPC
RTI
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TM in DFP Non Root SimNode

• SimNodeP provides RTI type functionality to
  SimNode
• SimNode must forward time stamped messages
  received from its children to SimNodeP
• LBTS(SNC) Time(SN) Lookahead(SN)

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TM in DFP Root SimNode

• Analogous to the single federation case with the
  SimNodeROOT acting as the RTI
• LBTS of SimNodeROOT equals the global minimum
  timestamp
• SimNodeROOT is the ultimate decision authority on
  time advances

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TM Process - DFPC

• Make time advance request to SimNode of timetSN
  LBTS(DFPC)RTI Lookahead(DFPC)SN
• Receive messages from SimNode with timestamp TS
  
• Make time advance request to RTI of timetRTI
  LBTS(DFPC)SN Lookahead(DFPC)RTI
• Receive messages from RTI with timestamp TS tRTI

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TM Process Non Root SimNode

• SimNode handles time advance requests from
  connected children
• Triggers its own time advance request to its
  parent (SimNodeP) of timet LBTS(SNC)
  Lookahead(SimNode)
• When time advance is granted from SimNodeP,
  SimNode can grant time advances to its children

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TM Process Root SimNode

• SimNodeROOT performs its time management process
  analogous to that of a RTI
• Time advance grants issued by SimNodeROOT trigger
  further time advance grants issued by the
  children of SimNodeROOT
• Finally, the DFPC is granted a time advance which
  triggers a time advance request in the federation
  to which it is connected, which allows time to
  progress

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Extensions

• Zero Lookahead
• Event Stepped Simulation
• Declaration Management

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Conclusion

• Systems of systems federations problematic when
  simulating multiple systems
• Hierarchical federation communities offer a
  solution to these problems
• Distributed federate proxy provides an
  architecture for communication in hierarchical
  federation communities
• Time management in distributed federate proxy
  possible with the root of the architecture
  becoming the central time authority

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Questions/Comments