A Hybrid HLA Time Management Algorithm Based on Both Conditional and Unconditional Information

The High-Level Architecture (HLA), which is the IEEE standard for distributed simulation, defines six service groups. The Time Management (TM) service group ensures a Time-Stamp-Ordered (TSO) message delivery sequence and correct time advancement of each simulation component (federate) in a HLA-based distributed simulation application (federation). To control time advancement of a federation, a distributed TM algorithm requires each regulating federate to periodically propagate its local time information to all constrained federates for their respective calculation of the Greatest Available Logical Time (GALT). The time information propagated is called conditional information or unconditional information depending on whether it can be guaranteed to be true conditionally or unconditionally. A traditional distributed TM algorithm can be either synchronous or asynchronous. In general, a synchronous algorithm utilizes conditional information while an asynchronous algorithm utilizes unconditional information. However, both synchronous and asynchronous algorithms have their own drawbacks and thus cannot be used for all federation scenarios. To resolve the drawback of each algorithm, this paper proposes a hybrid TM algorithm by combining synchronous and asynchronous algorithms. The three algorithms have been incorporated into a Run-Time Infrastructure (RTI) and experimental results show that the hybrid algorithm effectively combines the advantages of both synchronous and asynchronous algorithms. We also compare the proposed hybrid TM algorithm with the TM algorithm implemented in the Federated Simulations Development Kit (FDK), which also uses both conditional and unconditional information. The hybrid TM algorithm is more scalable than FDK’s TM algorithm with respect to the total number of federates in a federation, because FDK’s TM algorithm has the overhead of redundant GALT calculations.