مدیریت زمان هوشمند __ رابط هماهنگ سازی زمان واحد و یکپارچه برای شبیه سازی توزیع شده

Distributed simulation is a widely studied technique for the networked virtual environment. Among existing technologies, High Level Architecture (HLA) establishes a common distributed simulation framework that facilitates the interoperability and reuse of simulation components. However, some HLA services are very low-level and difficult to use, especially when a simulation is designed using a particular time synchronization mechanism. This paper describes an agent interface called Smart Time Management (STM). STM is used to unify the time management services of the time-stepped, event-driven, and optimistic time advancement in HLA specification. The capabilities of the STM include the followings: taking over events timestamp tagging work, maintaining a look-ahead value, and unifying different time advance approaches provided by the HLA Run Time Infrastructure (RTI). In addition, it adopts the time warp mechanism for optimistic simulation. In summary, STM presents a unified and scalable middle layer to allow the user to construct an HLA federation with a unanimous time management interface when solving the synchronization issue. The presented middle layer enables the user to deploy the conservative and optimistic synchronization mechanisms in a unanimous way.

Modeling and analyzing the timing behavior of the distributed simulation are of wide interests in the networked virtual environment. The parallel or distributed simulation refers to the execution of discrete-event simulation programs on a multiprocessor system or a network of workstations [1]. The researches of parallel/distributed simulation focus on how to achieve high-performance simulation while ensuring all events to be parallelly processed and still maintaining their causal relationships [2]. Over the years, two synchronization approaches for the parallel/distributed simulation have been proposed: the conservative synchronization [3] and [4] and the optimistic synchronization [5] and [6]. The High Level Architecture (HLA) specification was initiated by the Department of Defense, USA, to support interoperability among distributed simulators. It defines a standard architecture for modeling and simulation of a complex distributed simulation [7]. The HLA standard later becomes the international standard, IEEE 1516, for the distributed simulation. The implementation of the HLA specification [8] in the system side is called Run Time Infrastructure (RTI), whereas the application side is called the federate. Among the supported services of HLA specification, the synchronization mechanism was included in the RTI time management service to ensure the attributes/events sequence among distributed federates. In HLA terminology, each distributed node of a simulation is called a federate. To be more specific, the time management service provides both conservative and optimistic approaches to synchronize different federates within the federation [9]. In HLA terminology, a federation is a simulation that consists of a set of federates. However, it is not an easy task to use these synchronizing services to build a parallel/distributed federation. Even though RTI provides some synchronization interfaces for the distributed federates, several critical design issues must be carefully studied at the developing phase. Different types of federates require distinct synchronization schemes.

This paper presents the STM agent that integrates the conservative and optimistic simulations into a unified synchronization mechanism. STM encapsulates all the time synchronization factors into a basic model TMS. With the help of the STM, the federate developers can easily utilize the HLA Time Management services without concerning about time policies, message ordering definitions, or logical time advance strategies for the federate. In addition, the STM provides a smart optimistic synchronization method by creating a middle layer to shield the developer from the tedious work of employing the time management service on the RTI. Since the traditional optimistic simulation requires state saving, states roll back, and fossil-collection, which are complex for the designer and often difficult to implement the applications, the presented STM agent solved this problem by supporting smart rollback, generic state-saving, and efficient fossil-collection management for the optimistic federate. With the STM, different types of time-advancing federates of a federation can exchange messages by using a unified time advance function eventRequest to invoke the RTI. The STM will then smartly translate the request into an appropriate HLA Time Management service call. In this case, the STM presents a unified and scalable mechanism for the user to construct HLA federates with a unanimous time management interface to solve the synchronization issue. The STM helps the developer to simplify the process of building conservative and optimistic federates. All aspects of the time management issues are taken care of by the STM that interacts with the RTI and extended optimistic time warp functions. It relieves the simulation developers from the complexity of the synchronizing approach. The STM is capable of identifying which time advancement approach is used by a federate, as well as taking all suitable actions to ensure the federate processes internal/external events in the correct causality. The STM will accomplish tasks that are common to every synchronized federate. However, it does not depend on a specific federate behavior. Hence, the federate program code becomes simpler because most of the control and management of synchronization is handled by the STM.