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A high-level architecture (HLA) is a general purpose architecture for distributed computer simulation systems. Using HLA, computer simulations can interact (that is, to communicate data, and to synchronize actions) with other computer simulations regardless of the computing platforms. The interaction between simulations is managed by a Run-Time Infrastructure (RTI). HLA is an interoperability standard for distributed simulation used to support analysis, engineering and training in a number of different domains such as:
A high-level architecture consists of the following components:
Much of the interactions between federates involve objects and interactions which work in a publish-subscribe model. A federate can register an instance of an object and then change the attributes. Other federates that are subscribed to the object receive attribute value updates. Interactions work in a similar way, except that an interaction is only used once with a specified set of parameters values and then discarded.
The interface specification is divided into service groups:
The object model template (OMT) provides a common framework for the communication between HLA simulations. OMT consists of the following documents:
In 1.3 the FOM passed to the RTI by means of a file, called an FDD, in a Lisp-like syntax. In 1516 and 1516-2010 the file is an XML file.
Each FOM must contain a copy of the HLA standard Management Object Model, or MOM, which is a collection of classes and interactions
In order to ensure the proper interaction between simulations, a way of testing federate conformance is defined. This involves ensuring that every class and interaction listed in the SOM for a particular federate is used according to the usage described, "PublishSubscribe", "Publish", "Subscribe" or "None".
For HLA 1516-2010, instead of a single FDD that describes the entire FOM, the specification describes FOM modules which are merged to form the full FOM. By default, a federation is created by merging the HLAstandardMIM.xml FOM module with the module(s) provided by the federate that creates the federation. The standard MIM (MOM and Initialization Module) contains the MOM classes and the basic default data types. Any joining federate can add one or more FOM modules to extend the existing FOM.
The HLA rules describe the responsibilities of federations and the federates that join.
The Base Object Model (BOM), SISO-STD-003-2006 is a related standard by SISO to provide better reuse and composability for HLA simulations, and is highly relevant for HLA developers. It provides a way to specify conceptual models and how to map them to an HLA FOM. More information can be found at Boms.info.
FEDEP, IEEE 1516.3-2003, is a standardized and recommended process for developing interoperable HLA based federations. FEDEP is an overall framework overlay that can be used together with many other, commonly used development methodologies.
In spring 2007 SISO started revising the FEDEP. It has been renamed to Distributed Simulation Engineering and Execution Process (DSEEP) and is now an active standard IEEE 1730–2010 (instead of IEEE 1516.3).
HLA is defined under IEEE Standard 1516:
Machine-readable parts of the standard, such as XML Schemas, C++, Java and WSDL APIs as well as FOM/SOM samples can be downloaded from the IEEE 1516 download area of the IEEE web site. The full standards texts are available at no extra cost to SISO members or can be purchased from the IEEE shop.
Prior to publication of IEEE 1516, the HLA standards development was sponsored by the US Defense Modeling and Simulation Office. The first complete version of the standard, published 1998, was known as HLA 1.3.
HLA (in both the current IEEE 1516 version and its ancestor "1.3" version) is the subject of the NATO standardization agreement (STANAG 4603) for modeling and simulation: Modeling And Simulation Architecture Standards For Technical Interoperability: High Level Architecture (HLA).
SISO has developed a complementary HLA API specification known as the Dynamic Link Compatible (DLC) API for the IEEE 1516-2000 version of HLA. The DLC API addresses a limitation of the IEEE 1516 and 1.3 API specification, whereby federate recompilation was necessary for each different RTI implementation. Note that this API has since been superseeded by the HLA Evolved APIs, informally known as Evolved DLC APIs (EDLC).
The IEEE 1516 standard has been revised under the SISO HLA-Evolved Product Development Group and was approved 25-Mar-2010 by the IEEE Standards Activities Board. The revised IEEE 1516–2010 standard includes current DoD standard interpretations and the EDLC API, an extended version of the SISO DLC API. Other major improvements include:
Virtually all means of interconnecting Distributed Modeling and Simulation (DM&S) applications have alternatives and or disadvantages and the HLA is no exception.
In regards to the Distributed Modeling and Simulation (DM&S) industry the most often used alternative to the HLA is clearly Distributed Interactive Simulation (DIS), IEEE 1278.1-2012, a recently updated simulation protocol. Most HLA RTI vendors also feature DIS in their products. As for middleware applications that most closely match HLA features, such as the publish and subscribe feature (P&S) see Data Distribution Service (DDS) which shares many of the same characteristics including the fact that both are middleware implementations with a proprietary network transport. See A Comparison of HLA and DDS
HLA is defined as a set of services, provided by a C++ or Java API. There is no standardized on-the-wire protocol. Participants in a federation must use RTI libraries from the same provider and usually also of the same version in order for applications to interoperate.
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