1. Technical Overview

A high-level architecture consists of the following components:

• Interface specification, that defines how HLA compliant simulators interact with the run-time infrastructure (RTI). The RTI provides a programming library and an application programming interface (API) compliant to the interface specification.
• Object model template (OMT), that specifies what information is communicated between simulations, and how it is documented.
• Rules, that simulations must obey in order to be compliant to the standard.

1.1. Common HLA Terminology

• Federate: an HLA compliant simulation entity.
• Federation: multiple simulation entities connected via the RTI using a common OMT.
• Object: a collection of related data sent between simulations.
• Attribute: data field of an object.
• Interaction: event sent between simulation entities.
• Parameter: data field of an interaction.

Objects and Interactions

Much of the interactions between federates involve objects and interactions which work in a publish-subscribe model. A federate can register an object, which is instance of a class, and then change the values of the attributes of the object. Other federates that are subscribed to the class can discover the object and then 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.

1.2. Interface Specification

The interface specification is object oriented, with specifications for both C++ and Java programming languages plus Ada and FORTRAN for the 1.3 specification.

The interface specification is divided into service groups:

• Federation management: Defines how federates can connect to the RTI, create, join and manage federations, save and restore federation states and defines a system to synchronize federates to the same time.
• Declaration management: Defines how federates declare their intentions with regard to publication and subscription of classes and interactions.
• Object management: Defines how federates can utilize objects and interactions once they have ownership of them.
• Ownership management: Defines how federates divest and acquire ownership of registered objects.
• Time management: Defines how time is used in a federation and how it affects object and interaction updates, federate saves and other services.
• Data distribution management: Defines the various ways that object and interaction data is transferred from and to federates through the RTI.
• Support services: Defines various services to retrieve information about the current federation, such as classes and interactions.

1.3. Object Model Template

The object model template (OMT) provides a common framework for the communication between HLA simulations. OMT consists of the following documents:

• Federation object model (FOM). The FOM describes the shared object, attributes and interactions for the whole federation.
• Simulation object model (SOM). A SOM describes the shared object, attributes and interactions used for a single federate.

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.

Management Object Model

Each FOM must contain a copy of the HLA standard Management Object Model, or MOM, which is a collection of classes and interactions

Federation Conformance

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".

Evolved FOM Modules and MIM

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.

In principle, nothing changes for the federates. They call the same functions of the RTI as before. The difference is that elements of a FOM that are not needed do not have to be loaded and managed. In addition, if a federate joins late the additional information exchange requirements can be added when modular FOMs are used.

1.4. HLA Rules

The HLA rules describe the responsibilities of federations and the federates that join.^[1]

1. Federations shall have an HLA federation object model (FOM), documented in accordance with the HLA object model template (OMT).
2. In a federation, all representation of objects in the FOM shall be in the federates, not in the run-time infrastructure (RTI).
3. During a federation execution, all exchange of FOM data among federates shall occur via the RTI.
4. During a federation execution, federates shall interact with the run-time infrastructure (RTI) in accordance with the HLA interface specification.
5. During a federation execution, an attribute of an instance of an object shall be owned by only one federate at any given time.
6. Federates shall have an HLA simulation object model (SOM), documented in accordance with the HLA object model template (OMT).
7. Federates shall be able to update and/or reflect any attributes of objects in their SOM and send and/or receive SOM object interactions externally, as specified in their SOM.
8. Federates shall be able to transfer and/or accept ownership of an attribute dynamically during a federation execution, as specified in their SOM.
9. Federates shall be able to vary the conditions under which they provide updates of attributes of objects, as specified in their SOM.
10. Federates shall be able to manage local time in a way that will allow them to coordinate data exchange with other members of a federation.

1.5. Base Object Model

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.^[2]

1.6. Federation Development and Execution Process (FEDEP)

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.

1.7. Distributed Simulation Engineering and Execution Process (DSEEP)

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).

2. Standards

HLA is defined under IEEE Standard 1516:

• IEEE 1516–2010 – Standard for Modeling and Simulation High Level Architecture – Framework and Rules
• IEEE 1516.1–2010 – Standard for Modeling and Simulation High Level Architecture – Federate Interface Specification
• IEEE 1516.2-2010 – Standard for Modeling and Simulation High Level Architecture – Object Model Template (OMT) Specification
• IEEE 1516.3-2003 – Recommended Practice for High Level Architecture Federation Development and Execution Process (FEDEP)
• IEEE 1516.4-2007 – Recommended Practice for Verification, Validation, and Accreditation of a Federation an Overlay to the High Level Architecture Federation Development and Execution Process

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.

Previous version:

• IEEE 1516–2000 – Standard for Modeling and Simulation High Level Architecture – Framework and Rules
• IEEE 1516.1–2000 – Standard for Modeling and Simulation High Level Architecture – Federate Interface Specification
• IEEE 1516.1–2000 Errata (2003-oct-16)
• IEEE 1516.2-2000 – Standard for Modeling and Simulation High Level Architecture – Object Model Template (OMT) Specification

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.

2.1. STANAG 4603

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).^[3]

2.2. DLC API

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 superseded by the HLA Evolved APIs, informally known as Evolved DLC APIs (EDLC).

• Dynamic Link Compatible HLA API Standard for the HLA Interface Specification Version 1.3^[4]
• Dynamic Link Compatible HLA API Standard for the HLA Interface Specification (IEEE 1516.1 Version)^[5]

2.3. HLA Evolved

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:

• Extended XML support for FOM/SOM, such as Schemas and extensibility
• Fault tolerance support services
• Web Services (WSDL) support/API
• Modular FOMs
• Update rate reduction
• Encoding helpers
• Extended support for additional transportation (such as QoS, IPv6,...)
• Standardized time representations

3. Alternatives and Disadvantages

Virtually all means of interconnecting Distributed Modeling and Simulation (DM&S) applications have alternatives and or disadvantages and the HLA is no exception.

3.1. Alternatives

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 but with the distinct advantage of having an open on-the-wire protocol for system interoperability.^[6]

3.2. Disadvantages

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 ^[7].