Key Interoperability Technologies

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About HLA Print

High-Level Architecture for beginners

This is an introduction to the High-Level Architecture targeted at persons with no previous HLA knowledge and with only a basic understanding of computer programs.

What is HLA?

HLA (High-Level Architecture) is a standard for connecting several computer-based simulation systems so that they can run together and exchange information. Instead of building a big monolithic simulation system from scratch, the HLA allows you to combine existing simulation systems with new systems.

HLA enables you to reuse existing systems for new purposes. You can also mix different programming languages and operating systems. HLA supersedes several earlier standards such as DIS and ALSP.

Who uses HLA?

HLA was originally developed by the US Department of Defence. It is the prescribed standard for military simulation interoperability within the US. It is also the new standard for simulation interoperability within the NATO as well as in many countries, for example Sweden. The HLA can be used for any kind of simulation systems. There is a growing interest from non-military areas, for example manufacturing and transportation. The HLA standard has also become a non-military standard through IEEE.

What kind of simulation systems can interoperate through HLA?

HLA can be used to combine several simulations systems. Simulation systems can be entirely computer-based or involve real people. One type of simulation is called a virtual simulation where a real person operates simulated equipment, for example a flight simulator.

Another type of simulations is a constructive simulation where simulated people in a computer operate simulated equipment, for example in computer-generated forces. Yet another kind of simulation is a live simulation where real people operate real equipment, for example soldiers during a military exercise connected to other simulation systems using radio equipment.

HLA supports simulations developed for various purposes. Some examples are training, analysis and simulation based acquisition, where products are evaluated through a simulation of their capabilities.

What are the technical components of HLA?

In HLA you combine several simulation systems, called "federates", into one big simulation, called the "federation". To do this you need to do the following:

  1. You have to document the information that will be exchanged in a Federation Object Model (FOM). This is a file that describes the common language of the federation. It provides information about what object classes ("car"), attributes ("brand, speed") and interactions ("honk the horn") that will be exchanged within the federation. One of the reasons that HLA is so flexible is that you can develop a FOM to meet the needs of your particular federation. If, on the other hand, you have an application in a well-known area then there are several ready-made reference models that you can base your work upon.

  2. You have to exchange information between the participating systems (federates) using a piece of software called the Run-Time Infrastructure (RTI) that follows a standardised specification.

What does the Run-Time Infrastructure do?

The RTI lets the participating simulation systems (federates) connect to each other and exchange information. They can communicate what objects they have and what the attribute values are, as well as exchanging interactions. The RTI can synchronize time within the federation. Different types of simulations handle time in various ways. HLA supports several time management methods. Some examples are real-time, scaled real-time, event-based time and as-fast-as-possible.

The RTI also contains more advanced functions such as transferring the responsibility of updating an attribute between federates, managing data distribution and managing the federation.

The RTI is defined by a number of services that your program can use. These are described in the interface specification in the HLA standard. An RTI can be implemented using many different technologies (shared memory/network communications, centralized/distributed topology, etc). There are several RTIs targeted at different needs.

To ensure correct behavior of an RTI there are official certification procedures. A certified RTI is guaranteed to implement the full specification and behave according to the standard. There are currently two standards, HLA 1.3, the first standard (developed by US Department of Defence) and the new, improved, IEEE 1516 standard for HLA. Read more about the improvements in HLA 1516 here.

Where can I find more information about HLA?

In addition to this web site you can look at

  • MSCO. The US Department of Defense Modeling and Simulation Coordination Office (M&S CO) has a lot of information about HLA in the "Warfighter" section. This includes the HLA 1.3 standard, projects, organizations, HLA tools, RTI certification, etc.

  • SISO. The Simulation Interoperability Standards Organization arranges the SIW (Simulation Interoperability Workshop). A lot of practical HLA experiences are described in various papers.

  • IEEE. Provides the IEEE 1516 standard for HLA.

You should also consider attending HLA training arranged by Pitch and others. There is a book on HLA called "Creating Computer Simulation Systems: An Introduction to the High Level Architecture" by Frederick Kuhl (ISBN 0-13-022511-8) available from Amazon and others.