set of federates that connect to the same RTI together with a common FOM. Federation Execution: A session, where a set of federates execute together in a federation with a specific objective, using the same RTI and FOM. Federation Object Model (FOM): A document that specifies object classes, interaction classes, data types and additional data that is used for the information exchange in a federation. A FOM is an XML file that follows the format of the HLA Object Model Template and the associated XML Schema. Different FOMs are used for exchanging data for different application domains. There are standardized FOMs, called reference FOMs, that are commonly used as a starting point for FOM development. A FOM can be developed and extended in a modular way using FOM modules. Simulation Object Model (SOM): A document that specifies object classes, interaction classes, data types and additional data that a particular simulation publishes and/or subscribes to in a federation. A SOM is also an XML file that follows the format of the HLA Object Model Template and the associated XML Schema. SOMs can also be developed and extended in a modular way using SOM modules. Object: Objects are used to represent data that is persistent over some period of time and that have attributes that can be updated. They are defined in the FOM/SOM using an Object Class. Interaction: Interaction are used to represent instantaneous events with parameters. An interaction that has been sent cannot be updated (as opposed to object classes). They are defined in the FOM/SOM using an Interaction Class. Datatypes: The representation and interpretation of attribute and parameter data is specified in the FOM/SOM using HLA Datatypes. Publish: A federate that publishes an object class with a set of attributes can register and delete instances of that object class and update its attribute values. A federate that publishes an interaction class can send interactions of that interaction class, together with associated parameter values. Subscribe: A federate that subscribes to an object class with a set of attributes will discover registrations and deletions of instances of that object class and receive updates of subscribed attributes. A federate that subscribes to an interaction class will receive interactions of that interaction class, together with associated parameter values. Interface specification The RTI services are defined in the HLA Interface Specification. They are grouped into seven service groups. In addition to these services, the