Aggregate Level Simulation Protocol

In 1990, the Defense Advanced Research Projects Agency (DARPA) employed The MITRE Corporation to study the application of distributed interactive simulation principles employed in SIMNET to aggregate-level constructive training simulations. Based on prototype efforts, a community-based experiment was conducted in 1991 to extend SIMNET to link the US Army's Corps Battle Simulation (CBS) and the US Air Force's Air Warfare Simulation (AWSIM). The success of the prototype and users' recognition of the value of this technology to the training community led to development of production software. The first ALSP confederation, providing air-ground interactions between CBS and AWSIM, supported three major exercises in 1992.

By 1995, ALSP had transitioned to a multi-Service program with simulations representing the US Army (CBS), the US Air Force (AWSIM), the US Navy (RESA), the US Marine Corps (MTWS), electronic warfare (JECEWSI), logistics (CSSTSS), and intelligence (TACSIM). The program had also transitioned from DARPA's research and development emphasis to mainstream management by the US Army's Program Executive Office for Simulation, Training, and Instrumentation (PEO STRI)

ALSP developed and demonstrated key aspects of distributed simulation, many of which were applied in the development of HLA.

• No central node so that simulations can join and depart from the confederation at will
• Geographic distribution where simulators can be distributed to different geographic locations yet exercise in the same simulated environment
• Object ownership so each simulation controls its own resources, fires its own weapons and determines appropriate damage to its systems when fired upon
• A message-based protocol for distributing information from one simulation to all other simulations.
• Time management so that the times for all simulations appear the same to users and so that event causality is maintained – events should occur in the same sequence in all simulations.
• Data management permits all simulations to share information in a commonly understood manner even though each had its own representation of data. This includes multiple simulations controlling attributes of the same object.
• An architecture that permits simulations to continue to use their existing architectures while participating in an ALSP confederation.

In 1989, the Warrior Preparation Center (WPC) in Einsiedlerhof, Germany hosted the computerized military exercise ACE-89. The Defense Advanced Research Projects Agency (DARPA) used ACE-89 as a technology insertion opportunity by funding deployment of the Defense Simulation Internet (DSI). Its packetized video teleconferencing brought general officers of NATO nations face-to-face during a military exercise for the first time; this was well received. But the software application of DSI, distribution of Ground Warfare Simulation (GRWSIM), was less successful. The GRWSIM simulation was unreliable and its distributed database was inconsistent, degrading the effectiveness of the exercise.

DARPA was funding development of a distributed tank trainer system called SIMNET where individual, computerized, tank-crew trainers were connected over local area networks and the DSI to cooperate in a single, virtual battlefield. The success of SIMNET, the disappointment of ACE-89, and the desire to combine existing combat simulations prompted DARPA to initiate research that lead to ALSP.

DARPA sponsored the design of a general interface between large, existing, aggregate-level combat simulations. Aggregate-level combat simulations use Lanchestrian models of combat rather than individual physical weapon models and are typically used for high-level training. Despite representational differences, several principles of SIMNET applied to aggregate-level simulations:

• Dynamic configurability. Simulations may join and depart an exercise without restriction.
• Geographic distribution. Simulations can reside in different geographic locations yet exercise over the same logical terrain.
• Autonomous entities. Each simulation controls its own resources, fires its own weapons and, when one of its objects is hit, conducts damage assessment locally.
• Communication by message passing. A simulation uses a message-passing protocol distribute information to all other simulations.

The ALSP challenge had requirements beyond those of SIMNET:

• Simulation time management. Typically, simulation time is independent of wall-clock time. For the results of a distributed simulation to be "correct," time must be consistent across all simulations.[1]
• Data management. The schemes for internal state representation differ among existing simulations, necessitating a common representational system and concomitant mapping and control mechanisms.
• Architecture independence. Architectural characteristics (implementation language, user interface, and time flow mechanism) of existing simulations differed. The architecture implied by ALSP must be unobtrusive to existing architectures.

A conceptual framework is an organizing structure of concepts that facilitates simulation model development.[2] Common conceptual frameworks include: event scheduling, activity scanning and process interaction.

The ALSP conceptual framework is object-based where a model is composed of objects that are characterized by attributes to which values are assigned. Object classes are organized hierarchically in much the same manner as with object-oriented programming languages. ALSP supports a confederation of simulations that coordinate using a common model.

To design a mechanism that permits existing simulations to interact, two strategies are possible: (1) define an infrastructure that translates between the representations in each simulation, or (2) define a common representational scheme and require all simulations to map to that scheme.

The first strategy requires few perturbations to existing simulations; interaction is facilitated entirely through the interconnection infrastructure. However, this solution does not scale well. Because of an underlying requirement for scalability, the ALSP design adopted the second strategy. ALSP prescribes that each simulation maps between the representational scheme of the confederation and its own representational scheme. This mapping represents one of the three ways in which a simulation must be altered to participate in an ALSP confederation. The remaining modifications are:

• Recognizing that the simulation doesn't own all of the objects that it perceives.
• Modifying the simulation's internal time advance mechanism so that it works cooperatively with the other simulations within the confederation.

In stand-alone simulations, objects come into (and go out of) existence with the passage of simulation time and the disposition of these objects is solely the purview of the simulation. When acting within a confederation, the simulation-object relationship is more complicated.

The simulation-object ownership property is dynamic, i.e. during its lifetime an object may be owned by more than one simulation. In fact, for any value of simulation time, several simulations may own different attributes of a given object. By convention, a simulation owns an object if it owns the "identifying" attribute of the object. Owning an object's attribute means that a simulation is responsible for calculating and reporting changes to the value of the attribute. Objects not owned by a particular simulation but within the area of perception for the simulation are known as ghosts. Ghosts are local copies of objects owned by other simulations.

When a simulation creates an object, it reports this fact to the confederation to let other simulations create ghosts. Likewise, when a simulation deletes an object, it reports this fact to enable ghost deletion. Whenever a simulation takes an action between one of its objects and a ghost, the simulation must report this to the confederation. In the parlance of ALSP, this is an interaction. These fundamental concepts provide the basis for the remainder of the presentation. The term confederation model describes the object hierarchy, attributes and interactions supported by a confederation.

The object-based conceptual framework adopted by ALSP defines classes of information that must be distributed. The ALSP Infrastructure Software (AIS) provides data distribution and process coordination. Principal components of AIS are the ALSP Common Module (ACM) and the ALSP Broadcast Emulator (ABE).

If (an update passes all filter criteria)
|  If (the object is known to the simulation)
|  |  Send new attribute values to simulation
|  Else (object is unknown)
|  |  If (enough information is present to create a ghost)
|  |  |  Send a create message to the simulation
|  |  Else (not enough information is known)
|  |  |  Store information provided
|  |  |  Send a request to the confederation for missing data
Else (the update fails filter criteria)
|  If (the object is known to the simulation)
|  |  Send a delete message to the simulation
|  Else 
|  | Discard the update data

The ALSP communication scheme consists of (1) an inter-component communications model that defines the transport layer interface that connects ALSP components, (2) a layered protocol for simulation-to-simulation communication, object management, and time management, (3) a message filtering scheme to define the information of interest to a simulation, and (4) a mechanism for intelligent message distribution.

• Anita Adams, Gordon Miller, and David Seidel, November 1993, "Aggregate Level Simulation Protocol (ALSP) 1993 Confederation Annual Report", The MITRE Corporation. A history of the ALSP program in fiscal year 1993.
• William E. Babineau, Philip S. Barry, C. Zachary Furness, "Automated Testing within the Joint Training confederation (JTC)", Proceedings of the Fall 1998 Simulation Interoperability Workshop, Orlando, FL, September, 1998.
• MAJ John Bullington and Gordon Miller, September 1996, "Intelligence Simulation Support to the Joint Training Confederation: Implications for Future Development", TACSIM Project Office and The MITRE Corporation, published in the September 1996 edition of Phalanx, a MORS publication.
• Lydia P. Dubon, 1993, "Joining a Distributed Simulation Environment via ALSP," Proceedings of the 1993 Winter Simulation Conference.
• Laura Feinerman, Gordon Miller, David Prochnow, Richard Weatherly, Annette Wilson, and Anita Adams Zabek, "Aggregate Level Simulation Protocol (ALSP) Project 1994 Annual Report", dated March 1995, The MITRE Corporation. A history of the ALSP program in fiscal year 1994.
• Mary C. Fischer, April 1994, "Aggregate Level Simulation Protocol (ALSP) - Managing Confederation Development", U. S. Army Simulation, Training and Instrumentation Command. A paper presented on the 1994 Elecsim Internet Conference.
• Mary C. Fischer, Anita Adams, Gordon Miller, June 1994, "Aggregate Level Simulation Protocol (ALSP) - Training for the Future", U. S. Army Simulation, Training and Instrumentation Command and The MITRE Corporation. A paper presented at the Military Operations Research Symposium 62 meeting at the Air Force Academy in Colorado Springs, Colorado.
• Mary C. Fischer, December 1994, "Aggregate Level Simulation Protocol (ALSP) - Managing Confederation Development", U. S. Army Simulation, Training and Instrumentation Command. A paper presented at the 1994 Winter Simulation Conference in Orlando, Florida.
• Mary C. Fischer, April 1995, "Aggregate Level Simulation Protocol (ALSP) - Future Training with Distributed Interactive Simulations", U. S. Army Simulation, Training and Instrumentation Command. A paper presented at the 1995 International Training Equipment Conference on 25–27 April 1995 at The Hague in The Netherlands.
• Mary C. Fischer, September 1995, "Joint Simulated Battlefield", U. S. Army Simulation, Training, and Instrumentation Command, published in Proceedings of the 36th Defence Research Group (DRG) Seminar on Modeling and Simulation, 5–8 September 1995, Washington, D.C.
• Mary C. Fischer, October 1995, "Joint Simulated Battlefield Through Aggregate Level Simulation Protocol', U. S. Army Simulation, Training, and Instrumentation Command, published in Proceedings of the Southeastern Simulation Conference '95, Orlando, FL.
• Mary C. Fischer, March 1996, "Joint Training Confederation", U. S. Army Simulation, Training, and Instrumentation Command, published in Proceedings of the First International Simulation Technology and Training (SimTecT) Conference, 25–26 March 1996, Melbourne, Australia.
• Sean P. Griffin, Ernest H. Page, Zachary Furness, Mary C. Fischer, "Providing Uninterrupted Training to the Joint Training Confederation (JTC) During Transition to the High Level Architecture (HLA)",Proceedings of the 1997 Simulation Technology and Training (SimTecT) Conference, Canberra, Australia, 17–20 March 1997.
• George J. McFadden, "An Approach to Management of Enumerated Data in Federations", Proceedings of the Spring 2000 Simulation Interoperability Workshop, Orlando, FL, March, 2000.
• Gordon Miller and Anita Zabek, March 1996, "The Joint Training Confederation and the Aggregate Level Simulation Protocol", The MITRE Corporation, published in the June 1996 edition of Phalanx, a MORS publication.
• Ernest Page; Brad Canova; John Tufarolo (July 1997). "A Case Study of Verification, Validation and Accreditation for Advanced Distributed Simulation". ACM Transactions on Modeling and Computer Simulation. CiteSeerX 10.1.1.37.1829.
• David L. Prochnow; Ernest H. Page; Mary C. Fischer (3–7 March 1997). "Management of the Joint Training Confederation Family of Specifications". Proceedings of the Spring 1997 Simulation Interoperability Workshop. Orlando, FL. CiteSeerX 10.1.1.37.935.
• David L. Prochnow, Mary C. Fischer, "Unique Requirements for the Representation of Logistics in a Distributed Simulation Environment for Military Training", Proceedings of the Spring 1997 Simulation Interoperability Workshop, Orlando, FL, 3–7 March 1997.
• David L. Prochnow; Ernest H. Page; Bryan Youmans (9–13 March 1998). "Development of a Federation Management Tool: Implications for HLA". Proceedings of the Spring 1998 Simulation Interoperability Workshop. Orlando, FL. CiteSeerX 10.1.1.37.6101.
• David Seidel, March 1993, "Aggregate Level Simulation Protocol (ALSP) Program Status and History", The MITRE Corporation. A history of the ALSP program from its beginning in 1989 through 1992.
• John Tufarolo and Ernest Page, "Evolving the VV&A Process for the ALSP Joint Training Confederation", Proceedings for the 1996 Winter Simulation Conference, pp. 952–958, Coronado, CA, 8–11 December 1996.
• Richard Weatherly, David Seidel, and Jon Weissman, July 1991, "Aggregate Level Simulation Protocol", The MITRE Corporation. A paper presented at the 1991 Summer Computer Simulation Conference in Baltimore, Maryland
• Richard Weatherly, Annette Wilson, and Sean Griffin, December 1993, "ALSP - Theory, Experience, and Future Directions", The MITRE Corporation. A paper presented at the 1993 Winter Computer Simulation Conference in Los Angeles, California.
• Weatherly, Richard M.; Wilson, Annette L.; Canova, Bradford S.; Page, Ernest H.; Zabek, Anita A.; Fischer, Mary C. (1996). "Advanced distributed simulation through the Aggregate Level Simulation Protocol". Proceedings of HICSS-29: 29th Hawaii International Conference on System Sciences. pp. 407. CiteSeerX 10.1.1.37.4784. doi:10.1109/HICSS.1996.495488. ISBN 0-8186-7324-9.
• Annette Wilson and Richard Weatherly, April 1994, "New Traffic Reduction and Management Tools for ALSP Confederations", The MITRE Corporation. A paper presented on the 1994 Elecsim Internet Conference.
• Annette Wilson and Richard Weatherly, December 1994, "The Aggregate Level Simulation Protocol: An Evolving System", The MITRE Corporation. A paper presented at the 1994 Winter Simulation Conference in Orlando, Florida.