At the core of every AviationSimNet simulation is the SimCenter, a set of services listening for network connections from participating applications.  There’s one service for data communications, and another for voice communications.  All of the network connections use TCP/IP, a common networking protocol for communicating over the Internet and other local networks. The SimCenter acts as a network hub through which all messages are forwarded. Organizations may need to configure their corporate firewalls to allow outbound connections to specific TCP/IP ports on the SimCenter.  Even though connections are only established by participants to the SimCenter, messages are exchanged in both directions over those connections.  The SimCenter can be hosted on any computer, provided all of the expected organizations have network access to it. AviationSimNet participants never need to allow inbound connections to their network, nor do they establish any network connections directly with each other.

Simulation control and other message events are handled through HLA (High Level Architecture), an IEEE standard for executing distributed simulations over a computer network.  HLA provides simulation services such as time management, data publication and subscription, and object ownership. The types of data shared among participants are described in the Federation Object Model (FOM).  A  Runtime Infrastructure (RTI) is the middleware that implements the HLA standard as software libraries. AviationSimNet uses a commercial RTI that supports both Java and C++ platforms.  The AviationSimNet FOM contains the necessary objects and messages required for conducting many Air Traffic Management studies. The FOM is authored and extended as needed by the AviationSimNet community.

AviationSimNet organizations might already have some communications protocol for executing simulations inside their labs.  Gateway applications are typically put in place to bridge that internal protocol with the AviationSimNet federation. Each gateway is unique, and represents that organization’s handle for participating in AviationSimNet simulations.

Voice communications are conducted as simulated radio frequency transmissions via the Distributed Interactive Simulation (DIS) protocol, another IEEE standard.  DIS has specific Protocol Data Units (PDU) representing radios with frequencies, like those used by controllers and pilots in ATM.  Although DIS is locally transmitted over a UDP/IP network, the stream is converted to TCP to enable wide-area networking as described above.

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