Policella, Joseph; White, Joey; Shillington, Keith; CAE-Link Corporation; Fastrak Training Inc. (International Foundation for Telemetering, 1993-10)
      To model the "black boxes" in a command and telemetry simulation, it is important to preserve the abstraction of a one-to-one match between the real-world interfaces and the simulated interfaces. Everywhere a physical interface exists on the box, there needs to be a simulated interface. Preserving this abstraction allows the model to evolve more naturally with real-world design changes. In most command and telemetry systems, many different types of commands and telemetry can be sent over a single interface. This creates a problem in preserving the interface abstraction if the Ada language is used for implementation. Due to the fact that Ada is a "strongly typed" language, a different or overloaded operation needs to exist for each type of command or telemetry. However, by using a "discriminated variant record" to represent the commands and telemetry streams, a single operation can be used in the Ada specification. This not only preserves the abstraction but makes the software more maintainable by allowing the data list to change during the design of the "black box" without changing the Ada specification. As a result, "loose coupling" is achieved, a common set of commands and telemetry formats can be "inherited" to promote reuse, and overall system development and maintenance costs are reduced.