... of this model is to provide a framework for the coordination of standards development and allows both existing and evolving standards activities to be set within that common framework. ... hierarchy of seven layers. Each layer has a defined purpose and interfaces with the layers above it and below it. By laying down standards for each layer, some flexibility is allowed so that ... specifications and guidelines, which are ‘open’ to all. This allows equipment from any manufacturer, who claims to comply with that standard, to be used interchangeably on the standard network....