18.2 System Structure and Topology
This section describes ways of graphically showing the system structure and topology.  This section should be considered as informational rather than normative.
An AADL model represents a system as a set of component type and implementation declarations.  These declarations may be organized into packages and may have extends relationships.   This organizational structure can be shown in a library view.  This library view may show the nesting of package names, the content of packages as component types and component implementations, and the extends and implements relationship using the symbols shown in Figure 21.  The nesting of package names may be simply reflected in the package name label, or the packages themselves may be shown as a hierarchy based in the name nesting.   This hierarchy may be shown as a navigational tree (familiar to many through navigational browsers) or as structures similar to those shown for the instance hierarchy in Figure 30.  Packages are shown using the folder as graphical symbol. 
Actual systems have an instance hierarchy.  This system instance hierarchy is implicitly represented in the set of component types and component implementations with one system implementation identified as the root of the system instance.  The subcomponents of this root system implementation represent the components making up the system, and the subcomponents contained in the component implementations of these subcomponents recursively identify the nested component instances.
The system instance hierarchy may be shown as a navigational tree or as graphical structures as shown in Figure 30.  On the left the system hierarchy is shown in tree form with the containment association showing the containment relationship. On the right the system hierarchy is shown as a nesting of the graphical symbols to reflect the containment.  Both representations are permissible. 
graphic
Figure 30 Two Graphical Representations of System Instance Hierarchy
In many cases navigational tree views for showing hierarchical structures are combined with a second panel showing the content of the selected item in the navigational tree.  In case of the AADL, that panel may graphically show the content of a selected component implementation in the library view or of a selected component instance in the system instance view.  In this graphical view the connection topology among the subcomponents and between subcomponents and the features of the component containing them may be shown.