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One Model, Many Interests, Many Views
Nodes on a BDD represent elements (blocks). Nodes always include the element name and frequently
include additional information to emphasize design specifics:
• Operations – behavioral aspects allocated to the block. Operations describe
synchronous interactions where the requester waits for the request to be handled.
Operations reflect a subset of the allocated functions.
• Receptions – behavioral aspects allocated to the block. Receptions describe
asynchronous behaviors where the requestor can continue without waiting for a reply.
• Values – represent quantifiable characteristics of a block such as physical and
performance characteristics – weight, reliability, etc.
• Parts – are the hierarchical composition of the block (the children). This is classically
shown through connecting lines to lower-level blocks, but can be collapsed into the
body of the node and shown textually.
The lines on a BDD can reflect either a part-child relationship (in the direction of the arrow) or
a generalization / specialization relationship (per UML/SysML standards). When representing
decomposition in a part-child relationship, a filled diamond at the connection point with a parent
reflects the concept of composition (if the parent is destroyed, the part is destroyed as well). An open
diamond reflects the concept of reference (if the parent is destroyed, the child still exists). At the point
of connection to the child node, an optional label can be displayed, indicating the role the child plays
in the part. Likewise, multiplicity can be shown to indicate the part-child cardinality (the number of
elements).
Block definition diagrams can be considered more technical variants of a physical hierarchy diagram.
The diagram certainly has more breadth and depth than a classic hierarchy, and this mental model
leads to the following rule of thumb when considering its use. The greater technical content of the BDD,
including classification, block roles, and multiplicity, make the BDD an ideal replacement for the physical
hierarchy when dealing with systems engineers, software engineers, and subject matter experts who
crave the detailed, multi-level representation of system composition. For a more general audience, the
classic hierarchy diagram conveys the critical composition aspects in a satisfactory manner for their
needs and interests.
Interface and Physical Block Diagrams
Interface and physical block diagrams are traditional systems engineering box-and-line wiring diagrams
representing the logical interfaces and physical connections between components within a system or
system segment. The interface block diagram is often the first architectural block diagram that you
will develop, focusing first on the fact that logically, A must interface with B before crossing into the
details of how that connection is made. At higher levels, these block diagrams often include conceptual
communication graphics to enhance communication, leading to the name “architectural cartoons” or
“architoons.” At lower levels in the system hierarchy, graphics give way to boxes and lines, resulting in
a classic “system schematic.”