Two complementary functional aspects of the services provided by the system are behavior and communication. Behavior concerns the ordering of interactions in time; communication concerns the information exchange between systems during interactions. This part of the book describes communication notations.

• Chapter 15. Data flow diagrams (DFDs) are the flagship notation of Yourdon-style structured analysis. DFDs represent the system as a collection of communicating data stores and processes.

• Chapter 16. DFDs are generalized to communication diagrams by adding a few notational conventions. These diagrams are also related to a systems engineering approach that traces system properties to component properties that realize them.

• Chapter 17. I discuss the semantic options we have in formalizing communication diagrams (including DFDs). This chapter can be skipped if you do not want to make the meaning of communication descriptions precise.

• Chapter 18. Communication diagrams can be used to represent the communication structure of the environment of the system. The resulting diagram is called a context diagram. Structuring the context is modeling, not design. Here I give guidelines for context modeling and relate this to classes of problems we might want to solve with the SuD.

• Chapter 19. Communication diagrams can also be used to represent communication structures inside the system. Here I present guidelines for defining the requirements-level architecture of a system, which is the architecture it would have if perfect implementation technology...