Software Engineering

11.2.2 
Identify functional sequences
Each of the functional scenarios must be expressed in terms of the sequence of 
functional actions that are necessary to accomplish the scenario. The functional 
sequence is represented by a functional flow block diagram (FFBD). If further 
situations warrant the identification of alternative functional solutions, then the 
sequence should branch into multiple sequences. The control rules and criteria for 
each alternative branch must be specified.
The FFBD notation is a proven technique for depicting a sequence of func-
tional actions. However, it may need to be enhanced to permit the expression of 
control rules and alternative branching within the sequence of functions. It is neces-
sary to express the control decision logic and branch selection criteria as an integral 
description of the functional flow sequence diagram. An example of a simple FFBD 
is presented in 
Figure 11.2
 to depict the decomposition of the process application 
operational activity identified in Figure 8.3 in Chapter 8.

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