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Tuesday, March 1, 2011

6.4 Functional Analysis and Allocation

Figure 6-5, the list of 15 tasks in the post of December 29 titled Requirements Analysis, shows that Functional Analysis and Allocation is necessary to accomplish subtask 10, Define Functional Requirements. Functional analysis decomposes each of the high level functions of a system identified in requirements analysis into sets of lower level functions. The performance requirements and any constraints associated with the high level functions are then allocated to these lower level functions. Thus the top level requirements are flowed down to lower levels requirements via functions. This decomposition and allocation process is repeated for each level of the system. The objective is to define the functional, performance and interface design requirements. The result is called the Functional architecture and the collection of documents and diagrams developed is the Functional view.  
A function is an action necessary to perform a task to fulfill a requirement. Functions have inputs and outputs and the actions are to transform the inputs to the outputs. Functions do not occupy volume, have mass, nor are they physically visible. Functions are defined by action verbs followed by nouns; for example, condition power or convert frequency. A complex system has a hierarchy of functions such that higher level functions can be decomposed into sets of lower level functions just a physical system can be decomposed into lower level physical elements. A higher level function is accomplished by performing a sequence of sub-functions. Criteria for defining functions include having simple interfaces, having a single function each, i.e. one verb and one noun; having independent operations and transparency, i.e. each does not need to know the internal conditions of the others. There are both explicit and implicit or derived functions. Explicit functions are those that are specified or decomposed from specified functions or performance requirements. Implicit or derived functions are those necessary to meet other specified capabilities or constraints.
Sometimes inexperienced engineers ask why they have to decompose and allocate  functions for design elements at the subsystem or lower levels; they believe once they know the top level function, the performance requirements and any constraints defined in the requirements analysis task they can design the hardware and software without formal functional analysis/allocation. One simple answer is that items like time budgets and software lines of code are not definable from hardware or software; they are defined from the decomposed functions and sequences of functions. This is because hardware or software does not have time dimensions, only the functions the hardware or software performs have time attributes. Similarly the question of how many lines of code or bits of memory only have meaning in terms of the functions the software code is executing. Other reasons become more apparent as we describe functional design and design synthesis.
A diagram, simplified from Figure 6-4 and shown in Figure 6-19, helps understand both the question asked by inexperienced engineers and the answer to their question.

Figure 6-19 Developing a hardware/software system design is iterative and has multiple paths.

Figure 6-19 illustrates that although the primary design path is ultimately from requirements to hardware/software design; primary because when the design is complete the resulting hardware/software design fulfills the requirements, other paths are necessary and iteration on these paths is necessary to achieve a balanced design. The paths between requirements and functional design and between functional design and hardware/software design are necessary to:
  • Validate functional behavior
  • Plan modeling and simulation
  • Optimize physical partitioning
  • Facilitate specification writing
  • Facilitate failure analysis
  • Facilitate cost analysis and design to cost efforts
  • Facilitate concept selection
  • Define software budgets and CON-OPS timelines.
Although the path from requirements analysis to design synthesis isn’t formally shown in Figure 6-4 it is used when a team elects to design a product around a particular part, such as a state of the art digital processor or new and novel signal processing integrated circuit chip. However, having preselected a part doesn’t negate the need to define the functions performed by the part and verify that the performance requirements and interfaces are satisfied.
6.4.1 Decompose to Lower-level Functions – Decomposition is accomplished by first arranging the top level functions in a logical sequence and then decomposing each top level function into the logical sequence of lower level functions necessary to accomplish the top level functions. Sometimes there is more than one “logical” sequence. It is important to examine the decomposed functions and group or partition them in groups that are related logically. This makes the task of allocating functions to physical elements easier and leads to a better design, as will be explained in a later section. When more than one grouping is logical then trade studies are needed. Although the intent is not to allocate functions to physical entities at this point functions should not be grouped together if they obviously belong to very different physical elements. The initial grouping should be revisited during the later task of allocating functions to physical elements and this process is described in more detail in a following section.
The DoD SEF has an excellent description of functional analysis/allocation and defines tools used to include Functional Flow Block Diagrams (FFBD), Time Line Analysis, and the Requirements Allocation Sheet. N-Squared diagrams may be used to define interfaces and their relationships. Spreadsheets are also useful tools and are used later in various matrices developed for validation and verification. A spreadsheet is the preferred tool for developing a functions dictionary containing the function number, the function name (verb plus noun) and the detailed definition of each function and its sub functions. The list of function numbers and names in the first two columns of the functions dictionary can be copied into new sheets for the matrices to be developed in the design synthesis task.
Spreadsheets do not lend themselves to identifying the internal and external interfaces as well as FFBDs so the FFBD is the preferred tool for decomposition. Time Line Analysis and the Requirements Allocation Sheet are well described in Supplement 5 of the DoD SEF and need no further discussion here. Similarly the Integration Definition for Function Modeling (IDEFO) is a process-oriented model for showing data flow, system control, and the functional flow of life cycle processes. It is also well described in Supplement 5 of the DoD SEF. The collection of documents, diagrams and models developed using all of the tools is the Functional view. The functional architecture is the FFBDs and time line analyzes that describe the system in terms of functions and performance parameters.
Although the FFBD is discussed in the DoD SEF there are some additional important aspects of this tool that are covered in the next posting.

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