4.16 Elementary Method "Functional Decomposition" (FCTD)

#### 1 Identification/Definition of the Method

/Martin, 1987/ pp. 62-65; pp. 139-142

#### 2 Brief Characteristic of the Method

Objective and Purpose

The objective of the Functional Decomposition (FCTD) is to decompose a system step by step, beginning with the main function of a system and continuing with the interim levels down to the level of elementary functions. On each level abstractions are made from each corresponding lower level. All the subfunctions together form completely the decomposed function (functional hierarchy).

Means of Representation

Depending on the main focus of the application, different types of diagrams are preferably used to represent the functional hierarchy:

• ellipse diagram
• box diagram
• tree diagram
• bracket diagram

Operational Sequence

Based on a global system description, up to seven subfunctions should be generated from a function in every decomposition step. The principles of the functional decomposition has to be observed in every refinement step.

The refinement can be realized according to the decomposition features Object and Operation, and according to the decomposition types Decomposition and Classification.

#### 3 Limits of the Methods Application

- not applicable -

#### 4 Specification of the Methods Allocation

No. Activity Description
4.1 SD1.1 - Recording of Actual Status and Analysis A functional model of an already existing system is developed from a special user-oriented aspect.

Together with DFM - Data Flow Modeling, the method covers subproduct User Requirements.Actual Status and Current Analysis on the function side.

4.2 SD1.5 - User-Level System Structure A model-like hierarchical description of the total functionality of the system from a special user-oriented aspect is given.

The refinement ends after the functions on user-level can no longer be decomposed in a practical manner. These functions are referred to as elementary functions.

This method includes an overview for subproduct User Requirements.Description of the Functionality.

4.3 SD2.1 - Technical System Design The decomposition of the total functionality of the system into segments and/or SW Units will be described.

The static structure of the system is described for subproduct System Architecture.Solution Proposals.

4.4 SD3.3 - Definition of Requirements for the Functionality A model-like hierarchical description of the total functionality of the SW Unit from a technical aspect will be given.

The function overview for the subproduct Technical Requirements.Overall Function of Element is covered.

#### 5 Interfaces

No. Interface Observation Information in Annex 1
5.1 FCTD-DFM For the execution of the basic method Data Flow Modeling, the results of chapter Functional Structure are the input information for the definition of the functional level to be taken into consideration. 4.4 Interface DFM-FCTD
5.2 FCTD-ER The functional hierarchy in FCTD must be generated by interacting with the ER - E/R Modeling (allocation of E/R submodels to the functions in FCTD).

#### 6 Literature

/Balzert, 1977/ Systementwicklung, hierarchische Strukturen, Modularisierung, Datenabstraktion
/Martin, 1987/ Diagramming Standards for Analysts & Programmers
/Ploenzke, 1991/ Ploenzke Informatik

#### 7 Functional Tool Requirements

SSD05 - Supporting Function Structure

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