acsc 155 system analysis and design 3 process modelling
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ACSC 155 System Analysis and Design 3. Process Modelling. Process Modelling. A technique used to organize and document the system’s processes Decomposition Diagram Data Flow Diagrams (DFDs). Decomposition Diagram. A tool used to depict the breaking of a system into subcomponents.

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process modelling
Process Modelling
  • A technique used to organize and document the system’s processes
  • Decomposition Diagram
  • Data Flow Diagrams (DFDs)

Dr. Stephania Loizidou Himona

decomposition diagram
Decomposition Diagram
  • A tool used to depict the breaking of a system into subcomponents.
  • A technique that helps the analyst visualize and define hierarchies of objects (a hierarchical tree structure). The "thing" represented by the parent object in the diagram is composed of the "things" represented by the child objects.

Dr. Stephania Loizidou Himona

Decomposition diagrams can be used to show hierarchies of processes, functions, requirements, goals, entities, organizations, problems, families or virtually anything else.
  • They are a very powerful and general tool that allows us to partition large, complex groups of things into smaller, more manageable groups of things.

Dr. Stephania Loizidou Himona

The first thing you need to create a decomposition diagram is a root.

The root generally represents a complex abstraction of some kind.

Then, that root is decomposed into one or more children. The children, when taken as a whole, make up the entire root.

Each child is then decomposed into its children.

The process continues until the diagram shows the desired level of detail.

Dr. Stephania Loizidou Himona



Dr. Stephania Loizidou Himona

another example
Another example

Dr. Stephania Loizidou Himona

data flow diagrams dfds
Data Flow Diagrams (DFDs)
  • DFDs show how data flows around an information system.
  • They are a simple and powerful graphic technique which is both easily updated and easily understood by users.
  • One of the main diagrammatic techniques of SSADM (Structured System Analysis and Design Methodology).

Dr. Stephania Loizidou Himona

  • A hierarchical set of diagrams which is used to define:

- the boundary of the system to be developed

- the information flow to and from the system

- data flows within the system

- the functions used by the system

Dr. Stephania Loizidou Himona

dfds consists of the following symbols
DFDs consists of the following symbols
  • Process

Shows a transformation of data, also referred to as a function

n is the number of the process, indicates the level of the process

PN is the process name

Dr. Stephania Loizidou Himona



  • Data Flow

Data flow/Physical flow of data

  • External Entity

Source and/or destination of data (person, organisation unit, system or another organisation interacting with the system).

Dr. Stephania Loizidou Himona



  • Data Store

Storage of data

DSn – Data Store number

DSName – Data Store name

  • Plus, some additional symbols that show optionality and repetitions of various symbols

Dr. Stephania Loizidou Himona


P. – process

D.S. – Data Store

E.E. – External Entity

Plus, additionally, each data store must have at least one input flow and one output flow (read and write)

Dr. Stephania Loizidou Himona

How is DFD developed?
  • Identify inputs & outputs
  • Label all data flows
  • Label all processes
  • Identify data stores
  • Label all External Entities
  • Start again (be prepared)

Dr. Stephania Loizidou Himona

Data Flows between
  • External entity and Process
  • Data Store and Process
  • Process and Process

Note: Information (Data) held for any amount of time between processes is called a DataStore.

Dr. Stephania Loizidou Himona


DFD Level 0 (Context Diagram)

Dr. Stephania Loizidou Himona

level 1 dfd general case
Level 1 DFD (General Case)

Dr. Stephania Loizidou Himona

data flow diagrams numbering rules
Data Flow Diagrams – Numbering Rules

Dr. Stephania Loizidou Himona

level 1 dfd example part
Level 1 DFD (example part)

Dr. Stephania Loizidou Himona

level 2 dfd
Level 2 DFD

Each Process box in the Top Level diagram will itself be made up of a number of processes, and will need to be decomposed as a second level diagram.

Basically, one for each process.

Dr. Stephania Loizidou Himona

Each box in a diagram has an identification number derived from the parent -  in the top left corner. (The Context level is seen as box 0).
  • Any box in the second level decomposition may be decomposed to a third and then a fourth level. (Very complex systems may possibly require decomposition of some boxes to further levels).
  • Decomposition stopswhen a process box can be described with an Elementary Process Description using ordinary English. (Later on the process will be described  more formally  as a Function Description using, for example, pseudo code).

Dr. Stephania Loizidou Himona

  • A DFD is also known as a Process Model.
  • Process Modelling is an analysis technique used to capture the flow of inputs through a system (or group of processes) to their resulting output).
  • The model is fairly simple in that there are only four types of symbols – process, dataflow, external entity and datastore.

Dr. Stephania Loizidou Himona

Process Modelling is used to visually represent what a system is doing. It is much easier to look at a picture and understand the essence than to read through verbiage describing the activities.
  • System Analysts after talking with various users will create DFD diagrams and then show them to users to verify that their understanding is correct.
  • The process models can be created to represent an existing system as well as a proposed system.

Dr. Stephania Loizidou Himona

Process – an activity or a function that is performed for some specific reason; can be manual or computerised; ultimately each process should perform only one activity
  • Data Flow – single piece of data or logical collection of information like a bill
  • Data Store – collection of data that is permanently stored
  • External Entity – a person, organisation, or system that is external to the system but interacts with it

Dr. Stephania Loizidou Himona

Be aware of the basic rules for Process Modelling:

- A series of data flows always starts or ends at an external agent and starts or ends at a data store. Conversely, this means that a series of data flows cannot start or end at a process

- A process must have both data inflows and outflows

- All data flow must be labelled with the precise data that is being exchanged

- Process names should start with a verb and end with a noun

- Data flows are named with descriptive nouns

Dr. Stephania Loizidou Himona

rules continued
Rules - continued

- A data store must have at least one data inflow

- A data store cannot go between an external agent and a data store, but a process must be in between

- A data flow cannot go between two external entities, but a process must be in between

- A data flow cannot go between two data stores, but a process must be in between

- External agents and data flows can be repeated on a process model in order to avoid lines crossing but do not repeat processes

Dr. Stephania Loizidou Himona