In our course, we need to understand and be able to draw 2 types of Data Flow Diagrams, they are Context Level DFD’s and Level 1 DFD’s. In this blog, I will hopefully make it easier to understand the differences between the two types of DFD’s and help understand how to draw a DFD for the exam. Firstly I will look at Context level DFD’s and give an example. Then I will look at Level 1 DFD’s and give an example.
Context Level DFD’s
A context level DFD is the most basic form of DFD. It aims to show how the entire system works at a glance. There is only one process in the system and all the data flows either into or out of this process. Context level DFD’s demonstrates the interactions between the process and external entities. They do not contain Data Stores.
When drawing Context Level DFD’s, we must first identify the process, all the external entities and all the data flows. We must also state any assumptions we make about the system. It is advised that we draw the process in the middle of the page. We then draw our external entities in the corners and finally connect our entities to our process with the data flows.
Below is an example of a Context Level DFD.
Level 1 DFD’s
Level 1 DFD’s aim to give an overview of the full system. They look at the sytem in more detail. Major processes are broken down into sub-processes. Level 1 DFD’s also indentifies data stores that are used by the major processes.
When constructing a Level 1 DFD, we must start by examining the Context Level DFD. We must break up the single process into its sub-processes. We must then pick out the data stores from the text we are given and include them in our DFD. Like the Context Level DFD’s, all entities, data stores and processes must be labelled. We must also state any assumptions made from the text.
Below is an example of a Level 1 DFD.
Hopefully this blog has helped to understand the difference between Context Level and Level 1 DFD’s and made it easier to draw them. Please leave any questions or comments below.
Cathal Doyle Lecture 18.