1. G045 Lecture 07 DFD Context Diagrams (Data Flow Diagrams Level 0)
Mr C Johnston
ICT Teacher

2. Session Objectives Understand what a context diagram shows,
Know the common components of a context diagram,
Be able to draw context diagrams for given scenarios,

3. Data Flow Diagrams
Data Flow Diagrams (DFDs) show how data flows into, around and out of a system,
They are made up of:
External data sources, people or organisations the system interacts with (eternal entities)
Flows of data,
Processes,
Data stores
Diagrams are spilt into different levels each painting a more detailed picture of what happens within a system
At level 0 they show the external data sources and how they interact with the system,
At higher levels they show the how the data is adapted by the system and where it is stored.

4. Context (Level 0) Diagrams
Show the external entities and how they interact with the system,
Diagram takes the format of a box called _____ system with the entities scattered around the system box,
Shape of the box will depend on the notation being used:
If a rectangle is used for the system then ellipses are used for the external entities
If an ellipse is used for the system then rectangles are used for the external entities
One way arrows enter and exit the system joining it with the external entities to show the data flows,
A label is then given to the arrow showing what data is flowing.

5. Examples Example context diagram using ellipse for system notation
Example context diagram using rectangle for system notation

6. Drawing Diagrams
Start by reading a scenario and summarising what is going on into a table
Draw the diagram using the information in the table
Its convention that arrows go down the diagram in order of what happens
Data Flow
External Entity
Source / Recipient

7. Example
Reporting day data is complied by a system at Stringer. Staff enter grades into the system for each student which includes their ID, a current working at grade and effort for a subject. The system stores them, compares with past data and generates a grade report for parents. Summary information is also generated for year heads showing subject grades for each student in the year and one for form tutors summarising their tutees performance.
Data Flow
External Entity
Source / Recipient
Student ID, working grade, effort
STAFF
Source
Grade Information
PARENTS
Recipient
Year group summary
YEAR HEAD
Tutor group summary
TUTORS

8. REPORTING DAY SYSTEM STAFF PARENTS STUDENT DB YEAR HEAD FORM TUTORS
Student ID,
working grade
and effort
REPORTING
DAY
SYSTEM
PARENTS
Grade Information
Student ID
Student Details
STUDENT DB
YEAR HEAD
Year group summary
grades
Tutor group summary
grades
FORM TUTORS

9. Exercises
An insurance company system takes details from customers which includes personal, car and past claim data. The information acquired is complied and pasted onto an external broker which generates quotes. On return from the broker the best quote is formatted appropriately and sent to the customer.
The university and college admission system (UCAS) handles applications to higher education by students in the UK. Students submit data to the system which includes personal details, estimated grades, courses and institutions they want to apply for and a personal statement. A tutor also submits a reference for each student into the system. The system compiles each students application and forwards it onto the institution applied for. Upon receipt the staff at the university scrutinise the application and return to UCAS an offer or rejection – this is then passed onto the student.

10. INSURANCE SYSTEM UCAS SYSTEM CUSTOMER BROKER TUTOR STUDENT INSTITUTION
Personal, car,
past claim data
Customer Information
INSURANCE
SYSTEM
Best Quote
Quotes
TUTOR
Personal, est grades,
courses and inst. data
and personal statement
UCAS
SYSTEM
Reference
STUDENT
Offer / Reject
Response To
Applicant
Applicant
Information
INSTITUTION

11. Further Exercise
For each of the further exercises listed below, read the case study and draw a level 0 (context) data flow diagram to show the main events in the current / proposed system.
Further Exercise 1
Further Exercise 2

12. Further Exercise 1 Answer

13. Further Exercise 2 Answer

14. Assignment Link
Draw a context diagram for KASBAH NEWS’ Current System.
Start by drawing a table to identify all the external entities and dataflows,
Then draw your diagram,
Include a description of each external entity –
Who / what are they
How do they interact with the system,
Why are they necessary,
What information do they provide / receive and in what format

15. External Entity Description