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2.1 Data Flow Diagrams:

P ALIGN="JUSTIFY"> Structure analysis is highly graphic, relying mainly on diagrams rather than narrative text. Its primary tool is the data flow diagram, a graphic representation of a systems component processes and the interfaces between them. Data flow diagrams shown how data flow to, from, and within an information system and the processes that transform the data. Data flow diagrams also show where the data are stored. Data flow diagrams are constructed using four basic symbols, which are illustrated in figure 2.1.

1.Data flow symbol, an arrow showing the flow of data.

Data flows show the movement of data between processes, external entities and data stores. They always contain packets of data with the name or content of each data flow listed beside the arrow. The flows are of known composition and represent data that are manual or automated. Data flows consist of documents, reports, data form a computer file. These can be either inputs or outputs.

2. The process symbol, rounded boxes or bubbles depicting processes that transform the data.

Processes portray the transformation of input data flows to output data flows. An example Is a process that transforms a sales order into an invoice or calculates an employees gross pay from his time card. The convention for naming a process consists of

combining a strong verb with an object.

3. The store symbol, an open rectangle indicating where data are stored.

Data store are either manual or automated inventories of data.They consist of computer files or database, files cabinets, card files, or a binder of paper reports.

4.The external symbol, either a rectangle or a square indicating the sources or destinations of data.

External entities are originators or receivers of information. They consist of customer, supplier, or government agencies, which are external to the organization. External entities are sometimes called outside interface because they are outside the boundry or scope of the system treated by the data flow diagram. Given belw is data flow diagram.

Data flow Diagrams





Data collected from Data Flow Diagram of Foreign Student Department

Designation

Data Group Name

Data Units use in system

D1

Request for teacher general information

First_name, Last_name

Q1(D1)

Give teacher information from teachers file

First_name, Last_name

A1(Q1)

Answer to query Q1(D1)

First_name, Last_name, Position, Experience, Address, Telephone, Salary, Notes

D2

Requst for student general information

First_name, Last_name,

Q2(D2)

Give student general information

First_name, Last_name

A2(Q2)

Answer of query Q2(D2)

First_name, Last_name, Date of birth, passport Number, Country, City,Permanent Address, Present Address, Fees, Paid, Remaining

D3

Request for student test information

ID

Q3(D3)

Give student information

First_name, Last_name

A3(Q3)

Answer of query Q3(D3)

Fist_name, Last_name, Registration No., Group No.,Specilization

Q4(A3)

Request for student test info

First_name, Last_name

A4(Q4)

Answer of query Q4(A3)

Test ID, Course name, Marks, Type, Date

D4

Request for teaching staff Salary for one month

ID

Q5(D4)

Give information about faculty

ID

A5(Q5)

Answer to query Q5(A4)

Faculty name

Q6(A5)

Give teacher information according to there faculties

ID

A6(Q6)

Answer to query Q6(A5)

First_name, Last_name, Salary, position, Tax

A20

Calculate the Salary, taxes of all teaching staff

Total Salary, Total Tax

D5

Request for the sum of fees and taxes on it

ID

Q7(D5)

Give faculty information

ID

A7(Q7)

Answer to query Q7(D5)

Faculty name

Q8(A7)

Give department information

ID

A8(Q8)

Answer to query Q8(A7)

Department name

Q9(A8)

Give group detail

ID

A9(Q9)

Answer to query Q9(A8)

Group No, Year, Specilization

Q10(A9)

Give information about student

ID

A10(Q10)

Answer to query Q10(A9)

First_name, Last_name, Fees, Tax

A21

Calculate the fees and taxes on it and give sum

Total_fees, Total_taxes

D6

Request for faculty information

Name

Q11(D6)

Give detais of faculty

Name

A11(Q11)

Answer to query Q11(D6)

Name, Telephone, E_mail, Address

D7

Request for registration in University

First_name, Last_name, Passport No., Date of birth, Degrees

Q12(D7)

Verify availability

ID

A12(Q12)

Answer of query Q12(D7)

Group No., Total places, available

A13

Send information not registered

Report

A14

Send information places are available

Registration No,

Q13(A14)

Enroll student and put information in group, student

Registration No, First_name, Last_name,

A15(Q13)

Answer to query Q13(A14)

Registration No, First_name, Last_name, Group No., Year, Speciality, Department name,

D8

Book one place in hostel

First_name, Last_name, Registration No.,

Q14(D8)

Verify availability

Hostel No.

A16(Q14)

Answer to query Q14(D8)

Hostel No., Total places, avaliable

A18

Information about place is avaliable

Hostel No.

Q15(A18)

Book one place in hostel

Hostel No.,

A19(Q15)

Answer to query Q15(A18)

Hostel No., Room No, Payment ,Type, Address, Telephone

A17

No place avaliable

Report

Table 2.1 Data obtained from the data flow diagram

Representing Systems With Data Flow Diagrams

by Kenneth A. Kozar

Spring 1997


DATA FLOW DIAGRAMS

Data flow diagrams are a network representation of a system. They are the cornerstone for structured systems analysis and design. The diagrams use four symbols to represent any system at any level of detail. The four entities that must be represented are:
  • data flows - movement of data in the system
  • data stores - data repositories for data that is not moving
  • processes - transforms of incoming data flow(s) to outgoing data flow(s)
  • external entities - sources or destinations outside the specified system boundary

Data flow diagrams do not show decisions or timing of events. Their function is to illustrate data sources, destinations, flows, stores, and transformations. The capabilities of data flow diagramming align directly with general definitions of systems. Data flow diagrams are an implementation of a method for representing systems concepts including boundaries, input/outputs, processes/subprocesses, etc.

The data flow diagram is analogous to a road map. It is a network model of all possibilities with different detail shown on different hierarchical levels. The process of representing different detail levels is called "leveling" or "partitioning" by some data flow diagram advocates.

Like a road map, there is no start or stop point, no time or timing, or steps to get somewhere. We just know that the data path must exist because at some point it will be needed. A road map shows all existing or planned roads because the roads are needed. If we were to add usage patterns or great detail to the road network on a state road map, it would become unusable to plan routes. When we look at the road map of Illinois, we know we could find more detail of the Chicago vicinity, and then even find more detail of the street network within the Chicago vicinity.

See the data flow diagram of the systems development process. Note that the diagram is a detailing of the context diagram shown earlier. The data flows to and from the external entities shown on the context diagram all appear on the data flow diagram. Note that only four symbols are used.

Actual procedures, such as those to accomplish a specific task, would be detailed in a procedure specification. Procedures will present not only data manipulation, but control, such as deciding which path to take in performing a procedure. Data flow diagrams present all possible paths. Control, or how to select a specific path, is not shown on the data flow diagram. Procedures would be analogous to a "Trip-Tik" that the American Automobile Association uses for getting from a specific location to a specific destination. The "Trip-Tik" would have detailed maps if detail was needed to avoid getting lost.

Repetition, how many times a procedure is performed or when it is performed, is not included on the data flow diagram. Remember that the data flow diagram only presents a network of paths. It presents the network at a degree of detail that is understandable and usable for a select set of users. Different views at different levels of the same system are needed for users at different levels of authority and responsibility to understand and review their relevant system portion.

Detail that is not shown on the different levels of the data flow diagram such as volumes, timing, frequency, etc. is shown on supplementary diagrams or in the data dictionary. For example, data store contents may be shown in the data dictionary.

 

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