Describing systems with DFD & Levelling DFDs

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Describing systems with DFD & Levelling DFDs

An entire system is represented by one DFD which gives the system’s overview . It is called a context diagram. It gives little detail & is also known as the top level DFD.

Context diagram of mess management is shown

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Note: This diagram gives very little detail

LEVELLING DFD

A context diagram gives an overview, it should be split into major processes which give greater detail. Each major process is further split to give more detail. Each major process is further split to give more detail

WHY LEVEL DFD?

If a DFD is too detailed it will have too many data flows and will be large and difficult to understand. Therefore start from a broad overview. Expand the details - Idea similar to using procedures and linking these with a main program. Each DFD must deal with one aspect of a big system

EXPANDED DFD FOR HOSTEL MESS MANAGEMENT

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LEVELLING RULES

•If process p is expanded, the process at the next level are labeled as p.1,p.2 etc.

•All data flow entering or leaving p must also enter or leave its expanded version.

• Expanded DFD may have data stores

• No new external entity can appear in expanded DFD

• Keep the number of processes at each level less than 7.

ILLEGAL CONSTRUCTS IN DFD

• No loops are allowed in DFD

• A process cannot be a pure decision

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• A single data flow should not be split into many flows with different labels

• No data flow allowed between data stores

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