Difference between Logical DFD and Physical DFD
So far we considered data flow diagrams as logical models. They specify the logical process preformed on the data, i.e. the type of operations performed. Beside that logical DFDs doesn’t specify who does the operation, whether it is done manually or by computer and also where it is done. But physical DFD specifies these. For example, in figure 1(a), a physical DFD specifies that a checking clerk receives the items from vendors, checks them and rejects an item if it is not as per order. Accepted items are sent to a stores clerk who updates the inventory file. Figure 1(b), a logical DFD shows the type of operation preformed. Although the DFD is Physical or Logical but symbols used in the DFD are same.
A physical DFD can be taken as the first stage for the development of logical DFD; it can be easily drawn while gathering the facts at the first step of SDLC. It can be verified by the user.
As an example, we can consider the development of a physical and logical DFD for the process of getting a cheque cashed in a bank.
Starting to build DFD is very simple if you start from Physical DFD and reach to Logical DFD.
A customer presents a cheque to a clerk. The clerk checks a ledger containing all account numbers and makes sure whether the account number in the cheque is valid, whether adequate balance is there in the account to pay the cheque, and whether the signature is authentic. After the positive report of the above operation, clerk gives the token to the customer. The clerk also debits customer’s account by the amount specified in the cheque. If the cash cannot be paid due to an error in the cheque, the cheque is returned. The token number is written on the top of the cheque, customer gives the cheque to the cashier, when the token number is called out by the cashier the customer goes to cashier with the token. The cashier checks token number, takes customer’s signature, pays cash, enters cash paid in a ledger called day book, and files the cheque.
To explain this operation, I have modeled two different DFDs, a physical and logical. The physical DFD for this operation is given in Figure in 2(a).
Same physical DFD is converted into logical DFD, by taking the functions performed at each step as same. In logical DFD each process has a well defined operation. Further, in this diagram we do not include details such as clerks/cashiers performing operations. Logical DFD for this operation is shown in Figure 2(b).