DATA FLOW DIAGRAMS - EXERCISE Withdrawal of Cash from an Automatic Teller Machine An ATM Withdrawal To start an operation, operation, a card is inserted in the slot, and the PIN (personal identification identification number) is entered. The ATM reads information from the card, in particular, the card number and the expiry date, and accepts the PIN from the keyboard input. The card is verified for expiry date, and then if valid, the card number and the PIN are sent to the bank for card validation by communication circuit. The bank either sends confirmation or refusal. If confirmation is received, the user is invited to enter a transaction type. Assuming that the transaction is a withdrawal of cash, one more validation is performed to check that the amount nominated nominated is below or equal to the daily limit for withdrawals of $1000, that the user’s weekly withdrawals do not exceed the weekly limit of $5000, and that the account balance is in credit to cover the withdrawal. If all these are OK the card is returned, the money is dispensed and the withdrawal receipt is printed, with the date, account number, withdrawal amount and account balance. The bank’s account records are updated to reflect the transaction. If the card has expired or is invalid, the card is retained and the user is invited to contact their bank. If wrong PIN is entered three (3) times the card is returned and the transaction transaction is cancelled. If the amount entered exceeds the user’s account balance, or the daily or weekly limits, the user is asked to change the amount up to three (3) times or cancel the transaction. If a valid transaction amount has not been entered by the third attempt, the c ard is returned and the transaction is cancelled.
For the educator This is a useful exercise because understand the process flow as they have all used an ATM machine. This makes the analysis easy and encourages discussion. The exercise highlights what DFDs are NOT designed to do. DFDs do not model the order or dependency of events. For example, from the user’s perspective the number of attempts needed to enter the correct PIN is critical, but there is no mechanism for this to be made apparent apparent via this kind of system model. The problem domain provides provides sufficient detail for the DFD to be developed to level 2. State any assumptions
It is important that all assumptions about the system are discussed and clarified. Without a clear definition of assumptions it will be impossible to evaluate the level of understanding that students have gained when looking at their solutions. With a relatively complex scenario such as this, it is sometimes valuable for students to produce first-cut versions singly or in pairs. Group discussion may then be used to define a common set of assumptions. Students should then try to complete their DFD on their own. When a process is complex (as in Level 3) it should be broken to the next level. It is important that the number of incoming and outgoing flows provided in the Level 1 diagram for process no.3 is exactly the same in the level 2 diagram for that process.
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SOLUTION Level 0 DFD (Context Diagram) – ATM Cash Withdrawal Invalid Card
Card Details
Invalid PIN User
ATM Withdrawal System
PIN
User verdra w O ve ils details
T ran ransaction Am Amount
Receipt
Level 1 DFD – ATM Cash Withdrawal Card Details Card Type Details
PIN
CARDS
1. VERIFY CARD
Invalid card
Invalid PIN
Transaction amount
Card status
Valid Card Details
2. VERIFY PIN
Card status 3. VALIDATE AMOUNT
ACCOUNTS count Ac Accou Balance
Overdraw details Validated Transaction Details
Weekly Balance Daily Balance
Receipt
Transac Transaction tion details etails
4. TRANSACT WITHDRAWAL
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Card num, PIN
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Level 2 DFD – ATM Cash Withdrawal (Validate Amount)
Transaction amount
Card status
Transaction details 3.1 ACCEPT AMOUNT
Daily limit exceeded
Overdraw details 3.4 NOTIFY INVALID AMOUNT
Weekly limit exceeded
Daily balance 3.2 CHECK DAILY LIMIT
ACCOUNTS
Daily total OK 3.3 CHECK WEEKLY LIMIT Weekly total OK
Insufficient funds
3.5 CHECK ACCOUNT BALANCE Validated transaction details
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Weekly balance
Account balance
