Question $6[19]$

$6\mathrm{.}1$ Consider the following use case narration which describes the scenario of a specially designed

telephone box used to take telephone calls using a credit card. Draw a sequence diagram to

represent the above scenario. $(19)$

Use Case Name: Take a telephone call using the credit card

Participant: User, Credit card processing centre

Entry Conditions: User has a valid credit card.

Credit balance should be sufficient for the

minimum amount required to take a phone call.

Exit Conditions: User hangs the receiver.

Credit card is returned to the user.

Flow of events:

$1.$ The user inserts the credit card.

$2.$ Telephone box sends the message to the card centre.

$3.$ Card centre processes the card.

$4.$ Card centre sends the available credit to the Telephone box.

$5.$ Telephone box processes the available balance.

$6.$ Telephone box shows a message for the user to pick up the receiver.

$7.$ User picks up the receiver.

$8.$ Telephone box displays the number pad to the user on the screen.

$9.$ User dials the number and makes the phone call.

$10.$ Telephone box processes the duration and the amount charged.

$11.$ User ends the phone call by pressing the $$END$$ button$/$hanging the receiver.

$12.$ Telephone box displays the total phone call duration and the amount charged.

$13.$ Telephone box sends the amount to the card centre.

$14.$ Card centre charges the amount from the credit card.

$15.$ Card centre sends the confirmation to the telephone box.

$16.$ Telephone box displays the message$$ THANK YOU, COME AGAIN$.$

$17.$ Telephone box ejects the credit card.

Exceptional conditions and alternative flow of events:

When the credit is not sufficient to make a call

$5\mathrm{.}1$ Display error message and Go to $17$

When the amount exceeds the credit card balance

$10\mathrm{.}1$ Ends the phone call and Go to $12$