Answer all parts of this question.

Read the following case study.

A mobile telephone company is constructing a billing and invoicing system.  To use the phone system, individuals or companies must have a subscriber account which contains their subscriber ID, name, billing address, current balance and credit limit.  Subscribers are divided into business subscribers and personal subscribers and each can make payments by direct debit or cheque.  In the case of business subscribers who have a high average usage, the phone company can designate them as ‘platinum’ subscribers.  Platinum subscribes receive a 5% discount on all calls, although they may only make payments by direct debit.

 All payments made by a subscriber appears on a monthly statement, with each statement containing the month and year to which it relates.  Statements also contain the various charges made to a subscriber.  These charge items can be either for a phone call or an additional service.  For phone calls, the phone number dialled, call duration and charge are shown, whilst for a service, the service name and charge is shown.  All statement entries contain the date on which the charge or payment was recorded.

For each subscriber, there are a number of users for whom the subscriber is responsible.  Each user has a user ID.  Each user can have one or more mobile phone handsets, each of which has a handset ID. 

When a handset is instructed to make a call, it must first check that the subscriber has sufficient credit.  If there is insufficient credit, a message is displayed on the handset indicating that there is insufficient credit.  If the subscriber’s credit is OK and a call was successfully completed, a phone call record is created.  When a record of the call has been created, the cost of the call is added to the subscriber’s current balance.

End of case study

a.  Draw a class model for the above case study, showing only those attributes and operations necessary to implement the case study as described.  Addition of unnecessary classes, attributes or operations will result in the loss of marks.  All association roles should be named and the cardinality clearly shown.

 (30 marks)

b.   Draw a sequence diagram, including appropriate boundary and control objects, to show the messages resulting from a user wishing to make call from their handset.  Messages should be appropriately named and parameters shown.

(10 marks)

c.   Explain the term message passthrough, including the circumstances under which it is a good thing and the circumstances under which it is a bad thing.  Also indicate any message passthrough which occurs in the model you produced for part a) of this question  and indicate why it is appropriate.

(10 marks)

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