TEXTUAL REQUIREMENTS

The task is to design the software to support a computerized access control 
system.  The purpose of the system is to control the access
rights to an <SDT_LINK_ENDPOINT_BEGIN 17>office<SDT_LINK_ENDPOINT_END>.

An <SDT_LINK_ENDPOINT_BEGIN 18>entrance<SDT_LINK_ENDPOINT_END> leading to an office can have four different <SDT_LINK_ENDPOINT_BEGIN 33>securitylevel<SDT_LINK_ENDPOINT_END>s:
1. always unlocked 
2. requires a <SDT_LINK_ENDPOINT_BEGIN 20>card<SDT_LINK_ENDPOINT_END> to unlock
3. requires a card as well as a <SDT_LINK_ENDPOINT_BEGIN 21>code<SDT_LINK_ENDPOINT_END> to unlock
4. always locked

The security levels of an office entrance can be altered during the day.

Each <SDT_LINK_ENDPOINT_BEGIN 22>employee<SDT_LINK_ENDPOINT_END> working in the office has a card with a personal
code consisting of four digits. To enter through a <SDT_LINK_ENDPOINT_BEGIN 23>door<SDT_LINK_ENDPOINT_END> with security level 
three, the employee enters her card into a <SDT_LINK_ENDPOINT_BEGIN 24>cardreader<SDT_LINK_ENDPOINT_END> and types 
her personal code on a <SDT_LINK_ENDPOINT_BEGIN 25>keypad<SDT_LINK_ENDPOINT_END>. The time between consecutive keystrokes 
when typing the code is not allowed to exceed three seconds.To enter through 
a door with security level two the employee just enters her card into 
a cardreader. 

Each entrance leading into the office provides its own door as well as a
cardreader, keypad 
and <SDT_LINK_ENDPOINT_BEGIN 26>display<SDT_LINK_ENDPOINT_END> on the outside, and an <SDT_LINK_ENDPOINT_BEGIN 27>exitbutton<SDT_LINK_ENDPOINT_END> on the inside. The 
employee only needs the card and code to *enter* the office. To exit, 
the employee just presses the exit button and the door is unlocked for 
ten seconds.

All entrances communicate directly with a <SDT_LINK_ENDPOINT_BEGIN 28>centralcontrol<SDT_LINK_ENDPOINT_END>ler which
makes sure that a validation of the correctness of cards and codes is 
performed. The controller has
access to a <SDT_LINK_ENDPOINT_BEGIN 29>database<SDT_LINK_ENDPOINT_END> consisting of all cardnumbers and their corresponding
personal codes. If the card is valid and, in case of security level 
three, the corresponding code correct, the door is unlocked for ten 
seconds and the employee may enter. In case of an invalid or unregistered 
card, access to the office is not allowed. In case of an incorrect code, 
the employee 
is informed of this and must try again by entering the card into the 
cardreader and retyping the personal code.

The access control system must read its data, consisting of
cardnumbers with their corresponding personal code, from a 
database. The database is managed by using a
separate <SDT_LINK_ENDPOINT_BEGIN 30>managementsystem<SDT_LINK_ENDPOINT_END> that is not developed within the project.
The <SDT_LINK_ENDPOINT_BEGIN 31>systemoperator<SDT_LINK_ENDPOINT_END>, who is running the management system, is
authorized to register new employees, cards and codes, to change a
code if the employee wishes so, to delete employees from the
database if they are quitting and to change the	security level of a door. 
The system operator is also responsible for
initializing the access control system. All the actions mentioned above are
done using the management system.

The system must be able to recover from computer and connection
failures. If a connection between an entrance and the central controller
is lost, the door is locked from the outside not permitting anyone to
enter (i.e.security level four is set). It is, however, possible to open the 
door from the inside by means of the exit button.

The system must be extensible to include new functions and be easily
maintained.