How to set the access rights in project server 2003?

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I have installed a project server in demain. There is a access rights
scenario I don't know how to set.
There are 5 roles in a company. The organization charter is :
executive leader
|
department manager
|
project manager
|
developer

There are 1 executive leader, he is responsible for the management of two
departments.
There is one department manager in every department,and manage 5 project
managers.
Every project manager is responsible for 4 developers.

Requirement is following:
1. administrator, IT, he may do anything in project server.
2. executive leader, he may view all projects in server , but he can not
change them.
3. department manager can view and changer all projects in his department
4. project manager can create and change project, assign the task in server.
He can view his all projects.
5. developer can view his task and update the task.


My question is :
1/2/4/5 role all can be setup in project server easily by default setting.
But how can set 3? Where can I find some information about it ? I have read
the online help , but can not understand it.

Thanks.