First thing you need to do is either create or obtain a print of each floor. Now layout the locations of ac, ethernet, lighting ect. By doing this you will define the wiring needs.
Contact several telco firms and get quotes on running the cable for the ethernet and telephone system.
You will want to place one or more switch's on the 11th floor and run fiber optics to the 15th. This will minimize the line loss and any electrical interference that might be encountered in long runs.
You don't say if you are going to remodel or if it is new construction.
Take a close look at the electrical capabilities on both floors. Limiting the number of computers on a single circuit will save you some headaches in the furture.
By letting your electrician know which outlets are computers he can avoid placing them on the same phase as known emi/rfi producing equipment.
The computer room needs to have dedicated circuits, isolated grounds would be perfered.
The equipment that will be in the computer room will dictate the capacity of the enviroment control. This should be separate from the floor ac system if at all possible. Having some employee's sweat is a minor inconvience. You let the computer room starting getting hot that's a different story.
You can contact APC: http://www.apc.com and they will help you configure the ups requirements for your computer room.
If the moneys will let you look in to using a dedicated fire supression system in the computer room.
Good Luck Lee