Camera Normal { viewing Volume Collisions }

Hi Folks,

I have been using prespective camera for my project .iam navigating (walk thru) with the help of model view transformations.{ Rotation and Translation} while mouse move. I can enter inside the wall of the virtual room which i have created.
Now i need to add an collision detection algorithm for the above , which prevents me not to walk thru the walls. I need to calculate the ray which travesals from the position of the camera towards the center of the camera. Camera normal. Then i may look for some plane ray intersection.
Any one advise me how to compute the alogrithm for the walkthru.