I was thinking of sending a header when the previous page was not from www.mydomain.com so that the homepage with the frameset would appear.
Once the main frameset is opened visitors would click links and stay within the structure i intended.
Of course i am aware that visitors can escape the frameset from there by opening a new page in a new window, but i don't mind about that.
Is it possible then to check from what site a visitor came then ?