var no=1;

$(window).scroll(function () {

    if(no==1)

    {

        if ($(window).height() + $(window).scrollTop() == $(document).height()) {

            no=2;

            $.ajax({

                type: POST,

                url: request.php,

                data: datas,

                cache: false,

                success: function(html){



                }

            });

        }

    }

});

This does an ajax call when a user reaches at end of page.

You can specify a height at which it occurs.