public static function get_forum_discussion_posts_parameters() {
        return new external_function_parameters (
            array(
                'discussionid' => new external_value(PARAM_INT, 'discussion ID', VALUE_REQUIRED),
                'sortby' => new external_value(PARAM_ALPHA,
                    'sort by this element: id, created or modified', VALUE_DEFAULT, 'created'),
                'sortdirection' => new external_value(PARAM_ALPHA, 'sort direction: ASC or DESC', VALUE_DEFAULT, 'DESC')
            )
        );
    }