How to transform Laser Data stream from laser_frame to base_link?

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Where are you getting your /scan from? You will need to first figure out what the frame id in your scan msg is. You can do this by rostopic echo /scan/header in your terminal. Then you need to make sure that a link exists in your tf from laser_link -> base_link. You can use tf_echo or tf_monitor for this. If a transform exists, you can subscribe to your /scan topic and in the callback have this to transform your entire scan to base_link: (I am using PCL here)

tf2_ros::TransformListener* tf_listener_1;
tf2_ros::Buffer tf_buffer_1;
geometry_msgs::TransformStamped transform_stamped;

// Point Cloud callback
void cloud_cb (const sensor_msgs::PointCloud2ConstPtr& input)
{
  // initialize pcl objects
  pcl::PointCloud<pcl::PointXYZI>::Ptr input_cloud(new pcl::PointCloud<pcl::PointXYZI>);
  pcl::fromROSMsg(*input, *input_cloud);

  // Transform point cloud to base_link
  try
  {
    transform_stamped = tf_buffer_1.lookupTransform("base_link",
                                                    input->header.frame_id,
                                                    ros::Time(0),
                                                    ros::Duration(2));
  }
  catch (tf2::TransformException &ex)
  {
    ROS_ERROR("%s",ex.what());
    return;
  }
  // transform current point cloud
  Eigen::Affine3d pc_to_base_link = tf2::transformToEigen(transform_stamped);
  pcl::transformPointCloud(*input_cloud, *input_cloud, pc_to_base_link);

// input_cloud is now in the base_link
...
}