protected function calulateDataTransferRateValues()
  {
    $vals = array();
    
    // Works like this:
    // After the server resonded, the socket-buffer is already filled with bytes,
    // that means they were received within the server-response-time.
    
    // To calulate the real data transfer rate, these bytes have to be substractred from the received
    // bytes beofre calulating the rate.
    if ($this->data_transfer_time > 0 && $this->content_bytes_received > 4 * $this->socket_prefill_size)
    {
      $vals["unbuffered_bytes_read"] = $this->content_bytes_received + $this->header_bytes_received - $this->socket_prefill_size;
      $vals["data_transfer_rate"] = $vals["unbuffered_bytes_read"] / $this->data_transfer_time;
      $vals["data_transfer_time"] = $this->data_transfer_time;
    }
    else
    {
      $vals = null;
    }
    
    return $vals;
  }