code: def _subscribe(self):
          
          cmd = MAGIC + SUBSCRIBE + self._mac \
              + PADDING_1 + self._mac_reversed + PADDING_1
          status = self._udp_transact(cmd, self._subscribe_resp)
          if status is not None:
              self.last_subscribed = time.time()
              return status == ON
          else:
              raise S20Exception(
                  "No status could be found for {}".format(self.host))
docs: Subscribe to the device.
  
          A subscription serves two purposes:
          - Returns state (on/off).
          - Enables state changes on the device
            for a short period of time.