code: def strip_rate(self, idx):
          
          val, = struct.unpack_from('<B', self._rtap, idx)
          rate_unit = float(1) / 2    # Mbps
          return idx + 1, rate_unit * val
docs: strip(1 byte) radiotap.datarate
          note that, unit of this field is originally 0.5 Mbps
          :idx: int
          :return: int
              idx
          :return: double
              rate in terms of Mbps