code: def transmission_rate(self):
          
          sent = self.bytes_sent
          received = self.bytes_received
          traffic_call = time.time()
          time_delta = traffic_call - self.last_traffic_call
          upstream = int(1.0 * (sent - self.last_bytes_sent)/time_delta)
          downstream = int(1.0 * (received - self.last_bytes_received)/time_delta)
          self.last_bytes_sent = sent
          self.last_bytes_received = received
          self.last_traffic_call = traffic_call
          return upstream, downstream
docs: Returns the upstream, downstream values as a tuple in bytes per
          second. Use this for periodical calling.