def send_terrain_data(self):
        '''send some terrain data'''
        for bit in range(56):
            if self.current_request.mask & (1<<bit) and self.sent_mask & (1<<bit) == 0:
                self.send_terrain_data_bit(bit)
                return
        # no bits to send
        self.current_request = None
        self.sent_mask = 0
send some terrain data