def get_serialnumber(self):
        """
        :return: the serial number
        """
        command = const.CMD_OPTIONS_RRQ
        command_string = b'~SerialNumber\x00'
        response_size = 1024
        cmd_response = self.__send_command(command, command_string, response_size)
        if cmd_response.get('status'):
            serialnumber = self.__data.split(b'=', 1)[-1].split(b'\x00')[0]
            serialnumber = serialnumber.replace(b'=', b'')
            return serialnumber.decode() # string?
        else:
            raise ZKErrorResponse("Can't read serial number")
:return: the serial number