Initiate fetching of the TOC.

def start(self):
        """Initiate fetching of the TOC."""
        self._useV2 = self.cf.platform.get_protocol_version() >= 4

        logger.debug('[%d]: Using V2 protocol: %d', self.port, self._useV2)

        logger.debug('[%d]: Start fetching...', self.port)
        # Register callback in this class for the port
        self.cf.add_port_callback(self.port, self._new_packet_cb)

        # Request the TOC CRC
        self.state = GET_TOC_INFO
        pk = CRTPPacket()
        pk.set_header(self.port, TOC_CHANNEL)
        if self._useV2:
            pk.data = (CMD_TOC_INFO_V2,)
            self.cf.send_packet(pk, expected_reply=(CMD_TOC_INFO_V2,))
        else:
            pk.data = (CMD_TOC_INFO,)
            self.cf.send_packet(pk, expected_reply=(CMD_TOC_INFO,))