def signalStrength(self):
        """ Checks the modem's cellular network signal strength
        
        :raise CommandError: if an error occurs
        
        :return: The network signal strength as an integer between 0 and 99, or -1 if it is unknown
        :rtype: int
        """
        csq = self.CSQ_REGEX.match(self.write('AT+CSQ')[0])
        if csq:
            ss = int(csq.group(1))
            return ss if ss != 99 else -1
        else:
            raise CommandError()