public function startChannelCommercial($channelIdentifier, $accessToken, $length = 30)
    {
        $validLengths = [30, 60, 90, 120, 150, 180];

        if ($this->apiVersionIsGreaterThanV3() && !is_numeric($channelIdentifier)) {
            throw new InvalidIdentifierException('channel');
        }

        if (!in_array($length = intval($length), $validLengths)) {
            throw new UnsupportedOptionException('length', $validLengths);
        }

        return $this->post(sprintf('channels/%s/commercial', $channelIdentifier), ['length' => $length], $accessToken);
    }