public function getUnits($type = null)
    {
        if (!isset($type)) {
            return $this->units;
        }

        $filtered = array();
        foreach ($this->units as $key => $value) {
            if (isset($value['type']) && $value['type'] === $type) {
                $filtered[$key] = $value;
            }
        }

        return $filtered;
    }