https://blog.gongquiz.com/2021/01/12/%eb%8b%a4%ec%9d%8c-%ec%a4%91-%ec%84%a4%eb%b9%84%eb%b3%b4%ec%a0%84%ea%b4%80%eb%a6%ac%ec%97%90%ec%84%9c-%ec%84%a4%eb%b9%84%ec%9d%b4%eb%a0%a5%ec%b9%b4%eb%93%9c-mtbf%eb%b6%84%ec%84%9d%ed%91%9c-%ea%b3%a0/