https://purengom.com/2010/11/24/iphone-3gs-%eb%b0%b0%ed%84%b0%eb%a6%ac-%ea%b5%90%ec%b2%b4-%ed%94%84%eb%a1%9c%ea%b7%b8%eb%9e%a8%ec%97%90-%ea%b4%80%ed%95%b4/