Passive Monitoring is the way to check the site performance by server-side software. It can create the exact copy of incoming traffic for analysis, or it can operate on real traffic. Passive monitoring may collect statistics and watch over some important values, like traffic consumption, number of visitors etc. Also it is handy to analyze the real workflow on your site. However, only server-side errors could be caught. If case of broken Іnternet connection or other Internet-related errors, passive monitoring fails - as it shows only the server performance which is Ok. You'll notice the the lack of visitors, but this fact does not appear immediately. And this is the main difference between the Passive Monitoring (server-side) and Active Monitoring (third-party).