traffic management

hi guys,
I am curious about the fact that how the ISP (Internet Service providers) manage the volume based plans i.e. you are alloted a fixed amount of bandwidth for certain payment for a certain time frame afterthat your plan expires and you have to renew. Now my question is that how the ISP monitor and record such bandwidth management. Same question for the website traffic i.e. fixed GB of uploads and downloads and then your site is inaccessible. By using which tools and techniques these plans are implemented.

Thanks

Im no expert in this matter but, i did find this on google

http://www.bandwidthcontroller.com/features.html

hope it helps :)