I have a website (project) where users get logged in, do their work and then they log out.

I need to build a report that displays how much each person has used of data. (bandwidth, how much was downloaded in Kb, etc)

So the process may be like counting start of usage from user login to user logout.

I have seen a little about Webalizer and AWStats for something like this, But I am not sure how they work. I have tried Content-Length but some pages don't send content-length.I have also seen mod_bandwidth but still I am little confused.

This process is needed for my site because now, our company is thinking of charging per usage and also bandwidth allocation for each users (according to their membership).

I haven't worked with this type of tools, I am newbie in this matter. I have done only simple websites not any setting like this in Apache or Linux. My project is in Codeigniter.