FTP may not be counted in AWStates and it may be counted in plesk. Same goes for Plesk Control Panel Usage... I am sure that there are other things that can cause this too, but those are the only two things that I can think of at the moment.
Awstats counts only the domain it is installed on.
If you have subdomains or domain aliases, statistics for those might not be counted in Awstats. If Plesk monitors FTP, Mail or any other service for bandwidth then that will also be counted, but not in Awstats.
Basically, Awstats is only useful for telling you what happened on your website, Plesk will cover everything on your account.
The same goes for me here, but I'm using cpanel (and have my own server).
One of those things (I would say awstats) is not very accurate when it comes to bandwith.
No, they are not cheating, I think this goes on for everybody.
There's also ftp and mail traffic ...
from 500 MB to 1GB is a huge gap..... are you allowing uploads!!!! does AWstat count uploads?
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ehsen you can ask your web host how much bandwidth you USED so you know which one your traffic program is showing correct bandwidth allocated. I think awstat is useful, I always use that for checking traffic, hits and referrers.
12MB a day is 360MB a month. that's more than half of the missing bandwith. bringing the gap down to 140MB..
Items not counted on awstats (some of these have been mentioned).
Email Incoming, and outgoing. Even incoming spam that gets bounced counts. Assuming email is turned on on the server. If email is turned completely off on the server, this would be lowered significantly. TONS of spam gets sent to random email addresses at every domain.
Incoming http requests. - DDS, or mini-dds creates HUGE bandwidth usage in incoming requests that awstats wont report.
FTP as people mentioned Incoming and outgoing.
mysql traffic.. Incoming and outgoing.
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ehsen, AWStats is an open source, log file parser and analyzer and it's also suitable for analyzing web server and ftp server log files and I think it shows everything that goes on on your server and your Control Panel shows only what goes on on your site. I'm not sure but it's only my idea about it.