Hello,
We can setup 2 types of compression in Backup Exec, Software based and hardware based compression. Software based compression is taken care by the NTFS file system and Hardware based compression is taken care by the Drive.
Compression ratio depends on what type of files we are backing up...example : .mp3, .dll, .jpg...etc, will not get a good compression rate where as .doc, .txt, .ppt...etc will get 100% compression as far as my experience goes.....
Hope this helps.....