Avoid Fragmentation of ext3

[Andre Nitschke]

Hello,
i just want to know, how ext3 avoids fragmentation. Well, i think it works like
this (but i dont know...):
When the OS says to the filesystem, save the file, the file system looks, where
are free sectors laying together to use. when there is enough place the
filesystem try's to write the file without fragments. is there not enough
place, the fs wrote the file in the way, to create less fragemnts. some file
systems keep space after the file, for when the file grows. i dont know, works
ext3 in this way?
maybe somebody can explain it shortly.

thanks

frindly