My old mac ibook G4 has pasnther installed on it,i want to upgrade to tiger but the dvd-rom is not functional also being an old system its not possible to boot via usb,Is it possible to boot it using an external dvd-rom connected via usb?
To answer your post title, yes it is, as long as it's a FireWire drive properly formatted (Apple Partition Map) and has an OS on it the iBook supports. This is 10.4.11 on the first generations and 10.5.8 on later ones.
You can have the OS on the external pre-installed (must be done on another PowerPC mac) or you can use Disk Utility to copy an install image to it and make the external drive a bootable installer.
USB can not be used to boot off of on PowerPC Macs.