ISO—What's that all about?

by Lee Alexander

Introduction

Most of us are familiar with ISO as a two or three digit specification for the light sensitivity of photographic film. ISO is not an acronym for International Standards Organization; is derived from the Greek word isos, meaning equal. However, it is the International Standards Organization that names ISO-standards. It was founded in 1946 and is headquartered in Geneva , Switzerland . It comprises over 70 nations and ANSI (American National Standards Institute) is a member. The standards cover multiple disciplines; from the dimensions of screw threads, size and construction of gas cylinders, paper sizes, and even credit card dimensions. Imagine what your wallet would look like if every credit card was a different size.

Data CD and DVD File Systems

The file system for these optical media describes where and how on the disk the files and folders are located. This is not to be confused with the TOC (Table Of Contents) which describes the session and track layout of the disk.

ISO9660

This was the most common file system for most CDs and DVDs; however, it is the oldest and has some limitations. It only accommodates short filenames (8.3) and a directory structure which can only be eight levels deep. Due to these limitations is always accompanied by another file system, without the limitations. Some programs allow long file names and more directories deep. From Windows 95 OSR 2 it is not a problem which could be in other systems. There are variations on the ISO9660:

Joliet is an extension that permits long file names and a deeper directory structure .

Rock Ridge, another extension allowing long file names and deeper directory structure but is not supported by Windows. It was popular on Commodore CDs.

Who knows? This file system starts with its own volume descriptor which has all the characteristics of Joliet and points to all the tables of the ISO9660 file system. It is always accompanied by Joliet , which is the preferred system.

El Torito – Bootable CD/DVD is not really a File System, more of an extra feature of the ISO9660. CDs and DVDs can be made bootable but then the ISO9660 File System needs to be present alongside with the El Torito Volume Descriptor and a Virtual Floppy image. By just copying a Virtual Floppy image to a CD/DVD one does not make the CD/DVD bootable! The File System needs to be set up as well. Bootable CDs and DVDs need to be supported by the BIOS of your PC system, not the Operating System. Older BIOS do not handle bootable CDs well, let alone bootable DVDs.

UDF

The Universal Disk Format is an implementation of the ISO/IEC 13346 standard for storing files on optical media. Considered to be a replacement of the ISO 9660, it is widely used for rewritable optical media. UDF is developed and maintained by OSTA (Optical Storage Technology Association). Initially intended for use with ROM media, it increased in popularity when both CD-RW media and hardware for CD writing came down in price levels. Compared to iso-9660, it supports larger files, larger disks, more information about individual files and folders, and all the OS-specific data.

UDF Usage

DVD-Video media use UDF version 1.02. These discs contain a so-called UDF Bridge format, where both an ISO 9660 as well as a UDF 1.02 filesystem are present on the same disc, describing the same filesystem. All standard formats for video recording on DVD-style media use some version of the UDF filesystem.

Philips' DVD+VR format uses UDF 1.02 with an ISO 9660 bridge for DVD+R and DVD+RW. The DVD Forum's DVD-VR format uses UDF 2.00 for DVD-R, DVD-RW and DVD-RAM. Blu-ray and the DVD Forum's HD-DVD will be using UDF 2.50 or UDF 2.60.

When combined with packet writing technology, UDF has the advantage of allowing files to be added to and removed from a disc through the normal filesystem mechanisms. That is, the contents of a disc can be manipulated in the same way a hard disk, floppy disk, or USB flash drive might be. This even works for sequentially written media such as CD-R, although files that are deleted remain on the disc occupying space. In Windows the disc appears as a drive letter, while in Unix/Linux or Mac OS X it is mounted as a directory.

Typically for writing UDF in Windows, third-party burner software such as DirectCD, DLA, InCD, and WriteUDF is required. As shipped by Microsoft, Windows (before Windows Vista) can only read UDF discs, while current (2006) releases of Mac OS X, Linux, and several other Unices include UDF writing capability with no additional software required. Older versions of these operating systems may have limited reading capabilities, and may not support more recent versions of UDF.

Flavors of UDF

UDF revisions since 1.5 actually describe three different variations of the format. For lack of an official name from OSTA they are called flavors here.

Original flavor

Can be used on any type of disk that allows random read/write access, such as hard disks and DVD-RAM media. Entries point directly to the block numbers of their file contents. Any physical block on the disk may be chosen for allocation of new or updated files. Since this is the basic format, practically any OS or File System Driver claiming support for UDF should be able to read this format.

VAT flavor

For DVD-R and CD-R media every physical block can only be written to once, and writing must happen incrementally. This means that the original UDF format can only be written to CD-Rs by pre-mastering the data and then writing all data in one piece to the media, similar to the way ISO 9660 gets written to CD media. To allow CD-R to be used virtually like hard disks, meaning that you could add and modify files on a CD-R at will (so-called drive letter access on Windows), OSTA added the VAT flavor to the UDF standard.

The VAT is an additional structure on the disk that helps remapping physical blocks when files or other data on the disc gets modified. The nature of the write-once media leads to the effect that when a file gets first added and later deleted on the disk, the file's data remains still on the disk, although it does not appear in the directory any more. Additionally, incremental writing also means that eventually the disk will be full and free space cannot be recovered by deleting files.

Spared (RW) flavor

DVD-RW and CD-RW media have fewer limitations than DVD-R and CD-R media: While sectors can be rewritten at random, these sectors may "wear out" after a while, meaning that their data becomes unreliable after having been rewritten too often (typically after a few hundred rewrites with CD-RW).

The consequences of using these flavors are as follows:

When using the original format, in theory a disc driver may allow to rewrite any disc sector at random, meaning the RW is truly rewritable in the fashion of hard disks. However, because of the wear-out effect, this would soon lead to loss of data. For that reason, if an original UDF file system is written to RW media, the file system should lock (write protect) the UDF volume to prevent accidental modification by a computer, or better, disc drivers should never even attempt to provide random-write access to RW media unless they can assure that no data loss due to wear-out can happen.

When using the VAT flavor, RW media effectively appears as CD/-R or DVD-R media to the computer. However, the media may be erased again at any time.

Using the Spared (RW) flavor for DVD-RW and CD-RW media has fewer limitations than DVD-R and CD-R media. While sectors can be rewritten at random, these sectors may "wear out" after a while, meaning that their data becomes unreliable after having been rewritten too often (typically after a few hundred rewrites with CD-RW).

RW media can be erased entirely at any time, making it blank again, ready for writing a new UDF or other file system (e.g. ISO 9660 or CD Audio ) to it. The wear-out effect is not an issue here unless the disk gets erased more than a few hundred times or the Spared flavor was used.

When using the original format, in theory a disc driver may allow us to rewrite any disc sector at random, meaning the RW is truly rewritable in the fashion of hard disks. However, because of the wear-out effect, this would soon lead to loss of data. For that reason, if an original UDF file system is written to RW media, the file system should lock (write protect) the UDF volume to prevent accidental modification by a computer, or better, disc drivers should never even attempt to provide random-write access to RW media unless they can assure that no data loss due to wear-out can happen.

When using the VAT flavor, RW media effectively appears as CD/-R or DVD-R media to the computer. However, the media may be erased again at any time.

Finally, the Spared flavor works basically like the original flavor, but uses an extra Sparing Table to remap worn-out sectors. This allows us to reclaim most of the disk's space when deleting a file.

Understanding this Sparing Table is necessary to be able to read discs written in this flavor. The problem is that some existing versions of UDF File System software ignore this extra information and treat such UDF discs as if they had the original flavor. As long as the media has no worn-out sectors, this does not matter - the files can be read properly. But once sectors are remapped, a File System not paying attention to the Sparing Table will read outdated sectors, leading to retrieval of the wrong data.

Why a computer might not read a particular UDF disk

Even if a computer claims to be able to read UDF 1.50, it still may only support the original format but not the VAT or Spared flavors. An example is Mac OS X (10.4.5), which claims to support UDF 1.50 yet it can only mount disks of the original format properly (it cannot mount UDF disks with a VAT at all). While it appears to be able to mount CD-RWs written with a Sparing Table, it does not read its files correctly in the case that files are actually remapped.

Standalone DVD Player Compatibility

Some users have reported that video DVDs burned on their computers in UDF version 1.5 are not compatible with their set-top video DVD players. These players seem to be only compatible with UDF version 1.02. As some popular DVD burning programs such as Roxio' Easy CD & DVD Creator default to UDF version 1.5 when burning video DVDs, users have found it necessary to avoid using the Software Wizard and instead manually configure the burn to version 1.02.