CD Interchange Requirements
Successful data interchange requires conformance to mechanical, physical, dimensional, recording, format, EDC, ECC, coding, and optical characteristics of ISO/IEC 10149, Yellow Book, and Orange Book standards. Validation requires a wide variety of expensive tools and test equipment costing more than $100,000, although low cost testers are available that only check error rates.
File exchange requires conformance of volume and file structures to ISO 9660 and its optional extensions. Logical flaws are frequently observed because these structures usually are not validated. A recent study Why CD-Rs Fail revealed that 57 of 100 CD-R field failures contained logical flaws, while only 19 of 100 bad discs had high BLER. Read failures associated with logical flaws are unpredictable, since some operating systems may be forgiving while others are more sensitive to defects.
Field failures can be prevented only if logical defects are detected and corrected by pre-mastering organizations and by duplicators and replicators who process their gold masters. Until recently, comprehensive logical testing required manual sector reads, hex conversions, and sophisticated interpretation. New software from Arrowkey supports fast and accurate evaluation of logical CD quality, screening out flaws before they are copied onto large numbers of discs and distributed to users.
Physical and Logical CD Structure
The physical structure of every ISO/IEC 10149 disc is comprised of lead-in, pre-gap, digital data, post-gap, and lead-out. These areas consist of frames with sync, subcode, information, and parity bits. Data sectors, each containing sync, header, user data, and parity bytes, are distributed among the information bits of the digital data frames.
Logical volume and file structure requirements of ISO 9660 define the digital data region and support file interchange between systems. Improper descriptors, path tables, directories, gaps, or modes in this region can result in non-conforming discs that may be readable in some systems but will unpredictably fail in others.
Requirements of ISO 9660
ISO 9660 defines mandatory requirements for CD data interchange, and also provides for optional extensions to Windows, UNIX, Macintosh, and other environments. Three levels control file and directory name lengths, directory depths, and file fragmentation permissions. Each of these elements is managed by pre-mastering software that may not comply with the requirements of ISO 9660 and its extensions.
The ISO 9660 Primary Volume Descriptor in sector 16 contains important information, including the location of path tables and the root directory. Directory records must have precise structures, and allow optional system use bytes for use by extensions. Various character set, name length, path length, and order rules must be rigorously followed.
ISO 9660 Extensions
ISO 9660 specifies core requirements for a primary volume. It also defines optional elements of undefined content that allow multiple, non-conflicting extensions. These can support specific operating systems, booting, or other features.
The Joliet extension utilizes a supplementary volume descriptor together with associated path tables and directories to identify an alternate volume using ISO 9660 Level 2 characters and longer names. It is supported by Windows 95/98/NT/2000/XP, and also by current Macintosh and Linux systems.
Characteristics unique to Macintosh systems are supported by an HFS extension to ISO 9660. Hierarchial File Systems have a system identifier in the primary volume descriptor and a system area containing the HFS driver descriptor record and HFS partition maps. Boot blocks, two master directory blocks, volume bitmaps, extents overflow and catalog files, and HFS directory records and files are contained in the HFS partition. Directory records provide for associated files, and contain system use bytes containing special HFS information.
Hybrid discs can be mounted in various systems when the ISO 9660 primary descriptor coexists with an HFS partition and/or a Joliet descriptor. Files can be shared between them.
The Rock Ridge extension allows UNIX-specific long filenames, character sets, deep directories, access privileges, and file types that are supported by System Use Sharing and Rock Ridge Interchange Protocols. These extensively use ISO 9660 directory record system use areas.
The El Torito extension defines a boot record volume descriptor in sector 17 that points to a boot catalog containing entries enabling systems with the proper BIOS to boot directly from the CD. Use of XA Mode 2 sector structures impose specific requirements on descriptors and directories.
Sources of Logical Defects
The pre-mastering process manages logical CD structures and determines compliance with ISO 9660 and appropriate extensions. Many types of errors are frequently observed that result from flawed recording software or improper operator choices, such as failure to close a disc or unnecessary use of XA Mode 2. Faulty post-gaps occur so often that mastering facilities may automatically append another post-gap, occasionally introducing invalid formats. Improper character sets or erroneous path table and directory ordering are commonly observed. Descriptors may contain an improper volume size, or selective read failures may be caused by an incorrect volume size in only one of several descriptors. Bootable discs may not contain key El Torito elements. These and many other flaws create unacceptable interchange risks.
Summary
Intricate requirements of ISO 9660 and its various extensions place complex demands on the pre-mastering process. Improper software design or use can create logical defects resulting in field failures that can only be prevented by testing. Originators of each ISO image must qualify their pre-mastering process, while duplicators and replicators should screen masters to assure conformance of each copy.
New, affordable software from Arrowkey supports comprehensive analysis of logical CD quality. CD Structure quickly and accurately verifies conformance with ISO 9660 and its extensions, assuring successful interchange, or pinpoints flaws that require corrective action. Coupled with other test methods, logical testing provides confidence in CD discs and avoids expensive, embarrassing interchange problems.
CD Structure is available from Arrowkey, Inc., http://www.arrowkey.com/, 1-888-759-0600. Comprehensive training, testing, and analysis is available from Media Sciences, Inc.