Optical disks have several important advantages:

1. Large Capacity: Ranging from 300 MBs on a 3.5" disk, to over 6.5 GBs on a 12" disk. The use of a laser to write and read information supports the highest number of data bits per unit area of any commercially existing storage technology.

2. Random Access: Fast access to information. For example: A typical erasable optical disk drive has an average seek time of 27 ms.

3. No Wear: There is no physical contact between the read/write optical head and the optical disk. Thus disks can be read repetitively without any physical wear or potential loss of data.

4. Durability: Recorded data on the optical disk is protected by one or more outer disk layers and a sophisticated ECC technology.

5. Non Erasable: WORM disks, CD-ROM, and CD-R discs are non erasable. Once information has be recorded on the disk it can not be altered without destroying the original information.

6. Long Life: With proper handling and storage, optical disks have very long life's. Typical life times range from 10 to 100 years.

7. Economical: Very inexpensive. ~12 cents per MB for erasable, ~7 cents per MB for WORM, ~4 cents per MB for CD-R.

8. Removable: Disks are easily removed for security, archival, or the insertion of a different disk.

5.1 CD Standards

The physical CD disc and CD drives are highly standardized, as is the organization of information on CD discs; thus allowing the discs to be read on almost any computer system.

5.1.1 Physical CD Standards

There are five major physical CD standards. They are:

1. Red Book [Audio].
2. Yellow Book [CD-ROM].
3. Green Book [CD-I].
4. Orange Book [CD-R].
5. White Book [Video CD].

5.1.2 ISO 9660 File System Standard

Almost all CD-ROM's and CD-R's use some form of this standard. Based on the High Sierra file system, ISO 9660 presents the information stored on the CD as a random access hierarchical file system. This file system is only suitable for pre mastered discs.

5.1.3 ISO 9660 Extensions

Unfortunately, ISO 9660 does not translate well on all computers. To over come these limitations, several non official extensions have been adopted by industry. They include:

1. Rock Ridge ISO 9660 Extension: Rock Ridge basically adds support for long directory and file names. Thus removing the DOS like restrictions of ISO 9660.

2. Apple ISO 9660 Extension: An extension that supports the Macintosh resource fork.

3. Updatable ISO 9660 Extension: An extension that defines how to handle multisession CD-R discs.

5.2.4 Beyond ISO 9660

Given the ability of multisession recording with CD-R technology, a new standard for CD file systems has been proposed. ECMA 168, is a standard that is designed for CD-R discs. It incorporates several ISO 9660 extensions, including Updatable ISO 9660 and Rock Ridge, but is not backwards compatible with the ISO 9660 and it's extensions.

5.3 WORM Standards

Today there are few physical standards for WORM technology. Most WORM manufacturers have agreed not to agree on a standard. Some have enough market share that they are the unofficial standard. Others have pushed forward to create a standard for their own products.

5.4 Erasable Standards

Using much of the work done for 5.25" WORM standards, manufacturers set out to define a single 5.25" physical standard for MO disks. Their work has been highly successful and resulted in a number of standards that actually work. These standards include:

1. ANSI X3.212: An MO standard for single density, 650 MB, 5.25" disks.

2. ANSI X3.220: A CCW WORM standard for single density 5.25" disks.

3. ECMA 184: A standard for double density, 1.3 GB, MO and CCW WORM 5.25" disks which is backwards compatible with single density standards.

4. ECMA 201: An MO standard for single density, 128 MB, 3.5" disks.

5.5 WORM and Erasable File System Standards

Currently there is no market accepted WORM file system standard; but a standard does exist. ECMA 167, Volume and File Structure of Write-Once and Rewritable Media using Non-Sequential Recording for Information Interchange, is a standard that defines both an erasable and WORM file system structure suitable for all types of optical disks. Currently various organizations are developing implementations of this standard.