All optical disks are round platters. They come in different sizes and capacities. Commonly used types of optical disks are CD-ROM, WORM(CD-R),CD-RW,and DVD. They are described below.
CD-ROM stands for compact Disk-Read-Only Memory. It is a spin-off of audio CD technology and works much like audio CDs used in music systems. In fact, if your computer has sound card and speakers, you can play audio CDs in your computer.
CD-ROM disk is a shiny, silver color metal disk usually of 51/4-inch(12cm)diameter. It is made of polycarbonate plastic having thin layer of pure aluminum coating to make its surface reflective. For some vary good quality disks, gold layer is used. A thin layer of lacquer on top of the reflective surface protects it. It has storage capacity of about 650 Megabytes or 700 Megabytes in newer ones. It is called because of its large storage capacity on a compact-size disk and because it is a read-only storage medium. That is, these disks come pre-recorded and a user cannot alter information stored on them.
Since an optical disk mechanism reads a disk from the center to outer edge, several sizes are possible. Apart from the most common 120mm 74/80 minute disk having storage capacity of 650MB to 700MB, 80mm 21 minute disk (called Mini-CD) having 184MB storage is also available. They are usable with the disk drive of a 120mm disk. Mini-CDs are available in various fancy shapes like round, oval, triangle, etc., while ensuring that the storage area is not affected.
The fact that a normal user cannot write data to a CD-ROM disk does mean that this storage medium is not useful. In fact, many applications deal with huge volumes of data that rerely change. Hence, CD-ROM is preferable for such applications. For example,infopedia is a single CD-ROM disk that holds a complete encyclopedia, or a dictionary, or a world atlas, or a dictionary of quotations, or biographies of great people, or information about all educational institution of a particular type in a country, etc, Nowadays., CD-ROM disk is preffered storage medium for distribution of Electronic version of conference procedings,journals,magazines,books,etc.
New software products
Software upgrades
Multimedia applications such as video games, which involve integration of text, sound graphics, motion video, and animation.
For wide distribution and use of CD-ROM disks on different computers, the International Standards Organization (ISO) has evolved a standard for recording information on CD-ROM disks, called ISO 9660. This standard requires creation of a volume Table of contents (VTOC) on a disk, which allows storage and direct access of 1,40,000 files on the disk. This enables opening of any one of the 1,40,000 files with a single seek. Microsoft Corporation has developed MSCDEX (Microsoft CD Extension) software, which records data in ISO 9660 format on CD-ROM disks. All CD-ROM vendores have accepted the ISO 9660 format.