CD-ROM as a Storage Media

Typically each CD-ROM can hold up to 650MB of data (the equivalent of over 450 high-density 3.5-inch floppy disks) and does not cost much to produce. The physical medium of the CD-ROM is the same as that used for audio compact discs (CDs): a polycarbonate disc with an aluminized layer. A laser reads the data, which is encoded in microscopic pits on the aluminized layer. CD-ROM media are more robust and reliable than other magnetic media, such as floppy disks. All these factors make CD-ROM an attractive medium for distributing data and programs. In fact, most Linux books (including this one) bundle CD-ROMs with a complete Linux distribution that includes the operating system and lots of popular software. In this case, the CD-ROMs contain Red Hat Linux-a popular Linux distribution.

In that list of good properties of a CD-ROM-high capacity, low cost, and reliability-you don't see any mention of speed; the data-transfer rates of CD-ROM drives are not as fast as those of hard disk drives. When CD-ROM drives first appeared, they could transfer data at rates of approximately 150KB per second. These drives were known as single-speed (also referred to as 1X) CD-ROM drives. Double-speed (or 2X) CD-ROM drives, which provide data-transfer rates of 300KB per second, were soon widely available. Currently, most systems come with 24X, 32X, or even 40X CD-ROM drives, which can sustain data-transfer rates of up to 3,600KB per second and average access times of 95 milliseconds (compare this with hard-drive access times of around 10 milliseconds).

Most CD-ROM drives enable you to play audio CDs via an external headphone jack. Also, you will find an output line you can connect to the sound card so that you can play an audio CD on the speakers attached to the sound card.