Optical Media: cDs versus dvDs

Data in an optical media is read and written using laser beams. Compact discs (CDs) and digital video discs (DVDs) are optical disks. A DVD is an enhanced form of a CD. The two types of disks are physically the same size, but they differ in format. DVDs offer much greater capacity, which they achieve in two ways. First, DVDs have narrower tracks, so they can squeeze more tracks onto the same size disk. Both CDs and DVDs are read using light from a laser. But, the CD laser is red, while the DVD laser is blue. Red light has a longer wavelength than a blue light. The blue laser is thus able to produce a smaller beam, allowing it to focus on the tinier tracks of the DVD. The second way that DVDs achieve increased capacity over CDs is by using multiple layers of tracks. The blue laser is not only narrower, but also more powerful. Its beams can penetrate the plastic and focus at different depths. DVDs that are dual-layered actually have two sets of tracks on one side of the disk, one beneath the other. The laser beam can be focused on either the upper or lower layer. This doubles the capacity of one side of a DVD disk. It is also possible to put tracks on both sides of a DVD. A double layer double side (DLDS) DVD drive uses double layers and can read double-sided disks, giving it four times the capacity of a single layer single side (SLSS) drive.

CDs have two recordable formats, CD-R and CD-RW. While the Parsons and Oja textbook lists recordable DVDs, there are multiple standard formats for this. So, while CD-Rs and CD-RWs are readable on most CD or DVD drives, recordable DVDs may not be readable on some types of DVD drives. For this reason, users who record data on optical media for widespread distribution (for example, for releasing a new software package) may wish to limit themselves to CDs for now even though it has lower capacity than DVDs. DVDs are usually used as a medium for multimedia presentations that combine sound with graphics, such as movies.

Magnetic Media

Magnetic media range from some of the smallest capacity storage devices, floppy disks, to the largest capacity devices, hard disk drives. The floppy disk is no longer used to distribute operating systems, though, because of the increasingly large size of current operating systems. The last major distribution of Windows using floppy disks was Windows 95, which occupied 30 diskettes.

Zip disks, removable storage drives produced by Iomega, allow users to store much larger amounts of data than a floppy disk can hold—although Zip disks are physically not much bigger than floppy disks. These drives were once very popular, and many machines came with a Zip drive as standard equipment. But, their use declined with the wide availability of CD-RW and DVD-RW drives and the reduced cost of blank CD-R and DVD-R disks.

Smaller portable drives are being manufactured with larger capacities. For example, Mini USB storage device offers 512MB, 1GB, or 8GB of storage capacity on a storage device the size of a car key. Another portable storage device offered by Iomega is the pocket-size HDD Desktop external hard drive. It is available from 40GB all the way to 1TB of storage capacity. These devices can be connected to a USB or FireWire port.

Fixed (non-removable) hard disk drives are still the main storage medium for computers today. They can hold more data than any of the removable media types, optical or magnetic. On most personal machines, the operating system, application programs, and user data all reside on one hard drive. The smallest hard drives today, which are typically found in laptops, hold about 20 GB. When purchased separately, hard drives run from 20 GB up to around 750 GB, with the limit continuing steadily upward each year. Another important characteristic when comparing hard disk drives is the speed at which a disk drive rotates, since this limits the rate at which bits can be transferred between the drive and the computer. Slower drives spin at 4200 rpm (i.e. laptop computers); faster ones, at 15,000 rpm. If a hard drive is to be used in a performance-critical application, such as a database server, the data transfer rate is an important consideration. This rate will be affected by several other factors besides the rotation speed of the disk and latency (time it takes to access the first bit of data)—such as the type of disk controller selected and the type of memory architecture the computer uses. Refer to the sections on benchmarking in 2.1.1 Processor Basics and 2.5.2 Bottlenecks to find more information on this subject.