Computers use many different types of storage. The primary storage mechanism in most computers, random access memory (RAM), is extremely short term. The secondary storage device is the hard disk drive, used for long-term data storage. Both primary and secondary computer storage are fully accessible by computers, unlike CDs, which belong to a user-controlled type of storage device called offline storage.
What is Offline Storage?
Offline storage differs from on-board computer storage in that the computer cannot access it in the absence of direct human interaction. In the case of a CD, a user must insert the disk into a drive for the computer to access its data. The user can store the media in another location. Other types of offline storage include USBflash drives, floppy disks and other optical discs.
CDs belong to a family of storage devices known as optical discs. The name comes from how the device is created: using lasers to write and read data on the surface of the disk. The variety of CD types include CD-ROMs, which allows a computer to read data from a disk but not write to it. Alternatively, CD-RW discs are rewritable storage and can be read and written to several times over.
How CDs Work
When a user inserts a CD into an optical disc drive, it spins at high speed with constant rotation. This motion allows the computer to read data at a uniform rate as the laser moves across the disks surface. A mechanism within the drive sends a laser beam along a single track of the disc as it rotates. Though invisible to the human eye, the laser makes contact with tiny pits and landings that reflect the beam into a detector, which interprets the data on the disk.
Similar Storage Devices
Many types of offline storage devices function using the same principles of CDs and CD-ROMs. Digital video dsks (DVDs), are optical discs designed to hold 4.7 to 17 gigabytes of data, whereas the standard CD-ROM holds less than 1 gigabyte of data. Blu-rays are enhancements on the DVD standard, offering up to 27 gigabytes of storage space on each disk.