Optical storage devices

Common optical storage devices are a CD-A (CD-Audio), CD-ROM, CD-R, CD-RW and DVD (Digital Versatile Disc).


Diagram: Data stored optically on a CD
The lower surface of a CD is covered with microscopic bumps called 'pits' and 'lands'. These bumps, stamped onto the aluminium surface, represent the binary data. Each pit/land and land/pit step represents a single binary 1. Flat areas of pits and lands represent a sequence of binary 0s. Every binary 1 is followed by at least three 0s. As it is impossible to stamp two 1s next to each other.
Data is read by focusing a laser on the disc surface and measuring the changes in the reflected light caused by the pit/land boundaries. These changes are converted into the sequence of 1s and 0s.
CDs use a single track of data in the form of a spiral. if you stretched out the track it would half a micron wide (one millionth of a millimetre) and almost 5km long.


Writeable CDs use a similar method of data storage. CD-Rs have a layer of green dye above a reflective, gold coated surface. When writing data to a CD-R a heating laser is used to make the dye transparent in places along the track. This is the 'burning' process. These changes in the dye layer represent the 'pits' and 'lands'. These changes are permanent. A second reading laser is used to read data from the CD.


Similar to CD-R, CD-RW uses a dye layer that can be made transparent but can be changed back again to non-transparent. This is done by using a variety of heat temperatures. The dye changes when exposed to different heat temperatures.


Uses the same technology as CDs. The pits and lands are smaller and closer together so that much more data can be packed onto the surface. Both the top and bottom disc surfaces can be used with up to two separate data layers. This gives DVDs a data storage of up to 17 gigabytes, which is the equivalent of 26 CDs. DVD-RWs use a similar technology to CD-RWs to write, erase and change data.

Network storage devices

Computers linked together in networks to share data and resources are a common feature of many businesses and schools. In most networks there is one computer setup as a fileserver. This computer is responsible for managing and storing all the files used by the workstations, as well as protecting the system from unauthorised users. The fileserver does not need to be faster than normal computers, but will need a lot more storage space.

An example of a typical network with a file server

Flash storage devices

Commonly used in digital cameras, MP3 players, plug-in game cards for video game systems and the BIOS chips that control your PC. There are several types of flash storage. They have a higher cost compared to other devices.

Flash memory storage devices are fast and do not require any power to keep data in storage. Data is written into the chip by applying an electric charge to each memory location. The contents can be erased quickly.

CompactFlash and SmartMedia

These are types of flash storage devices popular in portable devices because of their small size and large data capacities. SmartMedia cards range from 2MB to 128MB. CompactFlash cards are physically bigger than SmartMedia cards, allowing them to store more data, 256MB+.

Flash RAM uses less power but requires a small electric current to 'remember' its stored data.