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Storage of Remotely Sensed Data 89

but only once. Once the CD is “burned,” its content becomes
permanent and cannot be erased. Unlike CD-R, special rewritable
CDs or CD-RW allow data to be written to them up to a thousand
times. Similar to CD-R, they can be extended, but not overwritten.
However, the writing session must be closed before its content can be
read in a CD drive. Usually, the rewrite speed is slower than the write
speed, both of which can reach hundreds of kilobytes per second.
Read speed is much faster, though. The limit of CD-ROM is that
special software and the CD-R or CD-RW drive are essential in saving
data to them. This process of data writing is cumbersome and lengthy
because all the data to be stored in a recordable or rewritable CD have
to be queued up first and then burned to a CD in one transaction.

3.2.2 Digital Versatile Disk (DVD)
Similar to a CD, a DVD is also an optical, read-only storage media
that is recordable and rewritable. It has the same physical dimension
as a CD. Thus, all CD drives can recognize DVDs. At least the newest
models of DVD drives are backward compatible with current CD
media. At a thickness of 1.2 mm, a DVD has four storage capacity
levels, 4.7, 8.54, 9.4, and 17.08Gb, depending on the disk structure.
This storage capacity is at least seven times larger than that of a CD.
Data recording can be single layered or double layered, single sided
or double sided. DVDs also offer fast random access like in hard
drives and CDs. Thanks to all of its similarities to a CD, a DVD is
considered to be a future CD replacement (eMag Solutions, 2006).

3.2.3 Memory Sticks
Universal Serial Bus (USB) memory sticks, also known as USB flash
drives or pen drives, are a recent addition to the vast range of storage
media. There are an increasing number of brands of USB memory
sticks on the market. All of them tend to have a standardized physical
size, typically 9 to 10 cm long by 2.5 cm wide by 1.2 cm thick (2″ by
0.25″ by 0.75″ net size) (Fig. 3.4). At such a compact size, as small as a
thumb, it is even more portable and less subject to physical damage
than a CD because it is encapsulated inside a plastic shield with no
parts moveable. This new generation of storage media has greatly
expanded storage capacity that usually ranges from 512Mb to 1Gb.
Larger capacities such as 2 and 4Gb are also available, but are
disproportionately more expensive. Like CDs, these media are also
write protectable. Content protection can be turned on or off at will
by switching a tiny latch on the side of the drive. Access speed is very
fast, as high as a few megabytes per second for reading and up to
1Mb/s for writing. Some USB memory sticks are shock resistant, and
can retain data for more than 10 years.
USB memory sticks are a convenient way of transferring a large
amount of data among different users and between different machines.