What is it?
Optical media like CDs and DVDs store data using
microscopic variations in the layers of a (mostly)
plastic disk. The variations are stamped ("pressed"
or "molded") onto the surface of the disk, or "burned" into it using
a laser, in a spiral pattern that runs from the inside to
the outside of the disk. By contrast, "magnetic" media
like hard drives store data using
microscopic variations in electrical charge on a (mostly)
CDs consist of a 1.2-millimeter (mm) clear polycarbonate
plastic disk, with a reflective metal coating on one side.
The standard diameter is 120 mm (with an 80-mm "mini"
format). A laser beamed through the plastic bounces
off the metal surface. Differences in reflection
caused by the variations in the plastic layer are
detected and translated. A standard 120-mm CD holds
about 700 megabytes of data.
DVDs consist of two half-thickness (0.6-mm) CD-like disks
glued back to back, with the reflective coating in the middle.
Using a shorter-wavelength laser allows higher data densities,
so that a single side of a DVD can hold about 4.7
gigabytes (4700 megabytes). Recording on both sides
of the DVD doubles this. (Such DVDs must be flipped
over, like an old-fashioned record.)
Optical disk types: ROM, R, RW, RAM, + and -, DL,
CDs and DVDs come in many formats, with a confusing array of
abbreviations. It's less complicated than it seems at
first, though not necessarily simple.
- The most basic form of data-holding CD or DVD is one that can
only be read. Formally, it is CD-ROM or
DVD-ROM. ROM stands for "read-only memory"
which translates also as "non-writeable." CD-ROMs
and DVD-ROMs are permanently pressed/molded when manufactured.
- A CD-R, DVD-R or DVD+R may be recorded (written on),
but only a single time. The R stands for "recordable." This
"write-once" process uses a light-sensitive dye recording
layer to which information is irreversibly written by means
of a laser heating and altering it, to create a pattern
of marks mimicking those of a prerecorded (pressed/molded)
CD or DVD. This is why the writing process is sometimes
- A CD-RW, DVD-RW or DVD+RW may be recorded (written
on) many times. The RW standards for "rewriteable." The
rewriting process employs a metallic "phase-change"
recording layer that can be repeatedly altered and
restored by the writing laser (approximately 1000 times).
CD or DVD drives capable of writing (as opposed
to just reading) can write to either R or
- Finally, DVD-RAM also uses phase-change technology but
can be rewritten roughly 100,000 times. With its hard
sectors, random access capabilities and optional cartridge,
DVD-RAM more closely resembles traditional hard drive storage
media than do DVD-RW and DVD+RW.
So what's the difference between "+" and "-" for
DVDs? Unfortunately, rival industry groups DVD
Forum and DVD+RW
Alliance could not agree on a data format. Fortunately,
most computers now incorporate "dual-standard" DVD drives,
which can handle either format. You'll see them referenced
as "DVD+/-RW" drives.
As noted, DVDs can use two sides to double recording
capacities. An alternative approach to doubling is provided
by "dual-layer" DVDs -- DVD-R DL and DVD+R DL, depending
on whether the - or + format is supported --
that have two data-holding layers on a single side with a
semi-reflective coating between. The focus of the laser
is changed to access the shallow or deep layer.
Such disks can hold about 8 gigabytes on a single side
(and 16 gigabytes on double-sided DLs). CDs come only
in the single-layer, single-side variety.
Optical drive types
Almost all computers now come with at least an optical drive
capable of reading CDs. Drives capable
of writing data to a CD-R or CD-RW disc -- usually
called CD-RW or CDRW drives -- are a bit more expensive.
We recommend a CD-RW drive for most systems, given the utility
of using CDs for data backup, storage and transfer.
Since data and software are still commonly exchanged
on CD, at least a CD-reading drive is essential.
Most systems now come with an optical drive that capable
of reading DVDs too. Since DVDs are increasingly
used for data (as well as, ubiquitously, for video), having
such a DVD-reading drive is often beneficial. Commonly a
CD-reading/writing and DVD-reading drive is combined into
a single "CD-RW/DVD" drive.
DVD-writing drives can be used for data backup, storage and
transfer. Blank DVD-R/+R discs now cost only about twice
as much as CD-Rs, but hold 6-7 times as much data; so
having a DVD-RW/+RW drive is something to consider if
you need to transfer or backup higher volumes
of data. As noted, most computers now have dual format
DVD drives, so it's not necessary for you to pick
a + or - format. (The price differential between blank
+/- DVD disks is small.) However, remember to purchase
the right kind of media -- + or - as appropriate
-- if you don't have a dual format drive.
RAM and DL DVD formats are relatively expensive, and not
for the typical user. All optical drive types
come in a range of speeds -- denoted by "X" numbers, such
as 16X, 32X, 48X -- denoting the speed of data reading and
writing. As you'd expect, faster drives are more
Data availability and integrity
Optical disks are considered highly stable -- manufacturers
claim life-spans ranging from 50 to 200 years for CD-Rs, 20
to 100 years for CD-RWs, and comparable periods for DVD-/+Rs
and -/+RWs. (Obviously this is only an estimate.
CDs were not in commercial use until the mid 1980s and DVDs
until the mid 1990s.)
Unlike magnetic media, optical storage is generally invulnerable
to electromagnetic damage. CDs and DVDs react badly
to extreme humidity and temperatures, however. Heat
can cause warping, rendering a disk unusable. Cold can
make the disk (more) fragile. Mechanical trauma, causing
damage to the metal reflective or the plastic data layers,
can render all or part of the data on a disk unusable.
Absent such environmental or physical assaults, the main
risk to optical storage comes from their portability.
Anything that can be carried around easily can also be lost
or stolen easily.
Loss or theft of a CD/DVD containing sensitive data
presents security issues. In general, you should not
carry around large amounts of sensitive data on any portable
storage media, for the same reason you shouldn't carry around
a large amount of money. It's too risky. Sensitive
data on CDs or DVDs should be kept locked away, just like
any other valuable possession.
As with folders on hard drives, data can be stored in encrypted
form on optical media to protect against unwarranted disclosures.
Encryption software typically requires setting a password
(that serves as the encryption "key"). The usual considerations
of password safety apply.
Optical disks containing obsolete data can be overwritten
for security if they are of the RW or RAM (rewriteable) variety.
But R and ROM (non-rewriteable) disks must be physically destroyed
to protect data. (For more, see our discussion of secure
data disposal methods.)
Alternatives to optical storage
High volume storage where re-writing is not needed, such
as for archival backup copies, can cheaply use write-once
CD-Rs, DVD-Rs and DVD+Rs. As noted, the long-term productive
life of optical media like CDs and DVDs is claimed to be many
decades. But you should not rely on optical media --
or any other kind of media for that matter -- for your only
backup copy of important data.
With each passing year, hard drives are able to deliver ever
greater capacity, at an ever cheaper cost per stored byte.
Hard drives can generally transfer data much faster than optical
drives, so they make sense for data which must be accessed
frequently as well as data which must be changed frequently.
Note, however, that the capacities and costs of solid-state
flash drives are also improving
each year, and they present a compelling alternative to hard
drives for smaller data volumes (less than 1-2 gigabytes).
Needless to say -- at least we hope it is needless to say
-- whatever data storage medium you use, be sure to keep it
in a physically secure place, as safe as possible from human
and environmental threats.
Optical formats of the future
The CDs and DVDs common today use a "red-laser" (infrared)
technology. The newest DVD formats use a higher-frequency
(shorter-wavelength) "blue-laser" technology that allows much
higher recording densities -- from 15 up to 50 gigabytes per
disk. Unfortunately, the rival "HD DVD" and "Blu-ray"
formats, backed by rival industry groups, are incompatible
with each other.
An overall description of how they work, with links to
technical detail on the various implementations (CDs, DVDs,
CD-R and CD-RW (Optical Storage Technology Association)
Just about everything you could ever want to know about CD
Recordable and Rewritable DVD (Optical Storage Technology
A similarly thorough treatment of the technical details of