Improve your computer literacy
802.11 refers to a family of specifications developed by the
IEEE for wireless LAN technology. 802.11 specifies an over-the-air
interface between a wireless client and a base station or
between two wireless clients. The IEEE accepted the specification
There are several specifications in the 802.11 family:
* o 802.11 - applies to wireless LANs and provides 1 or 2
Mbps transmission in the 2.4 GHz band using either Frequency
Hopping Spread Spectrum (FHSS) or Direct Sequence Spread Spectrum
* o 802.11a - an extension to 802.11 that applies to wireless
LANs and provides up to 54 Mbps in the 5GHz band. 802.11a
uses an orthogonal frequency division multiplexing encoding
scheme rather than FHSS or DSSS.
* o 802.11b (also referred to as 802.11 High Rate or Wi-Fi)
- an extension to 802.11 that applies to wireless LANS and
provides 11 Mbps transmission (with a fallback to 5.5, 2 and
1 Mbps) in the 2.4 GHz band. 802.11b uses only DSSS. 802.11b
was a 1999 ratification to the original 802.11 standard, allowing
wireless functionality comparable to Ethernet.
* o 802.11g - applies to wireless LANs and provides data
rates of over 20 Mbps in the 2.4 GHz band.
All you need
to know about DVDs
What does ‘DVD’ stand for?
‘DVD’ stands for Digital Versatile Disc, but some sources
say that it doesn’t stand for anything anymore.
Typical contents of a DVD movie
- Up to 133
minutes of high-resolution video, in letterbox or pan-and-scan
format, with 720 dots of horizontal resolution. (The video compression
ratio is typically 40:1 using MPEG-2 compression.)
presented in up to eight languages using 5.1 channel Dolby digital
in up to 32 languages
A DVD can also be used to store almost eight hours of CD-quality
music per side.
- DVD picture
quality is better, and many DVDs have Dolby Digital or DTS sound,
which is much closer to the sound you experience in a movie theatre.
- Many DVD
movies have an on-screen index, where the creator of the DVD has
labelled many of the significant parts of the movie, sometimes
with a picture. With your remote, if you select the part of the
movie you want to view, the DVD player will take you right to
that part, with no need to rewind or fast-forward.
- DVD players
are compatible with audio CDs.
- Some DVD
movies have both the letterbox format, which fits wide-screen
TVs, and the standard TV size format, so you can choose which
way you want to watch the movie.
- DVD movies
may have several soundtracks on them, and they may provide subtitles
in different languages.
How they are
DVDs are of
the same diameter and thickness as CDs, and they are made using
some of the same materials and manufacturing methods. Like a CD,
the data on a DVD is encoded in the form of small pits and bumps
in the track of the disc.
A DVD is composed
of several layers of plastic, totalling about 1.2 millimetres thick.
Each layer is created by injection moulding polycarbonate plastic.
This process forms a disc that has microscopic bumps arranged as
a single, continuous and extremely long spiral track of data.
Once the clear
pieces of polycarbonate are formed, a thin reflective layer is sputtered
onto the disc, covering the bumps. Aluminum is used behind the inner
layers, but a semi-reflective gold layer is used for the outer layers,
allowing the laser to focus through the outer and onto the inner
layers. After all of the layers are made, each one is coated with
lacquer, squeezed together and cured under infrared light. For single-sided
discs, the label is silk-screened onto the non readable side. Double-sided
discs are printed only on the non-readable area near the hole in
The data tracks
on DVD are incredibly small - just 740 nanometres separate one track
from the next (a nanometre is a billionth of a metre) and the lengthened
bumps that make up the track are each 320 nanometres wide, a minimum
of 400 nanometres long and 120 nanometres high.
dimensions of the bumps make the spiral track on a DVD extremely
long. If you could lift the data track off a single layer of a DVD,
and stretch it out into a straight line, it would be almost 7.5
miles long! That means that a double-sided, double-layer DVD would
have 30 miles (48 km) of data!
do DVDs outperform CDs?
Higher density data storage
Single-sided, single-layer DVDs can store about seven times more
data than CDs. A large part of this increase comes from the pits
and tracks being smaller on DVDs. Let’s try to get an idea
of how much more data can be stored due to the physically tighter
spacing of pits on a DVD.
The track pitch
on a DVD is 2.16 times smaller, and the minimum pit length for a
single-layer DVD is 2.08 times smaller than on a CD. By multiplying
these two numbers, we find that there is room for about 4.5 times
as many pits on a DVD. So where does the rest of the increase come
overhead, more area
On a CD, there is a lot of extra information encoded on the disc
to allow for error correction. This information is really just a
repetition of information that is already on the disc. The error
correction scheme that a CD uses is quite old and inefficient compared
to the method used on DVDs. The DVD format doesn’t waste as
much space on error correction, enabling it to store more real information.
Another way that DVDs achieve higher capacity is by encoding data
onto a slightly larger area of the disc than is done on a CD.
To increase the storage capacity even more, a DVD can have
up to four layers, two on each side. The laser that reads the disc
can actually focus on the second layer through the first layer.
Here is a list of the capacities of different forms of DVDs:
Capacity - 4.38 GB
Approx. Movie Time - 2 hours
Capacity - 7.95 GB
Approx. Movie Time - 4 hours
Capacity - 8.75 GB
Approx. Movie Time - 4.5 hours
Capacity - 15.9 GB
Time - Over 8 hours
You may be wondering why the capacity of a DVD doesn’t double
when you add a whole second layer to the disc. This is because when
a disc is made with two layers, the pits have to be a little longer
on both layers than when a single layer is used, thus reducing the
total number of pits that can be etched onto the disk. This helps
to avoid interference between the layers, which would cause errors
when the disc is played.
in by Thushara Perera
Source: Hardware Jungle