CD

Physical details

A CD is made from 1.2 mm thick (.047 inches), almost-pure polycarbonate plastic and weighs 15–

20 grams.From the center outward, components are: the center (spindle) hole, the first-transition area

(clamping ring), the clamping area (stacking ring), the second-transition area (mirror band), the

information (data) area, and the rim.

A thin layer of aluminum or, more rarely, gold is applied to the surface making it reflective. The metal is

protected by a film of lacquer normally spin coated directly on the reflective layer. The label is printed on

the lacquer layer. Common printing methods for CDs are screen-printing and offset printing.

CD data are stored as a series of tiny indentations known as "pits", encoded in a spiral track molded into

the top of the polycarbonate layer. The areas between pits are known as "lands". Each pit is approximately

100 nm deep by 500 nm wide, and varies from 850 nm to 3.5 µm in length.

The distance between the tracks, the pitch, is 1.6 µm. A CD is read by focusing a

780 nm wavelength (near infrared)semiconductor laser through the bottom of the polycarbonate layer. The

change in height between pits (actually ridges as seen by the laser) and lands results in a difference in

intensity in the light reflected. By measuring the intensity change with a photodiode, the data can be read

from the disc.

The pits and lands themselves do not directly represent the zeros and ones of binary data. Instead,Non-

return-to-zero, inverted (NRZI) encoding is used: a change from pit to land or land to pit indicates a one,

while no change indicates a series of zeros. There must be at least two and no more than ten zeros between

each one, which is defined by the length of the pit. This in turn is decoded by reversing the  eight-to-

fourteen modulation used in mastering the disc, and then reversing the Cross-Interleaved Reed-Solomon

Coding, finally revealing the raw data stored on the disc.

CDs are susceptible to damage from both normal use and environmental exposure. Pits are much closer to

the label side of a disc, enabling defects and contaminants on the clear side to be out of focus during

playback. Consequently, CDs are more likely to suffer damage on the label side of the disk. Scratches on

the clear side can be repaired by refilling them with similar refractive plastic, or by careful polishing.

Diagram of CD layers.

A. A polycarbonate disc layer has the data encoded by using bumps.

B. A shiny layer reflects the laser.

C. A layer of lacquer helps keep the shiny layer shiny.

D. Artwork is screen printed on the top of the disc.

E. A laser beam reads the CD and is reflected back to a sensor, which converts it into

electronic data

Disc shapes and diameters

The digital data on a CD begins at the center of the disc and

proceeds toward the edge, which allows adaptation to the different

size formats available. Standard CDs are available in two sizes. By

far, the most common is 120 mm in diameter, with a 74- or 80-minute audio capacity and a 650 or

700 MB data capacity. This diameter has been adopted by subsequent formats, includingSuper Audio

CD, DVD, HD DVD, and Blu-ray Disc. 80 mm discs ("Mini CDs") were originally designed for

CD singles and can hold up to 24 minutes of music or 210 MB of data but never became popular. Today,

nearly every single is released on a 120 mm CD, called a Maxi single.

Novelty CDs are also available in numerous shapes and sizes, and are used chiefly for marketing. A

common variant is the "business card" CD, a single with portions removed at the top and bottom making

the disk resemble a business card.

Understanding the CD: The Spiral

A CD has a single spiral track of data, circling from the inside of the disc to the outside. The fact that the

spiral track starts at the center means that the CD can be smaller than 4.8 inches (12 cm) if desired, and in

fact there are now plastic baseball cards and business cards that you can put in a CD player. CD business

cards hold about 2 MB of data before the size and shape of the card cuts off the spiral.

What the picture on the right does not even begin to impress upon you is how incredibly small the data

track is -- it is approximately 0.5 microns wide, with 1.6 microns separating one track from the next. (A

micron is a millionth of a meter.) And the bumps are even more miniscule.

Physical sizeAudio

Capacity

CD-ROM Data

CapacityNote

12 cm 74–99 min 650–870 MB Standard size

8 cm 21–24 min 185–210 MB Mini-CD size

85x54 mm -

86x64 mm~6 min 10-65 MB

"Business card"

size

Understanding the CD: Bumps

The elongated bumps that make up the track are each 0.5 microns wide, a minimum of

0.83 microns long and 125 nanometers high. (A nanometer is a billionth of a meter.)

Looking through the polycarbonate layer at the bumps, they look something like this:

You will often read about "pits" on a CD instead of bumps. They appear as pits on the aluminum side, but

on the side the laser reads from, they are bumps.

The incredibly small dimensions of the bumps make the spiral track on a CD extremely long. If you could

lift the data track off a CD and stretch it out into a straight line, it would be 0.5 microns wide and almost

3.5 miles (5 km) long!

CD player Components

The CD player has the job of finding and reading the data stored as bumps on the CD. Considering how

small the bumps are, the CD player is an exceptionally precise piece of equipment. The drive consists of

three fundamental components:

A drive motor spins the disc. This drive motor is precisely controlled to rotate between 200

and 500 rpm depending on which track is being read.

A laser and a lens system focus in on and read the bumps.

A tracking mechanism moves the laser assembly so that the laser's beam can follow

the spiral track. The tracking system has to be able to move the laser at micron

resolutions.

Inside a CD player

What the CD Player Does: Laser Focus

Inside the CD player, there is a good bit of computer technology involved in forming the data into

understandable data blocks and sending them either to the DAC (in the case of an audio CD) or to the

computer (in the case of a CD-ROM drive).

The fundamental job of the CD player is to focus the laser on the track of bumps. The laser beam passes

through the polycarbonate layer, reflects off the aluminum layer and hits an opto-electronic device that

detects changes in light. The bumps reflect light differently than the "lands" (the rest of the aluminum

layer), and the opto-electronic sensor detects that change in reflectivity. The electronics in the drive

interpret the changes in reflectivity in order to read the bits that make up the bytes.

What the CD Player Does: Tracking

The hardest part is keeping the laser beam centered on the data track. This centering is the job of

thetracking system. The tracking system, as it plays the CD, has to continually move the laser outward.

As the laser moves outward from the center of the disc, the bumps move past the laser faster -- this

happens because the linear, or tangential, speed of the bumps is equal to the radius times the speed at

which the disc is revolving (rpm). Therefore, as the laser moves outward, the spindle motor must slow the

speed of the CD. That way, the bumps travel past the laser at a constant speed, and the data comes off the

disc at a constant rate

Types Of CD

Recordable CD-Recordable Compact Discs, CD-Rs, are injection molded with a "blank" data spiral. A

photosensitive dye is then applied, after which the discs are metalized and lacquer-coated. The write laser

of the CD recorder changes the color of the dye to allow the read laser of a standard CD player to see the

data, just as it would with a standard stamped disc. The resulting discs can be read by most CD-ROM

drives and played in most audio CD players.

CD-R recordings are designed to be permanent. Over time the dye's physical characteristics may change,

however, causing read errors and data loss until the reading device cannot recover with error correction

methods. The design life is from 20 to 100 years, depending on the quality of the discs, the quality of the

writing drive, and storage conditions. However, testing has demonstrated such degradation of some discs

in as little as 18 months under normal storage conditions.This failure is known as CD rot. CD-Rs follow

the Orange Book standard.

Recordable Audio CD- The Recordable Audio CD is designed to be used in a consumer audio CD

recorder. These consumer audio CD recorders use SCMS (Serial Copy Management System), an early

form of digital rights management (DRM), to conform to the AHRA (Audio Home Recording Act). The

Recordable Audio CD is typically somewhat more expensive than CD-R due to (a) lower volume and (b) a

3% AHRA royalty used to compensate the music industry for the making of a copy.

High Capacity Recordable CD- A higher density recording format that can hold:

98.5 minutes of audio on a 12 cm disc (compared to about 80 minutes for Red Book audio).

30 minutes of audio on an 8 cm disc (compared to about 24 minutes for Red Book audio).

ReWritable CD- CD-RW is a re-recordable medium that uses a metallic alloy instead of a dye. The write

laser in this case is used to heat and alter the properties (amorphous vs. crystalline) of the alloy, and hence

change its reflectivity. A CD-RW does not have as great a difference in reflectivity as a pressed CD or a

CD-R, and so many earlier CD audio players cannot read CD-RW discs, although most later CD audio

players and stand-alone DVD players can. CD-RWs follow the Orange Book standard.

High Speed ReWritable CD- Due to technical limitations, the original ReWritable CD could be written

no faster than 4x speed. High Speed ReWritable CD has a different design that permits writing at speeds

ranging from 4x to 12x.

Original CD-RW drives can only write to original ReWritable CDs. High Speed CD-RW drives can

typically write to both original ReWritable CD discs and High Speed ReWritable CD discs. Both types of

CD-RW discs can be read in most CD drives.

Higher speed CD-RW discs, Ultra Speed (16x to 24x write speed) and Ultra Speed+ (32x write speed), are

now available.

DIAGRAMS-:

CD STRUCTURE- http://www.tanhowsay.com/Program/cdstruct.html

How the Compact Disc Work.- http://www.tanhowsay.com/Program/cdwork.html

DVD

Introduction

DVD initially stood for Digital Video Disc but now stands for Digital Versatile Disc. Like a CD, DVD is an optical storage system for read-only, recordable and rewritable applications. But, being similar to a CD in many ways, DVD is considered to be a CD future replacement.

The main features of the DVD formats can be summarized as follows:

• Backwards compatibility with current CD media (at least the newest models of DVD drives)

• Physical dimensions identical to compact disc with total thickness equal to 1.2 mm, but with

capacity at least 7 times larger than that of CD.

• Capacities of 4.7 GB, 8.54 GB, 9.4 GB, and 17.08 GB, depending on the disk structure.

• Single-layer/dual-layer and single/double sided options.

• DVD replication process is similar to that used for compact disks.

• A disc-based format means fast random access like in hard drives and CDs and unlike tapes.

• Designed from the outset for video, audio and multimedia. Meets the requirement for 133 minutes

of high quality video on one side of a disk.

• DVD-ROM for enhanced multimedia and games applications.

• DVD-Video for full length high quality movies on one disc.

• DVD-Audio for higher quality music, surround sound and optional video, graphics and other features.

• All formats use a common file system.

• Copy protection built into standard (unless it is broken...)

DVD and Blu-ray Discs: A Closer Look

f you place a CD, a DVD, and a Blu-ray Disc™ next to each other, they look nearly identical. But if you try to play a DVD in your CD player, or a Blu-ray Disc in your DVD player, it won't read it. It can't because each type of disc has a different physical structure, data format, and error correction system. Below we'll discuss what makes a DVD different from a CD. And on page 2, we'll talk about what makes Blu-ray unique.

DVD

Where a CD is a single 1.2-millimeter-thick disc, all DVDs use a "sandwich" design — two 0.6-millimeter discs bonded together. This sandwich construction allows DVD discs

to have information on both sides and on one or two layers per side. This design is also more structurally stable and resistant to disc warping.

Compared to CD, DVD uses smaller data pits and more closely-spaced pit rows of "tracks," as illustrated above. This increased data density makes it possible for each layer on a DVD to hold more than six times the amount of data on a CD. DVD players use a red laser in place of the infrared laser found in CD players. The red laser's shorter wavelength, combined with a special lens design, result in a narrower, more tightly focused laser beam that can easily read the smaller pits.

DVD disc types: It's about sides and layers

In order to fit multiple movie formats, plus the common extra material like interviews, commentaries, and outtakes, movie studios are making greater use of DVD's dual-sided and dual-layered capabilities. All DVD players can play all of these different disc types. Some newer players may be a little quicker at handling layer changes on dual-layer discs.

Single-sided, single-layer: Even DVD discs with a single information layer can hold the complete audio and video for a full-length movie, including Dolby® Digital 5.1 soundtracks in three different languages. Total capacity: 4.4 gigabytes (over 2 hours of video).

Double-sided, single-layer: Most discs of this type include a movie version formatted for a standard TV screen on one side, and a widescreen version on the other side. Only a few titles spread a single version over two sides. In these instances, you'll need to manually flip the DVD over when it reaches the end of side 1 (only a few recent mega changers are able to change DVD sides). Total capacity: 8.75 gigabytes (about 4.5 hours of video).

Single-sided, dual-layer: This disc type has two information layers, providing nearly twice the data capacity of a single-layer DVD. The layer closer to the player's laser pickup has a semi-transparent coating. The laser is able to shine through this layer to read the deeper layer beneath it, then re-focus to read the semi-transparent layer. One way to tell if a DVD is dual-layer is to look at the disc's playing surface — single-layer discs are silver while dual-layer discs are gold. Total capacity: 8 gigabytes (about 4 hours of video).

Double-sided, dual-layer: This disc type provides the maximum data capacity. It's basically two single-sided, dual-layer discs bonded together. So far, only a few movie transfers have been in this format (Ben-Hur is one). As with all double-sided DVDs, unless you own one of the dual-side-play mega changers mentioned above, you'll have to flip the DVD over yourself. Total capacity: 15.9 gigabytes (over 8 hours of video).

DVD Layers

DVD Configurations and Basic design

CD Players and CD-ROM drives use an infrared laser working at a wavelength of 780 nanometers. Since the wavelength is one of the parameters responsible for the beam diameter, which translates into smaller and denser bits, the new DVD Players and DVD-ROM drives use the red laser working at 650 nm and 635 nm wavelengths.

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, totaling about 1.2 millimeters thick. Each layer is created by injection molding polycarbonate plastic. This process forms a disc that has microscopic bumps arranged as a single, continuous and extremely long spiral track of data. More on the bumps later.

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 nonreadable side. Double-sided discs are printed only on the nonreadable area near the hole in the middle. Cross sections of the various types of completed DVDs (not to scale) look like this:

Each writable layer of a DVD has a spiral track of data. On single-layer DVDs, the track always circles from the inside of the disc to the outside. That the spiral track starts at the center means that a single-layer DVD can be smaller than 12 centimeters if desired

What the image below cannot impress upon you is how incredibly tiny the data track is -- just 740 nanometers separate one track from the next (a nanometer is a billionth of a meter). And the elongated bumps that make up the track are each 320 nanometers wide, a minimum of 400 nanometers long and 120 nanometers high. The following figure illustrates looking through the polycarbonate layer at the bumps.

You will often read about "pits" on a DVD instead of bumps. They appear as pits on the aluminum side, but on the side that the laser reads from, they are bumps.

The microscopic 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! To read bumps this small you need an incredibly precise disc- reading mechanism.

Types of discs

A DVD disc is comprised of two 0.6 mm discs bonded together. Each of these discs has two sides. It is

possible to use both sides of a disc for storing information. The DVD format provides several configurations of data layers, moving from 2D storage towards 3D storage. Each configuration is designed to provide additional storage capacity:

DVD-R, D-RW or RAM

Recordable and rewritable DVD drives are clearly more complex than the DVD-ROM drives, since they require lasers with different power levels for reading, erasing, and writing. DVD- R media operates on a principle similar to the CD-R principle. The laser burns marks in a special dye layer and locally changes its reflectivity. Since the DVD-R uses a shorter- wavelength laser, it is incompatible with the green recordable media of CD-R, and another laser is required to solve this problem.

With the rewritable DVD, compatibility problems occur with the laser power-levels, since the drive needs different laser powers to record, read and erase for both DVD and CD media. Some of today's DVD players can read all of the most important formats, such as DVD- RAM, DVD-ROM, DVD-Video, DVD-R, CD-Audio, CD-ROM, CD-R, CD-RW, and video CD but are, understandably, quite expensive.

DVD-ROM

Like a CD, a DVD-ROM is a pre-recorded disk. DVD-ROM is used to store general data, as well as video and audio information needed for multimedia applications and computer games. DVD-ROM satisfies the following requirements:

• Backward compatibility with CD-ROMs

• Forward compatibility with the future recordable (R) and rewritable (RW) disks

• Single format for computer and TV applications

• Single file system for all data types and media types

The backward compatibility of the DVD drives means that it will read both CD-ROM and CD-audio, which makes them a great replacement for CD-drives. Because of higher bit density and other advantageous features, even a 5x-speed DVD drive will read the CD at the rate equivalent to about 40x for the regular CD drive. For now, DVD drives are, in general, more expensive, and require special MPEG-2 hardware or software decoders to read the compressed data. To have the best video quality, the hardware approach is better unless the fastest processors are used. This clearly makes DVD-ROM a computer storage of the near future, especially for databases, multimedia, games, interactive video, etc.

DVD-Video and DVD-Audio

One of the reasons for the success of DVD technology is the DVD-Video formats. DVD video application is strongly dependent on data compression, since at the bit rate of 167 Mbps, the 4.7 gigabyte capacity of a standard DVD would be enough to store roughly 4 hours of digital video. This provides for the nominal 133 minutes of playing time for DVD-5. Longer movies should use a dual-layer technology (DVD-9). The data on the first layer start at the inside of the disk and end at the outside, where the data on the second layer start thus providing uninterrupted playback.

Two types of video compression standards could be used for DVD: MPEG-1 and MPEG-2, but only MPEG-2 video data can be copy protected and region coded (MPEG stands for the Moving Picture Experts Group). Therefore, the same techniques of copy protection as are currently used for CDs are being adopted for DVD.

Like all compression algorithms, MPEG-2 analyzes repetition in the video signal, called redundancy, and tries to get rid of it. MPEG-2 is capable of 'filtering' about 97% of the data in the video signal without significantly degrading the quality of the picture. This allows recording of 133 minutes on a 4.7 GB disk at a much lower bit rate than required by the digital video standard.

DVD's direct data access allows interactivity and direct access to the movie episodes or other information of the disk. On the other hand, to provide additional copy protection, most DVDs have so-called regional coding, making it impossible to play the same disk in different regions, since most DVD-Videos are made for a specific region or country and not for free world-wide use. There are 6 regions used for DVD-Video coding:

Name Description

DVD-R

A write once format, readable on the majority of DVD players. There are 2 separate versions, DVD-R(G) and DVD-R(A), which uses slightly different wavelength lasers to write to the disc. DVD-R(G) discs cannot be written in a DVD-R(A) writer (and vice versa), but both are playable in about 90% of DVD players. Disc capacity is 4700 million bytes (4.37 Gb).

DVD-RAM

This format is best suited for PC use, since it has better resistance against errors, has faster random access and is re-writable. It is compatible with fewer DVD players than other formats, making it less suitable for audio and video applications.

DVD-RW Also known as DVD-R/W or DVD-ER, this re-writable format is playable on many DVD players.

DVD+RW DVD+RW is a re-writable format which has good compatibility with newer DVD players.

DVD+RThe write once sister of DVD+RW, this format is more similar to DVD-R than DVD+RW, being dye based. It also has good compatibility with newer DVD players.

Technical Specifications DVD-Video

Feature Description

Video Resolutions720x480, 704x480, 352x480 and 352x240 (NTSC). 720x576, 704x576, 352x576, and 352x288 (PAL).

Video Compression MPEG-1 or MPEG-2

Video Bitrate Up to 9.8 Mbps variable bitrate (VBR)

Audio Compression MPEG-1 layer 2, MPEG-2, Dolby Digital (AC3), DTS, PCM (uncompressed audio)

Audio Bitrate (Dolby Digital)

64 kbps to 448 kbps

Audio Bitrate (MPEG) 32 kbps to 912 kbps

Audio Bitrate (DTS) 64 kbps to 1536 kbps

Surround Sound MPEG-2 5.1 or 7.1, Dolby Digital, Digital Theater Systems Digital Surround (DTS).

Maximum audio streams Up to 8 (each with up to 8 channels)

Other features Multiple camera angles, menus and interactive functionality

Still picture resolutions Up to 720 x 480 or 720 x 576

Denser Data

The increased capacity of DVD discs is not only a result of more layers. The information on a DVD disc is recorded more densely than on a conventional CD. By narrowing the track pitch - the width of the track which contains the pits - it is possible to fit more data on the disc. In developing DVD technology, the track pitch could be reduced to 0.74 micrometres from 1.6 micrometres of a conventional CD - less than half the previous width. Equally important was the shortening of the minimum pit length. On CDs, the minimum pit length is slightly more than 0.8 micrometres. On DVDs, it is 0.4 micrometres. In short, the three major developments for increasing data capacity are multi-layer capability, a narrower track width and a shorter pit.

Dual-layer Structure

The dual-layer structure allows data to be recorded on 2 layers of each disk side. The uppermost layer is semitransparent, allowing it and the lower fully reflective layer to be read using only one laser pickup. In order to read the lower layer the laser pickup is focused through the semi-transparent upper layer onto the lower layer.

To read the data on the upper layer, the laser pickup is simply refocused once more onto the upper layer. To achieve the maximum storage capacity, two dual layer discs are bonded together. Since both discs can store up to 8.5 GB, a total of 17 GB can be stored.

Dual Lens System

In order to read the tighter track widths on a DVD disc, lasers that produce a shorter wavelength beam of light are required. More accurate aiming and focusing mechanisms are also needed. DVD uses a red-light laser with a wavelength of 640nanometres that not only reads the pits but also guides the laser on the pitch track. Conventional CD technology utilizes an infrared laser with a wavelength of

780nanometres.

Backward compatibility with CDs means that one device must read and interpret both CDs and DVD discs. In order to solve the problem of reading differing track widths and pit lengths, a dual lens system was developed.

Different lenses must be used to achieve the optimum focus characteristics necessary for these different standards. The two lenses are rotated horizontally to read signals for each disc. In fact, the focusing mechanism is the technology that allows data to be recorded on two layers. To read the second layer, the reader simply focuses the laser a little deeper into the disc, where the second layer of data is recorded.

Of course, all of this is done automatically when different disc are put in the drives or players. The whole system is electronically controlled for maximum precision.

Red laser vs. Blue laser

Current DVD drives use red lasers (630 to 650 nm), and the "easiest" way to increase aerial density is to switch to shorter-wavelength lasers, i.e. blue or violet lasers with wavelengths as low as 400 nm. This will make possible about 15 GB of data per layer per side. To achieve, say, 45 GB of data per side per layer, even shorter, UV (Ultra Violet) range lasers will be needed. Still, compact, reliable, and inexpensive short-wavelength lasers are hard to make.

Three primary blue-laser technologies are available now:

• ZnSe lasers - ZnSe lasers brought the first success to the field, but these lasers have problems with relatively

short life-time at the required power levels, and also are at the green end of the blue range (460 to 520 nm).

• GaN lasers - GaN In-doped lasers have already demonstrated high reliability at wavelengths as short as 370

nm and are considered to be a very promising future technology.

• Second-Harmonic Generation (SHG) lasers - SHG lasers offer the best durability at the moment. This

technology either doubles the frequency of a given infrared laser or directly generates a second harmonic in the

blue portion of the spectrum. For example, for a given infrared laser with a wavelength of 850 nm, this technology

will double the laser light frequency (using a so-called distributed Bragg reflector or DBR), and produce blue light

at 425 nm.

Advantages of DVD

When it was developed, the only serious competition of DVD was from 3.5 inch floppies. The floppies had been in existence for quite a long time and had slowly been coming down in size from 10 inch to 5.5 inches and then to the

standard 3.5 inches.

High density advantage of DVD: The capacity of floppies remained at a measly 1.44 MB, whereas the CD could store 700 MB and the DVD could store initially 4.5 GB and now you can have double sided double layer storing up to 17 GB. This had the advantage of storing 11000 times more data at just double the size of floppy.

Cost advantage of DVD: The cost of blank DVD is just 4 to 10 times that of the floppy or the audio CD, but the data storage capacity is huge and thus the cost per bit of data stored comes down considerably. With the costs coming down rapidly, the cost advantage of DVD becomes further obvious.

Duplication advantage of DVD: With DVD writers becoming just as cheap as the CD writers, the cost of carrying of data with you has reduced considerably. You can carry the data as cheaply and easily as the floppies themselves. You can do this without bothering about the costs.

This makes transfer of data quick and easy. Imagine sending 17 GB of data over 256 KBPS modem or through floppies and you will understand the advantage immediately. You might require remaining connected over the internet for more than one year or sending 11000 floppies instead of just one DVD

DVD format compatibility

As you may know it, not all DVD recorders are compatible with each other. And some of them have compatibility issues with your home DVD player.

Blu-ray raises disc technology to a new level

The DVD format was certainly a huge leap compared to VHS tapes, but it has major shortcomings as an entertainment medium for the high-definition era. DVD's compression scheme and disc structure were designed for standard-definition video. As TVs have grown bigger and better, the limitations of the DVD format have become

more apparent. When watching DVDs on some of the better 1080p HDTVs with screens of 50" or larger, compression noise and artifacts are sometimes noticeable. Blu-ray, on the other hand, offers 1080p resolution for an incredibly smooth, detailed picture.

HD's much higher level of picture detail requires much more information. So, any high-definition format requires much higher data storage capacity (measured in gigabytes). Here's an example: a digital recorder with a 250GB hard drive can store about 200 hours of standard-definition video, but only about 30 hours of HD video. HD's superior picture quality also requires much faster data transfer rates (often called "bit rates") from the player to your TV (measured in megabits per second — Mbps). If the flow of information from a DVD player to a TV could be characterized as a babbling brook, the flow from a high-definition player would be a roaring river.

  DVD-Video Blu-ray Disc

Disc capacity (gigabytes)single-layer (4.7GB); dual-layer (8.5GB)

single-layer (25GB); dual-layer (50GB)

Maximum picture resolution (pixels)

720 x 480 (SDTV) 1920 x 1080 (HDTV)

Maximum data transfer rate for movie playback (Megabits per second)

11Mbps 54Mbps

Video codecs MPEG-2 AVC MPEG-4, VC-1, MPEG-2

Audio codecs Dolby Digital, DTSDolby® Digital, Dolby Digital Plus, Dolby TrueHD (lossless), DTS®, DTS-HD™ High Resolution Audio, DTS-HD Master Audio (lossless)

Content protectionContent Scrambling System (CSS) 40-bit, region coding

Advanced Access Content System (AACS) 128-bit, BD+, ROM Mark, region coding

As the chart makes clear, Blu-ray discs provide much greater data storage capacity and faster bit rates than standard DVD. Translation: much improved picture and sound quality.

Blu-ray is the next-generation digital video disc. It can record, store and play back high-definition video and digital audio, as well as computer data. The advantage to Blu-ray is the sheer amount of information it can hold:

A single-layer Blu-ray disc, which is roughly the same size as a DVD, can hold up to 27 GB of data -- that's more than two hours of high-definition video or about 13 hours of standard video.

A double-layer Blu-ray disc can store up to 50 GB, enough to hold about 4.5 hours of high-definition video or more than 20 hours of standard video. And there are even plans in the works to develop a disc with twice that amount of storage.

Discs store digitally encoded video and audio information in pits -- spiral grooves that run from the center of the disc to its edges. A laser reads the other side of these pits -- the bumps -- to play the movie or program that is stored on the DVD. The more data that is contained on a disc, the smaller and more closely packed the pits must be. The smaller the pits (and therefore the bumps), the more precise the reading laser must be.

Unlike current DVDs, which use a red laser to read and write data, Blu-ray uses a blue laser (which is where the format gets its name). A blue laser has a shorter wavelength (405 nanometers) than a red laser (650 nanometers). The smaller beam focuses more precisely, enabling it to read information recorded in pits that are only 0.15 microns (µm) (1 micron = 10-6 meters) long -- this is more than twice as small as the pits on a DVD. Plus, Blu-ray has reduced the track pitch from 0.74 microns to 0.32 microns. The smaller pits, smaller beam and shorter

track pitch together enable a single-layer Blu-ray disc to hold more than 25 GB of information -- about five times the amount of information that can be stored on a DVD.

Source: Blu-ray Disc Association

Each Blu-ray disc is about the same thickness (1.2 millimeters) as a DVD. But the two types of discs store data differently. In a DVD, the data is sandwiched between two polycarbonate layers, each 0.6-mm thick. Having a polycarbonate layer on top of the data can cause a problem called birefringence, in which the substrate layer refracts the laser light into two separate beams. If the beam is split too widely, the disc cannot be read. Also, if the DVD surface is not exactly flat, and is therefore not exactly perpendicular to the beam, it can lead to a problem known as disc tilt, in which the laser beam is distorted. All of these issues lead to a very involved manufacturing process.

Fitting more data on the disc

Even though high-definition video requires so much more data, high-def discs can easily hold even the longest movies on a single disc. Blu-ray discs can hold multiple hours of HD content, with plenty of room to spare for the bonus features you may have grown accustomed to with DVD. The developers of Blu-ray couldn't make the disc physically larger, so in order to significantly increase the information storage capacity, they increased the data density. The information pits got smaller, and the spacing of the pit rows got tighter (see illustration below). The discs also have a super-thin transparent protective coating, which places the data layer closer to the disc's surface and thus closer to the laser. In order to read these much smaller data pits, Blu-ray players use a blue-violet laser, which has a shorter wavelength and a smaller "beam spot" than the red laser used in DVD players. The players also spin the discs at higher speeds for even faster data transfer.

Putting high-definition video on a disc requires much higher storage capacity than DVDs allow. Compared to

DVD and HD DVD, Blu-ray discs have smaller data pits and more closely spaced pit rows.

Storing high-definition video requires much higher data density than standard DVDs allow. Blu-ray discs have smaller data "pits" and more closely spaced pit rows compared to DVDs and HD DVDs. Blu-ray players require a blue laser to read these smaller pits. In Blu-ray players, the laser's higher "numerical aperture" (NA) allows the beam to be focused to create a tighter spot for reading smaller pits.

How Blu-ray Reads Data

The Blu-ray disc overcomes DVD-reading issues by placing the data on top of a 1.1-mm-thick polycarbonate layer. Having the data on top prevents birefringence and therefore prevents readability problems. And, with the recording layer sitting closer to the objective lens of the reading mechanism, the problem of disc tilt is virtually eliminated. Because the data is closer to the surface, a hard coating is placed on the outside of the disc to protect it from scratches and fingerprints.

Source: Blu-ray Disc Association

The design of the Blu-ray discs saves on manufacturing costs. Traditional DVDs are built by injection molding the two 0.6-mm discs between which the recording layer is sandwiched. The process must be done very carefully to prevent birefringence.

1. The two discs are molded. 2. The recording layer is added to one of the discs.

3. The two discs are glued together.

Blu-ray discs only do the injection-molding process on a single 1.1-mm disc, which reduces cost. That savings balances out the cost of adding the protective layer, so the end price is no more than the price of a regular DVD.

Blu-ray also has a higher data transfer rate -- 36 Mbps (megabits per second) -- than today's DVDs, which transfer at 10 Mbps. A Blu-ray disc can record 25 GB of material in just over an hour and a half.

Codecs At the core of all recent digital entertainment forms is the concept of data "compression." Compression is needed to squeeze digital content so that it takes up a minimum of storage space. Compression is what made video formats like DVD and HDTV possible, as well as audio formats like Dolby Digital and MP3. The digital data is compressed for transmission or encoding on a disc, and then decompressed by your player. These compression/decompression technologies are often referred to as "codecs" for short.

MPEG-2 is the video codec used for DVDs and current HDTV content, including broadcast, cable and most satellite TV. Blu-ray also uses MPEG-2, as well as two newer, higher-efficiency codecs: AVC MPEG-4 and VC-1 (based on Windows Media Video 9). Because Blu-ray employs such high bit rates (54Mbps, compared to 19.2Mbps for over-the-air HDTV), the picture quality of Blu-ray discs is exceptionally clean, with fewer visible compression artifacts.

The expanded storage capacity of Blu-ray also makes it possible for these discs to offer dramatically improved sound quality. The fact that Dolby Digital sounds as good as it does is remarkable considering how aggressive the compression is for DVDs. High-definition discs have much more space available for soundtracks, and often feature new, higher-quality codecs from Dolby and DTS. One of Dolby's new formats, Dolby Digital Plus, offers up to 7.1-

channel surround sound for even more enveloping audio than standard 5.1-channel Dolby Digital. There are even "lossless" options, Dolby TrueHD and DTS-HD Master Audio, which deliver the closest possible reproduction of the movie studio's original master. Many Blu-ray titles feature multichannel LPCM soundtracks — uncompressed audio that should also match the quality of the studio master.

Advantages Of Blue Ray

1. Huge storage capacity Although Blue-ray can’t quite fit an entire series of HD-quality material on one disc, it could potentially fit an entire series of standard DVD quality stuff on one. That’s pretty good, considering the storage savings alone.

2. Mandatory Managed CopyIf you haven’t heard of Digital Rights Management (DRM) before, well, then this is a really good time to be check it out. DRM is the copyright protection scheme the media industry uses to prevent piracy, and the Blue-ray’s technology in this realm is actually quite exciting. The possibility exists for users to copy the content of a disc a limited number of times, similar to Apple’s iTunes system.

3. Backwards compatibilityThe Blue-ray Disc Association is encouraging manufacturers to make the players fully backwards compatible. That will allow users to both read and write on CDs, DVDs, and, obviously, Blue-ray discs.

4. Quality supportQuality support Sony and Philips might be the strongest backers of Blu-ray, but other major corporations have announced future plans to support the technology. Some of these include Apple, Dell and Panasonic.

5. In tune with the gaming ageKids will be as excited as you are about the Blue-ray player, especially since the device is included in Sony's Play station 3 console. That's some serious value, considering the price tag of the PS3 includes both the player and next-gen gaming technology for only $599 USD.

6. DurabilityAnother advantage is Blue-ray discs are physically more durable than regular blank CDs or DVDs. They are more resistant to scratches. This is why they're also ideal for computer data storage and console gaming.

7. HDTV SupportThe most noticeable advantage of Blue-ray discs over DVDs is currently being used, although it hasn't been completely explored. This is HD TV. The quality of the pictures on HD TVs will not be of the same quality

as that of a standard DVD player. You'll have superb audio and video quality. Imagine your audio system sounding ten times better than before.

Disadvantages Of Blue Ray Technology

1. Tha costThe greatest problem with the Blu-ray playeris it,s cost effectiveness. The blue ray discs are more expensive as compared to HD DVD that provide same features as that of DVD.

2. HD versus capacityLike a prize fighter lining up against an opponent who was once a good friend, this match is personal. Although Blu-ray is capable of anywhere from 50 GB of storage to 200 GB, as it stands the average Blu-ray disc can't hold much more than four or five hours of high definition content.

3. The competitionRemember Betamax? No? That's because the VCR completely eradicated the superior technology from Beta long before it could make a significant impact on mainstream movie viewers. That could potentially happen to the Blu-ray if more people buy into the HD-DVD player; it gives a high definition picture so close to that of a device twice its cost that consumers may just decide to save their money.

Comparison of Various Storage Devices

Sr No. Concept Tape Floppy Disk CD/DVD USB HDD

1 Technology Used

Cartridges BIOS Optical Media

CD-Infrared laser

DVD-Blue Laser

EEPROM EEPROM

2 Data Read/Write

Sequentially Sequentially Parallel Or Depending upon Cycle Rate

Parallel Or Depending upon Cycle Rate

Sequentially Or Parallaly

3 Storage Capacity

Very High

(1.5TB)

Very Low

(1.44MB)

Average Capacity as compared to low level and high level devices.

CD-700MB

DVD-

High

(Upto-256GB)

Very High

(Upto-1000GB)

17.04GB4 Transfer Rate High Low Average

CD-150KB/sec

DVD-66MB/Sec

High

Greater Than

60MB/Sec

Very High

3.0GB/Sec

5 Cost Medium High

Very Low Low High Very High

6 Portability Non-Portable Portable Portable Portable Portable/ Non-Portable

7 Current Trends/Usage

Depending Upon The Requirement

Obsolete Depending Upon The Requirement

Widely Used Widely Used

Comparison of Portable Devices

Sr. No. Concept Flash Drives Smart Cards Holographic Storage

1 Technology Used EEPROM Integrated Circuit Card(ICC)

(EEPROM)

Magnetic and Optical data storage

2. Read/ Write Cycles Parallel Or Depending upon Cycle Rate

Sequentially Sequentially

3. Storage Capacity 256GB 8K-256K 1.6TB

4. Transfer Rate 60MB/Sec 8K-12K Bits/Sec 6GB/Sec

5. Cost High(Depending Upon The size

Low Very High

($180)6. Security Highly Secured Not Secured Secured

with password protection

Introduction To Pendrives

Sometimes referred to as a jumpdrive, the pen drive is a portable USB flash memory device that can be used to quickly transfer audio, video, and data files from the hard drive of one computer to another. With a construction that is small enough to fit into a pocket, the pen drive derives its name from the fact that many of these USB drive devices resemble a small pen or pencil in size and shape.

A USB flash drive consists of a flash memory data storage device integrated with a USB (Universal Serial Bus) interface. Pen drives are classified as NAND style data storage devices. Utilizing a pen drive is a simple task. One end of the drive is equipped with a USB connector at one end. The connector is inserted into the USB port on a desktop or laptop and activated. Once the pen drive is in place, it is possible to drop and drag files into the memory of the drive, or forward the files to the drive. The process is no more difficult than attaching files to an email or copying files onto a disk.

Properties Of Pendrives:

1. Removable and Rewritable2. Physically much smaller than floppy disks.3. Storage capacity can be as large as 256GB.

USB flash drives are often used for the same purposes as floppy disks or CD-ROMs were. They are smaller, faster, have thousands of times more capacity, and are more durable and reliable because of their lack of moving parts

USB Flash drives use the USB mass storage standard, supported natively by modern operating systems such as Linux, Mac OS X, Windows, and other Unix-like systems.

Design Of Pendrive:

A flash drive consists of a small printed circuit board carrying the circuit elements and a USB connector, insulated electrically and protected inside a plastic, metal, or rubberized case which can be carried in a pocket or on a key chain, for example. The USB connector may be protected by a removable cap or by retracting into the body of the drive, although it is not likely to be damaged if unprotected. Most flash drives use a standard type-A USB connection allowing plugging into a port on a personal computer, but drives for other interfaces also exist.

USB flash drives draw power from the computer via external USB connection.

Working Of Pendrives:

a. Technology Used:

Flash memory combines a number of older technologies, with lower cost, lower power consumption and small size made possible by advances in microprocessor technology. The memory storage was based on earlier EPROM and EEPROM technologies. These had very limited capacity, were very slow for both reading and writing, required complex high-voltage drive circuitry, and could only be re-written after erasing the entire contents of the chip.

Hardware designers later developed EEPROMs with the erasure region broken up into smaller "fields" that could be erased individually without affecting the others. Altering the contents of a particular memory location involved copying the entire field into an off-chip buffer memory, erasing the field, modifying the data as required in the buffer, and re-writing it into the same field. This required considerable computer support, and PC-based EEPROM flash memory systems often carried their own dedicated microprocessor system. Flash drives are more or less a miniaturized version of this.

The development of high-speed serial data interfaces such as USB made semiconductor memory systems with serially accessed storage viable, and the simultaneous development of small, high-speed, low-power microprocessor systems allowed this to be incorporated into extremely compact systems. Serial access requires far fewer electrical connections for the memory chips than does parallel access, which has simplified the manufacture of multi-gigabyte drives.

b. Components Of Pendrive:

1.USB connector

2 USB mass storage controller device

3 Test points

4 Flash memory chips

5 Crystal oscillators

6 LED

7 Write-protect switch (Optional)

8 Space for second flash memory chip

Essential Components:

There are typically four parts to a flash drive:

Type-A USB connector – provides a physical interface to the host computer. USB mass storage controller – implements the USB host controller. The controller contains a

small microcontroller with a small amount of on-chip ROM and RAM.

NAND flash memory chip – stores data. NAND flash is typically also used in digital cameras.

Crystal oscillator – produces the device's main 12 MHz clock signal and controls the device's data output through a phase-locked loop.

Additional Components:

The typical device may also include:

Jumpers and test pins – for testing during the flash drive's manufacturing or loading code into the microprocessor.

LEDs – indicate data transfers or data reads and writes.

Write-protect switches – Enable or disable writing of data into memory.

USB connector cover or cap – reduces the risk of damage, prevents the ingress of fluff or other contaminants, and improves overall device appearance.

Uses Of Pendrives:

1. Personal data transport

The most common use of flash drives is to transport and store personal files such as documents, pictures and videos. Individuals also store medical alert information on MedicTag flash drives for use in emergencies and for disaster preparation.

2. Secure storage of data, application and software files

With wide deployment(s) of flash drives being used in various environments (secured or otherwise), the issue of data and information security remains of the utmost importance.

3. Application carriers

Flash drives are used to carry applications that run on the host computer without requiring installation. While any standalone application can in principle be used this way, many programs store data, configuration information, etc. on the hard drive and registry of the host computer.

4. System administration

Flash drives are particularly popular among system and network administrators, who load them with configuration information and software used for system maintenance, troubleshooting, and recovery.

Advantages and Disadvantages of Pen Drives

Advantages

1. they are resistant to scratches and dust.2. water proof and can function even after being submerged in water.3. They are ideal for transporting personal data or work files from one location to another.4. Removable and Rewritable5. Physically much smaller than floppy disks.

6. Storage capacity can be as large as 256GB.7. Power consumption of a typical flash drive is very low. 8. Pen drives have no moving parts meaning that most modern operating systems can read and write to

flash drives without any additional device turners. 9. Pen drives are much more tolerant of abuse than mechanical drives.

Disadvantages

1. they can sustain limited number of write and erase cycles

2. Most USB pen drives do not include a write-protect mechanism. Write-protection makes a device suitable for repairing virus contaminated host computers without risk of infecting the USB flash drive itself.

3. Due to the small size is that they are easily misplaced or lost.4. They have limited capacity when compared to disk drives even in 2.5inch form factors.

ISDN Technology

1. Overview of ISDN Technology

Before ISDN (Integrated Service Digital Network) was introduced, dedicated networks were required to provide services of different nature, e.g. POTS (Plain Old Telephone Service) analog service, packet service, telex service,

data service, etc. The PSTN (Public Switched Telephone Network) provides analog telephone services to customers; the PSTN (Public Switched Telephone Network) provides packet services to customers.

Different networks were required because of the very different transmission characteristics. Dedicated and isolated network requirements lead to a number of drawbacks: high costs, low efficiency, and inconvenience.

ISDN, based on the telephony network, was conceived of to provide multiple voice and non- voice services over a single network and a digital user network interface over regular phone lines, instead of dedicated and isolated user-network interfaces.

Using ISDN, users not only can do telephony, but can access additional benefits such as telecommuting, Internet access, and video conferencing. These services were not possible in large deployment with regular services provided by the phone companies. ISDN is an integrated solution for providing basic telephony and data services, whilst offering more telephony services such as supplementary services. Its proven technology continues to be deployed and hence must be tested and maintained.

2. Network Architecture

ISDN provides complete digital capabilities. Figure-1 shows the basic ISDN architecture, revealing the user network interface and network capabilities, as well as the signaling system in the network. An ISDN user can access the following services using an ISDN Terminal Equipment (TE):

• Packet-switched data

• Circuit-switched data

• Circuit-switched voice

• User-to-user signaling

There are three different types of signaling: user network, intra-network and user-to-user signaling. All three employ common-channel signaling technique. User-network signaling is used to control signaling between the user terminal equipment and the network. Intra-network signaling is used to control signaling between ISDN switches. User-to-user signaling is used between the end users and can be transparently transferred through the network.

2.1 User-Network Interface In dedicated networks, different types of user-network interfaces are required to support the service to be delivered. In ISDN, there are certain criteria to minimize the number of compatible interfaces required to support different applications: generosity, portability, independence, etc. The ITU-T has defined "reference configurations" for ISDN user-network interface. The configurations are based on association rules of functional groups and reference points. With the reference configurations, the interface requirement at different reference points is defined.

Figure-1 ISDN Architecture

Functional groups are sets of functions that may be needed in ISDN arrangements. Reference points are the conceptual points between two adjacent functional groups, along the access line. Functional groups and reference points are depicted in Figure-2.

2.2 Functional Groups

LT: Line Termination; a device at the exchange office terminates an ISDN circuit

NT1: Network Termination 1; a device at the customer premises (terminating an ISDN circuit) performs physical layer functions such as signal conversion synchronization; converts 2-wire U-Interface to 4-wire S/T Interface

Figure-2 User-Network Interface Reference Points

NT2: Network Termination 2; a device with intelligence at the customer premises, performs data link layer and network layer functions

NT: Network Termination; a device which performs the combined functions of NT1 and NT2

TA: Terminal Adapter; a device which allows non-ISDN equipment to connect with an ISDN l i ne

TE: Terminal Equipment; a user terminal which handles communications such as voice or data and supports protocol handling, maintenance functions, etc.

• TE1: Terminal Equipment, ISDN Ready Equipment (i.e. Digital ISDN Phone)

• TE2: Non-ISDN Ready Terminal Equipment

Figure-3 Another sample ISDN configuration illustrates relationships between devices and reference points.

2.3 Basic Rate Interfaces (BRI)

A typical configuration for ISDN Basic Rate Access in reference to functional groups is shown in Figure-4. A reference point is often referred to as an interface. The various interfaces are:

U: Full-duplex 2-wire interface, using echo-cancellation technique between the NT1 and the LT for basic rate ISDN. In most countries, a compression transmission line code called 2B1Q is used at this interface.

T: 4-wire interface between a NT1 and NT2

S: 4-wire interface connects an NT (or NT2) to a TE or TA

R: Non-ISDN interface between a non-ISDN compatible terminal and a TE2

Figure-4 Groupings & Interfaces

2.4 Primary Rate Interface (PRI)

ISDN Primary Rate Interface (PRI) service offers 23 B channels and one D channel in North America and Japan, yielding a total bit rate of 1.544 Mbps (the PRI D channel runs at 64 Kbps). ISDN PRI in Europe, Australia, and other parts of the world provides 30 B channels plus one 64-Kbps D channel and a total interface rate of 2.048 Mbps. The PRI physical-layer specification is ITU-T I.431.

3. Standards and OSI Model

The OSI (Open Standard Interconnection) concept was developed for computer-to-computer communications. Although ISDN was developed based on telephony network, its implementation requires the support from data terminal communications to make non-voice service possible. The OSI model was adopted to develop a suite of ISDN related standards. The standards also ensure interoperability and compatibility between equipment in a multi- vendor environment.

The Layer 1 characteristics of the user-network interface at S- and T-reference points (for the basic rate interface) are defined in ITU-T I.430. Layer 1 characteristics of the user-network interface at the primary rate interface, are defined in ITU-T I.431.

The other two upper layers, Layer 2 and Layer 3, are defined to enable that signaling be accomplished independently of the type of user-network interface involved. The characteristics of Layer 2 and Layer 3 are specified in ITU-T Q.921 and Q.931 respectively.

Figure-5 OSI & ISDN Models and Standards

3.1 Layer 1 (ITU-T I.430, I.431)

• Encoding of digital data for transmission across the interface

• Full-duplex transmission of B-channel data

• Full-duplex transmission of D-channel data

• Multiplexing of channels to form basic or primary access transmission structure

Activation and deactivation of the physical circuit

• Power feeding from network termination to the terminal

• Faulty terminal isolation

• D-channel contention access; this is needed when there is a multi-point configuration

for basic rate access

3.2 Layer 2 LAP-D (ITU-T I.441, Q.921)

• Conveys user information between Layer 3 entities across ISDN using the D-channel

• Layer 2 employs Link Access Protocol on the D-channel (LAP-D)

• The LAP-D service will simultaneously support multiple logical LAP-D connections to enable:

o Multiple terminals at the user network installation

o Multiple Layer 3 entities

• The LAP-D supports two types of multiple frame operation:

o Unacknowledged operation: Layer 3 information is transferred in unnumbered frames. Error detection is used to discard damaged frames, but there is no error control or flow control.

o Acknowledged operation: Layer 3 information is transferred in frames that include sequence numbers and that are acknowledged. Error control and flow control procedures are included in the protocol. This type is also referred to in the standard as multiple-frame operation. The Un-acknowledge and Acknowledge operations may coexist on a single D-channel.

LAP-D

Link Access Protocol - D channel (LAP-D) is the Layer 2 protocol used. This is almost identical to the X.25 LAP-B protocol. Here is the structure of a LAP-D frame:

Flag Address Control Information CRC Flag

Flag (1 octet) - This is always 7E16 (0111 11102)

Address (2 octets)

1 2 3 4 5 6 7 8

SAPI (6 bits) C/R EA0

TEI (7 bits) EA1

SAPI (Service access point identifier), 6-bits (see below) C/R (Command/Response) bit indicates if the frame is a command or a response EA0 (Address Extension) bit indicates whether this is the final octet of the address or not TEI (Terminal Endpoint Identifier) 7-bit device identifier (see below) EA1 (Address Extension) bit, same as EA0

Control (2 octets) - The frame level control field indicates the frame type (Information, Supervisory, or Unnumbered) and sequence numbers (N(r) and N(s)) as required. Information - Layer 3 protocol information and User data CRC (2 octets) - Cyclic Redundancy Check is a low-level test for bit errors on the user data. Flag (1 octet) - This is always 7E16 (0111 11102)

SAPIs

The Service Access Point Identifier (SAPI) is a 6-bit field that identifies the point where Layer 2 provides a service to Layer 3. See the following table:

SAPI Description

0 Call control procedures

1 Packet Mode using Q.931 call procedures

16 Packet Mode communications procedures

32-47 Reserved for national use

63 Management Procedures

Others Reserved for Future Use

TEIs

Terminal Endpoint Identifiers (TEIs) are unique IDs given to each device (TE) on an ISDN S/T bus. This identifier can be dynamic; the value may be assigned statically when the TE is installed, or dynamically when activated.

TEI Description

0-63 Fixed TEI assignments

64-126 Dynamic TEI assignment (assigned by the switch)

127 Broadcast to all devices

3.3 Layer 3 (ITU-T I.450, I.451, Q.931)

• Defines the D-channel call control signaling.

• Specifies the procedures for establishing connections on the B-channels that share the same interface to ISDN

as the D-channel

• Provides user-to-user control signaling over the D-channel

• Packet switching signaling is also available using X.25 Layer 3 protocol. This is the same for using B-channel

packet switching service. Layer 3 provides higher layer information for supporting various ISDN functions.

• Two basic types of user terminals are supported by ISDN: Functional and Stimulus

o Functional terminals are intelligent devices and can employ the full range of ITU-T Q.931 messages and parameters for call control. All signaling information is sent in a single control message (en bloc sending).

o Stimulus terminals are devices with a rudimentary signaling capability. A simple digital telephone is an example of a stimulus terminal.

4. Channel Types

Different channel types are used to convey information across the user-to-network interface according to their specific purposes and requirements.

• B-channel: 64 kbit/s channel to carry user information (i.e., digitized voice or data)

• D-channel: 16 kbit/s channel for the BRI or 64 kbit/s channel for the PRI. Mainly used to carry signaling

information for connection control. Since signaling information transmission does not occupy the channel all

the time, it allows packet- switched service user information to be conveyed over the D-channel to maximize

utilization.

• H-channels: Higher bit rate channels to support wider bandwidth applications, such as video conferencing, etc.

Only available at primary rate.

H0 channel: 384 kbit/s

o H1 channel: 1536 kbit/s (H11) and 1920 kbit/s (H12)

5. Access Interfaces

ITU-T I.412 defines different interface structures for ISDN user-network physical interfaces at the S- and T- ISDN reference points.

• Basic interface structure

• Primary rate B-channel interface structure • Primary rate H-channel interface structure

• Primary rate interface structures for mixtures of Band H -channels 0

5.1 Basic Interface Structure

A typical configuration for ISDN Basic Rate Access is shown in Figure-6, illustrating the U- and S/T-interfaces.

• Composed of two B-channels and one 16 kbit/s D-channel, i.e., 2B+D

• The two B-channels may be used independently

Figure-6 Typical BRI Circuit

5.2 Primary Rate B-Channel Interface Structure

A typical configuration for ISDN Primary Rate Access is shown in Figure-7. This illustrates

the use of E1 primary rate connecting a PBX to the central office.

• E1 interface (2.048 Mbit/s)

• HDB3 Coding, PCM-31 framing

• Composed of thirty B-channels and one 64 kbit/s D-channel, i.e., 30B+D

Figure-7 Typical PRI Circuit

All thirty B-channels are always present at the user-network interface, but the number

of B-channels supported by the network may be fewer.

• For multiple interfaces, the D-channel in one structure may carry signaling information for B-channels

in another primary rate structure without an activated D- channel. The time slot for the non-activated D-

channel may or may not be used to provide one additional B-channel over this structure.

5.3 Primary Rate H-Channel Interface Structure

• For primary rate at 2048 kbit/s, the 1920 kbit/s H12-channel structure is defined

• Composed of one 1920 kbit/s H12-channel and one 64 kbit/s D-channel

• In a multiple interface arrangements, a single D-channel may carry signaling information for channels in

another interface.

5.4 Primary Rate Interface Structures for Mixtures of B- and H0-Channels

• Consists of one 64 kbit/s D-channel

• In multiple interface arrangements, a single D-channel may carry signaling information for channels in

another interface.

• Any mixture of B- and H0-channels

6. S/T-Interface Transmission

In European and most Asian countries, ISDN service provision via basic rate access interface structure is at the S-reference point, which becomes the service provision boundary between the user and the network. The physical

interface is defined in ITU-T I.430. Telephone companies are responsible for the NT1 equipment provision at the customer premises.

6.1 S/T-Interface Characteristics

• 8-wire interface

• Two symmetrical wire pairs, one for each direction of signal transmission

• Two wire pairs for power feed

• Overall transmission bit rate of 192 kbit/s, including 144 kbit/s 2B+D channels and 48

kbit/s overhead information for synchronization, activation and deactivation, and D- channel contention resolution in multi-point configuration

7. U-Interface Transmission

The U-interface is between the network side of the NT1 and the line termination of the ISDN exchange form (part of the access digital section of the basic rate access). In some countries, e.g., the US, ISDN service provision, according to the basic interface structure, is at this U reference point, which becomes the service provision boundary between the user and the network.

It is up to the customer to select an NT1, which converts the 2-wire U-interface into the S/T- interface. Regenerative repeaters can be used to extend the local loop. The maximum local loop distance without a U repeater can be up to 5,486 meters, as per ETSI ETR 080. The twisted pair needs to be pre-qualified to ensure that the 2B1Q transmission can be handled.

7.1 U-Interface Characteristics

The transmission system characteristics at this interface are defined in ITU-T G.961, ETSI ETR 080, and ANSI T1.601. These are summarized as follows:

• 2 B-channels and 1 D-channel with a total bit rate of 144 kbit/s

• Overhead at 16 kbit/s

o 12 kbit/s for synchronization

8. ISDN Services

The concept of ISDN is to provide different services over a unified digital network. ISDN services are grouped in three service categories:

• Bearer service

• Tele-service

• Supplementary service

8.1 Bearer Service

This is a type of service provided by the ISDN network, offering the capability for the transmission of signals between user-network interfaces. A bearer service is limited to the three lower layers of the OSI model. The bearer services are the basic services provided by the ISDN and include:

• 64 kbit/s unrestricted

• 3.1 kHz audio

• Speech

8.2 Tele-services

Tele-service is a type of telecommunication service that is offered at the user-terminal interface rather than at the S/T-interface points, as the bearer services are. Therefore, the service includes the capability of the network and the terminal equipment functions.

Examples of Tele-services are:

• Telephony

• Teletex

• Fax

• Videotext

8.3 Supplementary Services

A supplementary service adds value to the basic functions of a telecommunication service. Since it complements an existing service, a supplementary service cannot exist on its own. Here are just a few examples of available supplementary services:

• Direct Dialing In (DDI)

• Multiple Subscriber Number (MSN)

• Calling Line Identification Restriction (CLIP)

• Connected Line Identification Presentation (COLP)

• Sub-addressing (SUB)

• Call Transfer (CT)

• Call Forwarding Busy (CFB), etc.

Networks may or may not offer one or more of the supplementary services defined by ITU-T.

9. Basic Call Control Procedures

There are three phases in a basic call control procedure:

• Call set up

• User data transfer

• Call Clear-down

Figure-10 depicts the call control procedure of an ISDN circuit switched call. It shows the message types at the user-network interface throughout the process. The call is from an ISDN user connected to one exchange, to a user connected to another exchange. The two exchanges are interconnected via Signaling System No.7 (SSN) link.

If the calling terminal equipment places the outgoing call with en-block dialing, the SETUP message includes the bearer capability, low level compatibility, high level compatibility, and called party number. If, however, the call is made with overlap dialing, then each of the individual digits of the called party number is sent as INFORMATION packets.

The exchange examines the called party number in the SETUP message and returns a CALL PROCEEDING message when the number is complete and valid. If the called party number received is incomplete, the exchange will send the SETUP ACK and ask the user for additional called party number information.

The Originating Exchange sends the call setup request via Signaling System No. 7 to the Terminating Exchange, which in turn sends a SETUP message at the user network interface to the Called TE. On receipt of the SETUP message, the Called TE will check the SETUP message to see if it is compatible with the bearer capability, low and high level compatibility specified in the SETUP message.

.

Figure-10 Basic Call Sequence

Next, the Called TE will return with the ALERTING message to confirm compatibility. This generates the alerting tone at the Originating TE. Once the call is answered, the Called TE sends a CONNECT message. The Terminating Exchange acknowledges the CONNECT message with the CONNECT ACK to the answering TE, and also relays this message to the originating party.

The Calling TE may or may not return with an optional CONNECT ACK. At this time, the designated B-channel path is connected. This completes the Call Setup phase and the User Data Transfer phase begins. On completion of

the User Data Transfer, one of the parties, either the Calling TE or the Called TE, can initiate a Call Clear-down. In Figure-10, the Calling TE initiates the Clear-down by sending a DISCONNECT message which includes the cause and location of the Call Clear-down and clears-down the B-channel connection.

In response to the DISCONNECT message, the Originating Exchange will return to the Originating TE a RELEASE message. The Originating TE completes the Call Clear-down phase by sending a RELEASE COMPLETE message.

Multiple Devices

Previously, it was necessary to have a separate phone line for each device you wished to use simultaneously. For example, one line each was required for a telephone, fax, computer, bridge/router, and live video conference system. Transferring a file to someone while talking on the phone or seeing their live picture on a video screen would require several potentially expensive phone lines.

ISDN allows multiple devices to share a single line. It is possible to combine many different digital data sources and have the information routed to the proper destination. Since the line is digital, it is easier to keep the noise and interference out while combining these signals. ISDN technically refers to a specific set of digital services provided through a single, standard interface. Without ISDN, distinct interfaces are required instead.

What is Bluetooth, WiFi and WiMAX?

Bluetooth, WiFi and WiMAX are wireless technologies which allow devices to inter-connect and communicate with each other. Radio waves are electomagnetic waves and have different frequencies. These technologies are radio frequencies. Similar to the analogue radio, or FM radio. Bluetooth works on 2.45GHz frequency. WiFi works in two frequency bands 2.4GHz and 5GHz. WiMAX works in two frequency bands, 2 - 11GHz and 10 - 66GHz. See chart below for a comparison of these technologies.

Bluetooth

Named after the Danish king, Harold Bluetooth,was the first to emerge, several devices like mobile phones, headsets, keyboards, medical equipment and even cars now come with this feature. Due to its low cost, manufacturers are willing to implement this technology in most devices. It is designed for short range communications with a range of about 10m. As a result, it consumes less power and are suited for very small battery powered devices and portable devices. Problems associated when devices communicate via infrared or cables are removed. Infrared requires a line of sight, bluetooth only needs to be in reasonable vicinity. As cables are not required, it would be less cumbersome carrying a personal bluetooth device and space would be less cluttered. As bluetooth devices automatically communicate with each other, it requires very little from the user. Bluetooth allows for a wireless Personal Area Network (PAN) with it's short range. See chart below for a comparison of these technologies.

Working

When you use computers, entertainment systems or telephones, the various pieces and parts of the systems make up a community of electronic devices. These devices communicate with each other using a variety of wires, cables, radio signals and infrared light beams, and an even greater variety of connectors, plugs and protocols.

There are lots of different ways that electronic devices can connect to one another. For example:

Component cables Electrical wires

Ethernet cables

WiFi

Infrared signals

The art of connecting things is becoming more and more complex every day. In this article, we will look at a method of connecting devices, called Bluetooth, that can streamline the process. A Bluetooth connection is wireless and automatic, and it has a number of interesting features that can simplify our daily lives.

The Problem

When any two devices need to talk to each other, they have to agree on a number of points before the conversation can begin. The first point of agreement is physical: Will they talk over wires, or through some form of wireless signals? If they use wires, how many are required -- one, two, eight, 25? Once the physical attributes are decided, several more questions arise:

How much data will be sent at a time? For instance, serial ports send data 1 bit at a time, while parallel ports send several bits at once.

How will they speak to each other? All of the parties in an electronic discussion need to know what the bits mean and whether the message they receive is the same message that was sent. This means developing a set of commands and responses known as a protocol.

Bluetooth offers a solution to the problem.

WiFi

WiFi or Wireless Fidelity, has a range of about 100m and allows for faster data transfer rate between 10 - 54Mbps. There are three different wireless standards under WiFi, 802.11a, 802.11b and 802.11g. 802.11 being the wireless standard set by The Institue of Electrical and Electronic Engineers (IEEE). WiFi is used to create wireless Local Area Networks (WLAN). The most widely used standard is 802.11b and 802.11g is expexcted to grow rapidly. These two standards are relatively inexpensive and can be found providing wireless connectivity in airports, railway stations, cafes, bars, restaurants and other public areas. The main difference between the two is the speed. 802.11b has data transfer rate of upto 11Mbps and 802.11g has a rate of upto 54Mbps. 802.11g is a relatively new and has yet to be adopted widely. 802.11a is more expensive and as a result it not available for public access. See chart below for a comparison of these technologies.

Definition: Wi-Fi is the industry name for wireless LAN (WLAN) communication technology related to the IEEE 802.11 family of wireless networking standards. To some, the term Wi-Fi is synonymous with 802.11b, as 802.11b was the first standard in that family to enjoy widespread popularity. Today, however, Wi-Fi can refer to any of the established standards: 802.11a, 802.11b, 802.11g and 802.11n.

Wi-Fi (pronounced /ˈwaɪfaɪ/) is a trademark of the Wi-Fi Alliance that may be used with certified products that belong to a class of wireless local area network (WLAN) devices based on the IEEE 802.11 standards. Because of the close relationship with its underlying standard, the term Wi-Fi is often used as a synonym for IEEE 802.11 technology.

The Wi-Fi Alliance is a global, non-profit association of companies that promotes WLAN technology and certifies products if they conform to certain standards of interoperability. Not every IEEE 802.11-compliant device is submitted for certification to the Wi-Fi Alliance, sometimes because of costs associated with the certification process and the lack of the Wi-Fi logo does not imply a device is incompatible with Wi-Fi devices.

Today, an IEEE 802.11 device is installed in many personal computers, video game consoles, smartphones, printers, and other peripherals, and virtually all laptop or palm-sized computers.

Uses

Internet access

A Wi-Fi enabled device such as a personal computer, video game console, mobile phone, MP3 player or personal digital assistant can connect to the Internet when within range of a wireless network connected to the Internet. The coverage of one or more interconnected access points — called a hotspot — can comprise an area as small as a few rooms or as large as many square miles covered by a group of access points with overlapping coverage. Wi-Fi technology has been used in wireless mesh networks, for example, in London.

In addition to private use in homes and offices, Wi-Fi can provide public access at Wi-Fi hotspots provided either free of charge or to subscribers to various commercial services. Organizations and businesses such as airports, hotels and restaurants often provide free hotspots to attract or assist clients. Enthusiasts or authorities who wish to provide services or even to promote business in selected areas sometimes provide free Wi-Fi access. As of 2008 there are more than 300 metropolitan-wide Wi-Fi (Muni-Fi) projects in progress. There were 879 Wi-Fi based Wireless Internet service providers in the Czech Republic as of May 2008.

Routers that incorporate a digital subscriber line modem or a cable modem and a Wi-Fi access point, often set up in homes and other premises, provide Internet-access and internetworking to all devices connected (wirelessly or by cable) to them. One can also connect Wi-Fi devices in ad hoc mode for client-to-client connections without a router. Wi-Fi also enables places that would traditionally not have network to be connected, for example bathrooms, kitchens and garden sheds.

Airport Wi-Fi

In September of 2003, Pittsburgh International Airport became the first airport to allow and offer free Wi-Fi throughout its terminal. It is now commonplace.

City-wide Wi-Fi

In the early 2000s, many cities around the world announced plans for a city wide Wi-Fi network. This proved to be much more difficult than their promoters initially envisioned with the result that most of these projects were either canceled or placed on indefinite hold. A few were successful, for example in 2005, Sunnyvale, California became the first city in the United States to offer city wide free Wi-Fi. Few of the Municipal Wi-Fi firms have now entered into the field of Smart grid networks.

Campus-wide Wi-Fi

Carnegie Mellon University built the first wireless Internet network in the world at their Pittsburgh campus in 1994, long before the Wi-Fi standard was adopted.

Direct computer-to-computer communications

Wi-Fi also allows communications directly from one computer to another without the involvement of an access point. This is called the ad-hoc mode of Wi-Fi transmission. This wireless ad-hoc network mode has proven popular with multiplayer handheld game consoles, such as the Nintendo DS, digital cameras, and other consumer electronics devices. A similar method is a new specification called Wi-Fi Direct which is promoted by the Wi-Fi Alliance for file transfers and media sharing through a new discovery and security methodology.

Future directions

As of 2010 Wi-Fi technology had spread widely within business and industrial sites. In business environments, just like other environments, increasing the number of Wi-Fi access-points provides redundancy, support for fast

roaming and increased overall network-capacity by using more channels or by defining smaller cells. Wi-Fi enables wireless voice-applications (VoWLAN or WVOIP). Over the years, Wi-Fi implementations have moved toward "thin" access-points, with more of the network intelligence housed in a centralized network appliance, relegating individual access-points to the role of mere "dumb" radios. Outdoor applications may utilize true mesh topologies. As of 2007 Wi-Fi installations can provide a secure computer networking gateway, firewall, DHCP server, intrusion detection system, and other functions.

The Wi-Fi name

The term Wi-Fi suggests Wireless Fidelity, compared with the long-established audio equipment certification term High Fidelity or Hi-Fi. Wireless Fidelity has often been used, even by the Wi-Fi Alliance itself in its press releases

and documents; the term may also be found in a white paper on Wi-Fi from ITAA. However, based on Phil Belanger's statement, the term Wi-Fi was never supposed to mean anything at all.

The term Wi-Fi, first used commercially in August 1999, was coined by a brand consulting firm called Interbrand Corporation that had been hired by the Alliance to determine a name that was "a little catchier than 'IEEE 802.11b Direct Sequence'." Mr Belanger also said, Interbrand invented Wi-Fi as a play on words with Hi-Fi, and also created the yin yang-style Wi-Fi logo. The term Wireless Fidelity was used later as an explanation of what Wi-Fi means.

The Wi-Fi Alliance initially used an advertising slogan for Wi-Fi, "The Standard for Wireless Fidelity", but later removed the phrase from their marketing. Despite this, some documents from the Alliance dated 2003 and 2004 still contain the term Wireless Fidelity. There was also no official statement for dropping the term.

The yin yang logo indicates that a product had been certified for interoperability.

Working of wifi

A wireless network uses radio waves, just like cell phones, televisions and radios do. In fact, communication across a wireless network is a lot like two-way radio communication. Here's what happens:

1. A computer's wireless adapter translates data into a radio signal and transmits it using an antenna. 2. A wireless router receives the signal and decodes it. The router sends the information to the Internet using a

physical, wired Ethernet connection.

The process also works in reverse, with the router receiving information from the Internet, translating it into a radio signal and sending it to the computer's wireless adapter.

The radios used for WiFi communication are very similar to the radios used for walkie-talkies, cell phones and other devices. They can transmit and receive radio waves, and they can convert 1s and 0s into radio waves and convert the radio waves back into 1s and 0s.

Advantages and challenges

A keychain size Wi-Fi detector.

Operational advantages

Wi-Fi allows local area networks (LANs) to be deployed without wires for client devices, typically reducing the costs of network deployment and expansion. Spaces where cables cannot be run, such as outdoor areas and historical buildings, can host wireless LANs.

Wireless network adapters are now built into most laptops. The price of chipsets for Wi-Fi continues to drop, making it an economical networking option included in even more devices. Wi-Fi has become widespread in corporate infrastructures.

Different competitive brands of access points and client network interfaces are inter-operable at a basic level of service. Products designated as "Wi-Fi Certified" by the Wi-Fi Alliance are backwards compatible. Wi-Fi is a global set of standards. Unlike mobile phones, any standard Wi-Fi device will work anywhere in the world.

Wi-Fi is widely available in more than 220,000 public hotspots and tens of millions of homes and corporate and university campuses worldwide.The current version of Wi-Fi Protected Access encryption (WPA2) is considered secure, provided a strong passphrase is used. New protocols for Quality of Service (WMM) make Wi-Fi more suitable for latency-sensitive applications (such as voice and video), and power saving mechanisms (WMM Power Save) improve battery operation.

Limitations

Spectrum assignments and operational limitations are not consistent worldwide. Most of Europe allows for an additional 2 channels beyond those permitted in the U.S. for the 2.4 GHz band. (1–13 vs. 1–11); Japan has one more on top of that (1–14). Europe, as of 2007, was essentially homogeneous in this respect. A very confusing aspect is the fact that a Wi-Fi signal actually occupies five channels in the 2.4 GHz band resulting in only three non-overlapped channels in the U.S.: 1, 6, 11, and three or four in Europe: 1, 5, 9, 13 can be used if all the equipment on a specific area can be guaranteed not to use 802.11b at all, even as fallback or beacon. Equivalent isotropically radiated power (EIRP) in the EU is limited to 20 dBm (100 mW).

Reach

Large satellite dish modified for long-range Wi-Fi communications in Venezuela

Wi-Fi networks have limited range. A typical wireless router using 802.11b or 802.11g with a stock antenna might have a range of 32 m (120 ft) indoors and 95 m (300 ft) outdoors. The new IEEE 802.11n however, can exceed that range by more than double. Range also varies with frequency band. Wi-Fi in the 2.4 GHz frequency block has slightly better range than Wi-Fi in the 5 GHz frequency block. Outdoor ranges - through use of directional antennas - can be improved with antennas located several kilometres or more from their base. In general, the maximum amount of power that a Wi-Fi device can transmit is limited by local regulations, such as FCC Part 15 in USA.

Wi-Fi performance decreases roughly quadratically as distance increases at constant radiation levels.

Due to reach requirements for wireless LAN applications, power consumption is fairly high compared to some other standards. Technologies such as Bluetooth, that are designed to support wireless PAN applications, provide a much shorter propagation range of <10m (ref. e.g. IEEE Std. 802.15.4 section 1.2 scope) and so in general have a lower power consumption. Other low-power technologies such as ZigBee have fairly long range, but much lower data rate. The high power consumption of Wi-Fi makes battery life a concern for mobile devices.

A number of "no new wires" technologies have been developed to provide alternatives to Wi-Fi for applications in which Wi-Fi's indoor range is not adequate and where installing new wires (such as CAT-5) is not possible or cost-effective. One example is the ITU-T G.hn standard for high speed Local area networks using existing home wiring (coaxial cables, phone lines and power lines). Although G.hn does not provide some of the advantages of Wi-Fi (such as mobility or outdoor use), it's designed for applications (such as IPTV distribution) where indoor range is more important than mobility.

Due to the complex nature of radio propagation at typical Wi-Fi frequencies, particularly the effects of signal reflection off trees and buildings, Wi-Fi signal strength can only be predicted generally for any given area in relation to a transmitter.This effect does not apply equally to long-range Wi-Fi, since longer links typically operate from towers that broadcast above the surrounding foliage.

Mobility

Speed vs. Mobility of wireless systems: Wi-Fi, HSPA, UMTS, GSM

Because of the very limited practical range of Wi-Fi, mobile use is essentially confined to such applications as inventory taking machines in warehouses or retail spaces, barcode reading devices at check-out stands or receiving / shipping stations. Mobile use of Wi-Fi over wider ranges is limited to move, use, as for instance in an automobile moving from one hotspot to another (known as Wardriving). Other wireless technologies are more suitable as illustrated in the graphic.

Data security risks

The most common wireless encryption standard, Wired Equivalent Privacy or WEP, has been shown to be easily breakable even when correctly configured. Wi-Fi Protected Access (WPA and WPA2) encryption, which became available in devices in 2003, aimed to solve this problem. Wi-Fi access points typically default to an encryption-free (open) mode. Novice users benefit from a zero-configuration device that works out of the box, but this default is without any wireless security enabled, providing open wireless access to their LAN. To turn security on requires the user to configure the device, usually via a software graphical user interface (GUI). Wi-Fi networks that are unencrypted can be monitored and data (including personal information) may be recorded, but may be protected by other means, such as a virtual private network or by secure Hypertext Transfer Protocol (HTTPS) and Transport Layer Security.

Population

Many 2.4 GHz 802.11b and 802.11g access points default to the same channel on initial startup, contributing to congestion on certain channels. To change the channel of operation for an access point requires the user to configure the device.

Channel pollution

Standardization is a process driven by market forces. Interoperability issues between non-Wi-Fi brands or proprietary deviations from the standard can still disrupt connections or lower throughput speeds on all users' devices that are within range, to include the non-Wi-Fi or proprietary product. Moreover, the usage of the ISM band in the 2.45 GHz range is also common to Bluetooth, WPAN-CSS, ZigBee and any new system will take its share.

Wi-Fi pollution, or an excessive number of access points in the area, especially on the same or neighboring channel, can prevent access and interfere with the use of other access points by others, caused by overlapping channels in the 802.11g/b spectrum, as well as with decreased signal-to-noise ratio (SNR) between access points. This can be a problem in high-density areas, such as large apartment complexes or office buildings with many Wi-Fi access points. Additionally, other devices use the 2.4 GHz band: microwave ovens, security cameras, ZigBee devices, Bluetooth devices and (in some countries) Amateur radio, video senders, cordless phones and baby monitors, all of which can cause significant additional interference. It is also an issue when municipalities, or other large entities such as universities, seek to provide large area coverage. This openness is also important to the success and widespread use of 2.4 GHz Wi-Fi.

What is a Wi Fi Hotspot?

A Wi Fi hotspot is defined as any location in which 802.11 (wireless) technology both exists and is available for use to consumers. In some cases the wireless access is free, and in others, wireless carriers charge for Wi Fi usage. Generally, the most common usage of Wi Fi technology is for laptop users to gain Internet access in locations such as airports, coffee shops, and so on, where Wi Fi technology can be used to help consumers in their pursuit of work-based or recreational Internet usage.

Hardware

Standard devices

An embedded RouterBoard 112 with U.FL-RSMA pigtail and R52 mini PCI Wi-Fi card widely used by wireless Internet service providers (WISPs) in the Czech Republic.

OSBRiDGE 3GN - 802.11n Access Point and UMTS/GSM Gateway in one device.

USB wireless adapter

A wireless access point (WAP) connects a group of wireless devices to an adjacent wired LAN. An access point is similar to a network hub, relaying data between connected wireless devices in addition to a (usually) single connected wired device, most often an ethernet hub or switch, allowing wireless devices to communicate with other wired devices.

Wireless adapters allow devices to connect to a wireless network. These adapters connect to devices using various external or internal interconnects such as PCI, miniPCI, USB, ExpressCard, Cardbus and PC Card. Most newer laptop computers are equipped with internal adapters. Internal cards are generally more difficult to install.

Wireless routers integrate a Wireless Access Point, ethernet switch, and internal Router firmware application that provides IP Routing, NAT, and DNS forwarding through an integrated WAN interface. A wireless router allows wired and wireless ethernet LAN devices to connect to a (usually) single WAN device such as cable modem or DSL modem. A wireless router allows all three devices, mainly the access point and router, to be configured through one central utility. This utility is usually an integrated web server that is accessible to wired and wireless LAN clients and often optionally to WAN clients. This utility may also be an application that is run on a desktop computer such as Apple's AirPort.

Wireless network bridges connect a wired network to a wireless network. This is different from an access point in the sense that an access point connects wireless devices to a wired network at the data-link layer. Two wireless bridges may be used to connect two wired networks over a wireless link, useful in situations where a wired connection may be unavailable, such as between two separate homes.

Wireless range extenders or wireless repeaters can extend the range of an existing wireless network. Range extenders can be strategically placed to elongate a signal area or allow for the signal area to reach around barriers such as those created in L-shaped corridors. Wireless devices connected through repeaters will suffer from an increased latency for each hop. Additionally, a wireless device connected to any of the repeaters in the chain will have a throughput that is limited by the weakest link between the two nodes in the chain from which the connection originates to where the connection ends.

Distance records

Distance records (using non-standard devices) include 382 km (237 mi) in June 2007, held by Ermanno Pietrosemoli and EsLaRed of Venezuela, transferring about 3 MB of data between mountain tops of El Aguila and Platillon. The Swedish Space Agency transferred data 310 km (193 mi), using 6 watt amplifiers to reach an overhead stratospheric balloon.

Embedded systems

Embedded serial-to-Wi-Fi module

Wi-Fi availability in the home is on the increase. Examples of remote monitoring include security systems and tele-medicine. In all these kinds of implementation, if the Wi-Fi provision is provided using a system running one of operating systems mentioned above, then it becomes unfeasible due to weight, power consumption and cost issues.

Increasingly in the last few years (particularly as of early 2007), embedded Wi-Fi modules have become available that incorporate a real-time operating system and provide a simple means of wirelessly enabling any device which has and communicates via a serial port.This allows the design of simple monitoring devices, for example, a portable ECG device monitoring a patient at home. This Wi-Fi-enabled device can communicate via the Internet.

These Wi-Fi modules are designed so that implementers need only minimal Wi-Fi knowledge to provide Wi-Fi connectivity for their products.

Network security

The main issue with wireless network security is its simplified access to the network compared to traditional wired networks such as ethernet. With wired networking it is necessary to either gain access to a building, physically connecting into the internal network, or break through an external firewall. Most business networks protect sensitive data and systems by attempting to disallow external access. Thus being able to get wireless reception provides an attack vector, if encryption is not used or can be defeated.

Attackers who have gained access to a Wi-Fi network can use DNS spoofing attacks very effectively against any other user of the network, because they can see the DNS requests made, and often respond with a spoofed answer before the queried DNS server has a chance to reply.

Securing methods

A common but unproductive measure to deter unauthorized users is to suppress the AP's SSID broadcast, "hiding" it. This is ineffective as a security method because the SSID is broadcast in the clear in response to a client SSID query. Another unproductive method is to only allow computers with known MAC addresses to join the network. The fault with this method is MAC addresses can often, but not always, be set by a user with minimal effort (MAC spoofing). If the eavesdropper has the ability to change his MAC address, then they may join the network by spoofing an authorized address.

Wired Equivalent Privacy (WEP) encryption was designed to protect against casual snooping, but is now deprecated. Tools such as AirSnort or Aircrack-ng can quickly recover WEP encryption keys. Once it has seen 5-10 million encrypted packets, AirSnort can determine the encryption password in under a second; newer tools such as aircrack-ptw can use Klein's attack to crack a WEP key with a 50% success rate using only 40,000 packets.

To counteract this in 2002, the Wi-Fi Alliance approved Wi-Fi Protected Access (WPA) which uses TKIP as a stopgap solution for legacy equipment. Though more secure than WEP, it has outlived its designed lifetime, has known attack vectors and is no longer recommended.

In 2004, the full IEEE 802.11i (WPA2) encryption standards were released. If used with a 802.1X server or in pre-shared key mode with a strong and uncommon passphrase WPA2 is still considered secure, as of 2009.

To keep your network private, you can use one of the following methods:

WiFi Protected Access (WPA) is a step up from WEP and is now part of the 802.11i wireless network security protocol. It uses temporal key integrity protocol (TKIP) encryption. As with WEP, WPA security involves signing on with a password. Most public hotspots are either open or use WPA or 128-bit WEP technology, though some still use the vulnerable WEP approach.

Media Access Control (MAC) address filtering is a little different from WEP or WPA. It doesn't use a password to authenticate users -- it uses a computer's physical hardware. Each computer has its own unique MAC address. MAC address filtering allows only machines with specific MAC addresses to access the network. You must specify which addresses are allowed when you set up your router. This method is very secure, but if you buy a new computer or if visitors to your home want to use your network, you'll need to add the new machines' MAC addresses to the list of approved addresses. The system isn't foolproof. A clever hacker can spoof a MAC address -- that is, copy a known MAC address to fool the network that the computer he or she is using belongs on the network.

Piggybacking

Piggybacking refers to access of a wireless Internet connection by bringing one's own computer within the range of another's wireless connection, and using that service without the subscriber's explicit permission or knowledge.

During the early popular adoption of 802.11, providing open access points for anyone within range to use was encouraged to cultivate wireless community networks, particularly since people on average use only a fraction of their downstream bandwidth at any given time.

Recreational logging and mapping of other people's access points has become known as wardriving. It is also common for people to use open (unencrypted) Wi-Fi networks as a free service, termed piggybacking. Indeed, many access points are intentionally installed without security turned on so that they can be used as a free service. Providing access to one's Internet connection in this fashion may be contrary to the Terms of Service or contract with the ISP. These activities do not result in sanctions in most jurisdictions; however, legislation and case law differ considerably across the world. A proposal to leave graffiti describing available services was called warchalking. In a Florida court case, owner laziness was determined not to be a valid excuse.

Piggybacking is often unintentional. Most access points are configured without encryption by default, and operating systems such as Windows XP SP2, Mac OS X or Ubuntu Linux may be configured to automatically connect to any available wireless network. A user who happens to start up a laptop in the vicinity of an access point may find the computer has joined the network without any visible indication. Moreover, a user intending to join one network may instead end up on another one if the latter's signal is stronger. In combination with automatic discovery of other network resources (see DHCP and Zeroconf) this could possibly lead wireless users to send sensitive data to the wrong middle man when seeking a destination (see Man-in-the-middle attack). For example, a user could inadvertently use an insecure network to log in to a website, thereby making the login credentials available to anyone listening, if the website is using an insecure protocol like HTTP.

WiMAX

WiMAX is Worldwide Interoperability for Microwave Access. The IEEE standard for WiMAX is 802.16 and falls under the category of wireless Metropolitan Area Network (WMAN). WiMAX operates on two frequency bands, 2 - 11GHz and 10 - 66GHz and has a range of about 50km with speeds of upto 80Mbps. This enables smaller wireless LANs to be interconnected by WiMAX creating a large wireless MAN. Networking between cities can be achieved without the need for expensive cabling. It is also able to provide high speed wireless broadband access to users. As it can operate in two frequency bands WiMAX can work by line-of-sight and non-line-of-sight. At the 2 - 11GHz frequency range it works by non-line-of-sight, where a computer inside a building communicates with a tower/antenna outside the building. Short frequency transmissions are not easily disrupted by physical obstructions. Higher frequency transmissions are used for non-line-of-sight service. This enables to towers/antennae to communicate with each other over a greater distance. Due to infrastructure and costs involved it would be more suited to provide the backbone services for ISPs and large corporations providing wireless networking and internet access. See chart below for a comparison of these technologies.

Think about how you access the Internet today. There are basically three different options:

Broadband access - In your home, you have either a DSL or cable modem. At the office, your company may be using a T1 or a T3 line.

WiFi access - In your home, you may have set up a WiFi router that lets you surf the Web while you lounge with your laptop. On the road, you can find WiFi hot spots in restaurants, hotels, coffee shops and libraries.

Dial-up access - If you are still using dial-up, chances are that either broadband access is not available, or you think that broadband access is too expensive.

The main problems with broadband access are that it is pretty expensive and it doesn't reach all areas. The main problem with WiFi access is that hot spots are very small, so coverage is sparse.

What if there were a new technology that solved all of these problems? This new technology would provide:

The high speed of broadband service Wireless rather than wired access, so it would be a lot less expensive than cable or DSL and much easier to

extend to suburban and rural areas

Broad coverage like the cell phone network instead of small WiFi hotspots This system is actually coming into being right now, and it is called WiMAX. WiMAX is short for Worldwide Interoperability for Microwave Access, and it also goes by the IEEE name 802.16.

OR

WiMAX is a wireless digital communications system, also known as IEEE 802.16, that is intended for wireless "metropolitan area networks". WiMAX can provide broadband wireless access (BWA) up to 30 miles (50 km) for fixed stations, and 3 - 10 miles (5 - 15 km) for mobile stations. In contrast, the WiFi/802.11 wireless local area network standard is limited in most cases to only 100 - 300 feet (30 - 100m).With WiMAX, WiFi-like data rates are easily supported, but the issue of interference is lessened. WiMAX operates on both licensed and non-licensed frequencies, providing a regulated environment and viable economic model for wireless carriers.

WiMAX can be used for wireless networking in much the same way as the more common WiFi protocol. WiMAX is a second-generation protocol that allows for more efficient bandwidth use, interference avoidance, and is intended to allow higher data rates over longer distances.

The IEEE 802.16 standard defines the technical features of the communications protocol. The WiMAX Forum offers a means of testing manufacturer's equipment for compatibility, as well as an industry group dedicated to fostering the development and commercialization of the technology.

WiMax.com provides a focal point for consumers, service providers, manufacturers, analysts, and researchers who are interested in WiMAX technology, services, and products. Soon, WiMAX will be a very well recognized term to describe wireless Internet access throughout the world

WiMax, also known as 802.16, looks to combine the benefits of broadband and wireless. WiMax will provide high-speed wireless Internet over very long distances and will most likely provide access to large areas such as cities.

Wireless Technology Comparison Chart

Bluetooth WiFi (a) WiFi (b) WiFi (g) WiMAX

Standard 802.15 802.11a 802.11b 802.11g 802.16

Frequency (GHz) 2.45 5 2.4 2.4 2 - 66

Speed (Mbps) 0.72 54 11 54 80

Range 10m 50m 100m 100m 50km

Advantages Low Cost Speed Low Cost Speed Speed, Range

Disadvantages Range Cost Speed Cost, Range Cost

They transmit at frequencies of 2.4 GHz or 5 GHz. This frequency is considerably higher than the frequencies used for cell phones, walkie-talkies and televisions. The higher frequency allows the signal to carry more data.

They use 802.11 networking standards, which come in several flavors:

o 802.11a transmits at 5 GHz and can move up to 54 megabits of data per second. It also uses orthogonal frequency-division multiplexing (OFDM), a more efficient coding technique that splits that radio signal into several sub-signals before they reach a receiver. This greatly reduces interference.

o 802.11b is the slowest and least expensive standard. For a while, its cost made it popular, but now it's becoming less common as faster standards become less expensive. 802.11b transmits in the 2.4 GHz frequency band of the radio spectrum. It can handle up to 11 megabits of data per second, and it uses complementary code keying (CCK) modulation to improve speeds.

o 802.11g transmits at 2.4 GHz like 802.11b, but it's a lot faster -- it can handle up to 54 megabits of data per second. 802.11g is faster because it uses the same OFDM coding as 802.11a.

o 802.11n is the newest standard that is widely available. This standard significantly improves speed and range. For instance, although 802.11g theoretically moves 54 megabits of data per second, it only achieves real-world speeds of about 24 megabits of data per second because of network congestion. 802.11n, however, reportedly can achieve speeds as high as 140 megabits per second. The standard is currently in draft form -- the Institute of Electrical and Electronics Engineers (IEEE) plans to formally ratify 802.11n by the end of 2009.

Other 802.11 standards focus on specific applications of wireless networks, like wide area networks (WANs) inside vehicles or technology that lets you move from one wireless network to another seamlessly.

WiFi radios can transmit on any of three frequency bands. Or, they can "frequency hop" rapidly between the different bands. Frequency hopping helps reduce interference and lets multiple devices use the same wireless connection simultaneously.

What is a Local Area Network?

A Local Area Network (LAN) is a high-speed communications system designed to link computers and other data processing devices together within a small geographic area, such as a workgroup, department, or building.

Local Area Networks implement shared access technology. This means that all of the devices attached to the LAN share a single communications medium, usually a coaxial, twisted-pair, or fiber-optic cable.

A physical connection to the network is made by putting a network interface card (NIC) inside the computer and connecting it to the network cable. Once the physical connection is in place, the network software manages communications between stations on the network.

To send messages to and from computers, the network software puts the message information in a packet. (If the message to be sent is too big to fit into one packet, it will be sent in a series of packets.) In addition to the message data, the packet contains a header and a trailer that carry special information to the destination. One piece of information in the header is the address of the destination.

The NIC transmits the packet onto the LAN as a stream of data represented by changes in electrical signals. As it travels along the shared cable, each NIC checks its destination address to determine if the packet is addressed to it. When the packet arrives at the proper address, the NIC copies it and gives its data to the computer. Since each individual packet is small, it takes very little time to travel to the ends of the cable. After a packet carrying one message passes along the cable, another station can send its packet. In this way, many devices can share the same LAN medium.

Each LAN has its own unique topology, or geometric arrangement. There are three basic topologies: bus, ring, and star. Most LANs are a combination of these arrangements.

In a bus topology all of the devices are connected to a central cable or backbone.

In a ring topology the devices are connected in a closed loop so that each device is connected to two others, one on either side. This kind of topology is robust; that is, one device's failure will probably not cause total network failure.

In a star topology the devices are all connected to a central hub, which forwards data towards its final destination. The NCI-Frederick LAN infrastructure is standardized on the star topology. If the data's destination is within the local star segment, the hub will forward data directly to the destination device; if the data's destination is outside the local star segment, the hub forwards the data to a router.

Depending on the topology and media that are used, as well as the protocols (formats for transmitting data) that are implemented, a LAN can permit data transfer rates of up to 100 Million bps.

The following characteristics differentiate one LAN from another:

topology : The geometric arrangement of devices on the network. For example, devices can be arranged in a ring or in a straight line.

protocols : The rules and encoding specifications for sending data. The protocols also determine whether the network uses a peer-to-peer or client/server architecture.

media : Devices can be connected by twisted-pair wire, coaxial cables, or fiber optic cables. Some networks do without connecting media altogether, communicating instead via radio waves.

WAN

Definition: WAN stands for Wide Area Network. As its name suggests, it is a computer network that covers a far wider area than a LAN (Local Area Network). WANs cover cities, countries, continents and the whole world.

A WAN is formed by linking LANs together. For example, several major LANs in a city can connect together forming a WAN.

When networks connect to form a bigger network (a bigger WAN), the resulting network is called an internetwork, which is generically abbreviated to ‘an internet’. Now when all WANs in the world connect forming a global internet, we call it The Internet, which everyone knows! That’s why the Internet is always written with a capital I. It is the biggest WAN we have.

OR

A wide area network (WAN) is a computer network that covers a broad area (i.e., any network whose communications links cross metropolitan, regional, or national boundaries ). This is in contrast with personal area networks (PANs), local area networks (LANs), campus area networks (CANs), or metropolitan area networks (MANs) which are usually limited to a room, building, campus or specific metropolitan area (e.g., a city) respectively.

WANs are used to connect LANs and other types of networks together, so that users and computers in one location can communicate with users and computers in other locations. Many WANs are built for one particular organization and are private. Others, built by Internet service providers, provide connections from an organization's LAN to the Internet. WANs are often built using leased lines. At each end of the leased line, a router connects to the LAN on one side and a hub within the WAN on the other. Leased lines can be very expensive. Instead of using leased lines, WANs can also be built using less costly circuit switching or packet switching methods. Network protocols including TCP/IP deliver transport and addressing functions. Protocols including Packet over SONET/SDH, MPLS, ATM and Frame relay are often used by service providers to deliver the links that are used in WANs. X.25 was an important early WAN protocol, and is often considered to be the "grandfather" of Frame Relay as many of the underlying protocols and functions of X.25 are still in use today (with upgrades) by Frame Relay.

Difference Between Lan AND Wan:

LAN (Local Area Network) is a computer network covering a small geographic area, like a home, office, or group of buildings

Network in an organisation can be a LAN. Typically owned, controlled, and managed by a single person or organization LANs have a high data transfer rate

Wan-->WANs (like the Internet) are not owned by any one organization but rather exist under collective or distributed ownership and management

WANs tend to use technology like ATM, Frame Relay and X.25 for connectivity over the longer distances

WANs have a lower data transfer rate as compared to LANs

Have a large geographical range generally spreading across boundaries and need leased telecommunication lines

Definition: WLANs provide wireless network communication over short distances using radio or infrared signals instead of traditional network cabling.

A WLAN typically extends an existing wired local area network. WLANs are built by attaching a device called the access point (AP) to the edge of the wired network. Clients communicate with the AP using a wireless network adapter similar in function to a traditional Ethernet adapter.

Network security remains an important issue for WLANs. Random wireless clients must usually be prohibited from joining the WLAN. Technologies like WEP raise the level of security on wireless networks to rival that of traditional wired networks.

Also Known As: wireless LAN

Examples:

For WLANs that connect to the Internet, Wireless Application Protocol (WAP) technology allows Web content to be more easily downloaded to a WLAN and rendered on wireless clients like cell phones and PDAs.

Benefits of WLANs

Wireless LANs offer the following productivity, service, convenience, and cost advantages over traditional wired networks:

Mobility-Wireless LAN systems can provide LAN users with access to real-time information anywhere in their organization. This mobility supports productivity and service opportunities not possible with wired networks.

Installation Speed and Simplicity-Installing a wireless LAN system can be fast and easy and can eliminate the need to pull cable through walls and ceilings.

Installation Flexibility-Wireless technology allows the network to go where wire cannot go.

Reduced Cost-of-Ownership-While the initial investment required for wireless LAN hardware can be higher than the cost of wired LAN hardware, overall installation expenses and life-cycle costs can be significantly lower. Long-term cost benefits are greatest in dynamic environments requiring frequent moves, adds, and changes.

Scalability-Wireless LAN systems can be configured in a variety of topologies to meet the needs of specific applications and installations. Configurations are easily changed and range from peer-to-peer networks suitable for a small number of users to full infrastructure networks of thousands of users that allows roaming over a broad area.

Range/Coverage

The distance over which RF waves can communicate is a function of product design (including transmitted power and receiver design) and the propagation path, especially in indoor environments. Interactions with typical building objects, including walls, metal, and even people, can affect how energy propagates, and thus what range and coverage a particular system achieves. Most wireless LAN systems use RF because radio waves can penetrate many indoor walls and surfaces. The range (or radius of coverage) for typical WLAN systems varies from under 100 feet to more than 500 feet. Coverage can be extended, and true freedom of mobility via roaming, provided through microcells.

WWAN - Wireless Wide Area Network

Meaning of WWAN - "Wireless Wide Area Network", enables users to establish wireless connections over remote private or public networks using radio, satellite and mobile phone technologies instead of traditional cable networking solutions like telephone systems or cable modems.

These connections can be maintained over large geographical areas, such as cities or countries, through the use of satellite systems or multiple antenna sites maintained by wireless service providers.

Schools or a businesses in rural areas benefit from Wireless WANs because it is more cost effective than laying long cable or fibre links. The same can be said for institutions in built up urban areas. Installing cable into an existing location could be very disruptive, so a wireless alternative is more cohesive.

Examples of Wireless WANs are Digital Cellular Phone and Data Services, Satellite Modems or a computer hooked up with a wireless WAN card and used in a city or geographical area that has WWAN deployed. There are cities that offer wireless internet services to the anyone who lives within it's reach. Some universities offer WWAN services to students who can hook up to the school network from outside of the building or across town. 

The main WWAN technologies are GSM (Global System for Mobile Communication), GPRS (General Packet Radio Service), UMTS (Universal Mobile Telecommunication System), and CDMA (Code Division Multiple Access).

How Municipal WiFi Works

Imagine you are a journalist writing about a tense hostage, and you’re on a tight deadline. You do not have time to

return to the desktop, and if you leave, you miss on the development of history. Fortunately, you have wireless

Internet access – you can write your story and files without leaving the stage.

Part of your article describes how the police can access real-time flow of wifi security cameras. Their patrol cars,

officers monitor the situation and map of the building, including entrances, exits and hiding places. They use this

information to plan what to do. They also have a secure network that connects them to a hostage negotiator.

When the situation is over, everyone believes that municipal wireless networks and the information it conveys,

contributed to a peaceful solution. In this article you will discover amazing that these networks can do – also

potentially provide free or cheap Internet access. You can also read more about the technology behind them and

why municipal WiFi may be wrong.

Wireless Basics

The first days of the Internet at home using a modem connected to a computer to dial a number and maintain a

connection. It was heavy and slow. The modem is now faster, more people have realized how painfully slow data

transmission was in the days of 300 baud. Finally, users who can afford a price increase could have broadband

access through Digital Subscriber Lines (DSL), cable and satellite.

Broadband is faster than dial-up, but until recently, plug your computer into an outlet or a piece of equipment.

Wireless networks or Wi-Fihas changed all that. Wireless networks using 802.11 standard networks for

communication devices. In a wireless network, data travels from place to place via radio waves. You still have to

physically connect awireless router to a modem, but you can move your computer from one location to another.

802.11 networks using unlicensed radio spectrum to send and receive data. Many other parts of the spectrum, as

well as groups who bear the signals of radio and television, requires a license. The unlicensed spectrum is available

to anyone who has the right equipment. In case of wireless network is a wireless router and wireless technology in

the device you are using.

What is the difference between Wi-fi and Wi-max? Explain the working of both the technologies.

Ans. In fact WiFi (technically standard 802.11) and WiMAX (802.16) don't compete for broadband users or applications today. That's partly because WiFi is widely deployed and WiMAX is still largely an unfulfilled promise and partly because the two protocols were designed for very different situations. However, if WiMAX is eventually widely deployed, there will be competition between them as last mile technologies.

Wifi, like a cordless phone, is primarily used to provide a connection within a limited area like a home or an office. WiMAX is used (or planned to be used) to provide broadband connectivity from some central location to most locations inside or outside within its service radius as well as to people passing through in cars. Just like mobile phone service, there are likely to be WiMAX dead spots within buildings.

From a techie POV, the analogy is apt at another level: WiFi, like cordless phones, operates in unlicensed spectrum (in fact cordless phones and WiFi can interfere with each other in the pitiful swatch of spectrum that's been allocated to them). There are some implementations of WiMAX for unlicensed spectrum but most WiMAX development has been done on radios which operate on frequencies whose use requires a license.

Some more subversive types (they're subversive so I can't link to them) say that WiMAX is what you get when bellheads (not a nice term) try to reinvent WiFi the way they'd like it to be. It's true that WiMAX is much more a command and control protocol than WiFi. Oversimplified, in a WiFi environment every device within reach of an access point shouts for attention whenever it's got something to transmit. In that chaos, some signals tromp on other signals; the more powerful devices and those closer to the access point tend to get more than their share of airtime like the obnoxious kid who always has his hand up in the front of the class. In WiMAX devices contend for initial attention but then are assigned times when they may ask to speak. The protocol allows the operator more control over the quality of service provided—bellheads like control.

But it's not clear that more control means better service than contentious chaos (I'm talking about technology but the same may apply to economies or bodies politic). The Internet and its routing algorithms are chaotic; the routers just throw away packets if they get to busy to handle them. Bellheads (and even smart people like Bob Metcalfe) were sure that design or lack thereof wouldn't scale. They were wrong.

Same people said that voice would never work over the Internet—there's no guarantee of quality, you see. They were wrong although it's taken awhile to prove it. Now HD voice is available on the Internet but NOT on the traditional phone network (although it could be).

Lovers of an orderly environment and those who like to keep order were absolutely sure that WiFi couldn't work once it became popular. Not only is it chaotic; it also operates in the uncontrolled environment of unlicensed frequencies along with cordless phones, bluetooth headsets, walkie-talkies and the occasional leaky microwave oven. But somehow it's become near indispensable even in places where a city block full of access points contend for the scarce frequencies.

WiMAX may be too well-controlled for its own good. Moreover, if it is used only in regulated spectrum where most frequencies are idle most of the time AND licenses for the frequencies have to be purchased, it will be even less efficient than if it could contend for unlicensed spectrum.

By the way, WiFi CAN operate at distances as great as WiMAX but there are two reasons why it doesn't. One reason is that radios operating in the unlicensed frequencies are not allowed to be as powerful as those operated with licenses; less power means less distance. These regulations are based on the dated assumption that devices can't regulate themselves—but the assumption MAY be correct over great enough distances. The second reason why WiFi access points don't serve as wide an area as WiMAX access points are planned to do is the engineering belief that the problem of everybody shouting at once, even if it's surmountable in a classroom, would be catastrophic in a larger arena. Maybe.

New licensed spectrum is being made available for WiMAX and other technologies NOT including WiFi—for example, the valuable 700MHz frequencies currently used by analog over the air TV. WiMAX could have a good run because it is allowed to operate in that efficient spectrum while WiFi will eventually run out of the pitifully little spectrum that's been allocated to it. That's policy and politics and not engineering but could still be a reason for WiMAX success.

Working Of Wimax

Wimax stands for Worldwide Interoperability for Microwave Access. Wimax technology is a telecommunications technology that offers transmission of wireless data via a number of transmission methods; such as portable or fully mobile internet access via point to multipoints links. The Wimax technology offers around 72 Mega Bits per second without any need for the cable infrastructure. Wimax technology is based on Standard that is IEEE 802.16, it usually also called as Broadband Wireless Access. WiMAX Forum created the name for Wimax technology that was formed in Mid June 2001 to encourage compliance and interoperability of the Wimax IEEE 802.16 standard. Wimax technology is actually based on the standards that making the possibility to delivery last mile broadband access as a substitute to conventional cable and DSL lines

WiMAX has the potential to do to broadband Internet access what cell phones have done to phone access. In the same way that many people have given up their "land lines" in favor of cell phones, WiMAX could replace cable and DSL services, providing universal Internet access just about anywhere you go. WiMAX will also be as painless as WiFi -- turning your computer on will automatically connect you to the closest available WiMAX antenna.

Working of WiFi

Wi-Fi is a wireless technology used in home networks and to connect mobile phones, computers and many such electronic devices over a wireless network. It is the 802.11 IEEE standard. How does Wi-Fi work?

The wireless adapter of a computer translates data into radio signals and transmits the signals over an antenna. The transmitting antenna is generally connected to a DSL or a LAN-based Internet connection. The wireless router in the network receives the signals and decodes them. The router uses an Ethernet connection to transmit the information over the Internet. The places or spots, which offer Wi-fi access to the Internet, are known as Wi-Fi hotspots.The Wi-Fi signals have a range of about 120 feet indoors and 300 feet outdoors. With an increase in the distance between the user and the signal, the connection speed decreases. Wi-Fi connections allow you to get rid of the clutter of wires. WI-fi networks are easy to setup. Wi-Fi makes it possible for LANs to be deployed without cabling. This reduces the costs incurred in the network setup and expansion.

Plain old telephone service

Plain old telephone service (POTS) is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in most parts of the world. The name is a retronym, and is a reflection of the telephone service still available after the advent of more advanced forms of telephony such as ISDN, mobile phones and VoIP. POTS has been available almost since the introduction of the public telephone system in the late 19th century, in a form mostly unchanged to the normal user despite the introduction of Touch-Tone dialing, electronic telephone exchanges and fiber-optic communication into the public switched telephone network (PSTN).

The system was originally known as the Post Office Telephone Service or Post Office Telephone System in many countries. The term was dropped as telephone services were removed from the control of national post offices.

POTS services include:

bi-directional, or full duplex, voice path with limited frequency range of 300 to 3400 Hz: in other words, a signal to carry the sound of the human voice both ways at once;

call-progress tones, such as dial tone and ringing signal;

subscriber dialing;

operator services, such as directory assistance, long distance, and conference calling assistance;

a standards compliant analog telephone interface including BORSCHT functions

Public switched telephone network

The public switched telephone network (PSTN) is the network of the world's public circuit-switched telephone networks. Originally a network of fixed-line analog telephone systems, the PSTN is now almost entirely digital in its core and includes mobile as well as fixed (plain old telephone service, POTS) telephones.

Class 1 (regional center)

The class 1 office was the Regional Center (RC). Regional centers served three purposes in the North American toll network (a) their connections were the "last resort" for final setup of calls when routes between centers lower in the hierarchy were not available (b) they were initially staffed by engineers who had the authority to block portions of the network within the region in case of emergencies or network congestion - although these functions were transferred after 1962 to the Network Control/Operations Center and the distributed Network Management Centers (see below) (c) they provided collection points (until the development of more advanced computer hardware and software for toll operators) for circuits that would be passed along to one of the international overseas gateways (which operated as special centers outside the formal North American hierarchy). The regional centers updated each other on the status of every circuit in the network. These centers would then reroute traffic around the trouble spots and keep each informed at all times.

Class 2 (sectional center)

The class 2 office was the Sectional Center (SC). The sectional center typically connected major toll centers within one or two states or provinces, or a significant portion of a large state or province, to provide interstate or interprovincial connections for long-distance calls. At various times, there were between 50 and 75 active class two offices in the network.

Class 3 (primary center)

The class 3 office was the Primary Center (PC). Calls being made beyond the limits of a small geographical area where circuits are not connected directly between class 4 toll offices would be passed from the toll center to the primary center. These locations use high usage trunks to complete connection between toll centers. The primary center never served dial tone to the user. The number of primary centers in the network fluctuated from time to time, ranging between 150 and 230.

Class 4 (toll center)

The class 4 office is the Toll Center (TC), Toll Point (TP), or Intermediate Point (IP). A call going between two end offices not directly connected together, or whose direct trunks are busy, is routed through the toll center. The toll center is also used to connect to the long-distance network for calls where added costs are incurred, such as operator handled services. This toll center may also be called the tandem office because calls have to pass through this location to get to another part of the network. Toll centers might have been operated either as interstate facilities, under the operation of AT&T Long Lines (GTE in a few cases), or by local telephone companies, handling long distance traffic to points within a particular operating company territory. Class 4 offices continue to exist, although with considerable changes, as they handle local exchange company interconnections, locally charged or long distance rated, or provide facilities for connection to long distance company points of presence.

Class 5 (local exchange)

The class 5 office is the local exchange or end office. It delivers dial tone to the customer. The end office, also called a branch exchange, is the closest connection to the end customer. Over 19,000 end offices in the United States alone provide basic dial tone services.

In modern times only the terms Class 4 and Class 5 are much used, as any tandem office is referred to as a Class 4. This change was prompted in great part by changes in the power of switches and the relative cost of transmission, both of which tended to flatten the switch hierarchy. The breakup of the Bell System, and the need for each of the surviving regional operating companies to handle long distance interconnections, also promoted the inclusion of inter-regional and international processing through larger Class 4 offices.

BD Applications- High Definition Television Recording- High Definition Video Distribution- High Definition Camcorder Archiving- Mass Data Storage- Digital Asset Management and Professional Storage

The Blu-ray Disc format was designed to offer the best performance and features for a wide variety of applications. High Definition video distribution is one of the key features of Blu-ray Disc, but the format's versatile design and top-of-the-line specifications mean that it is suitable for a full range of other purposes as well.

High Definition Television RecordingHigh Definition broadcasting is vastly expanding in the U.S. and Asia. Consumers are increasingly making the switch to HDTV sets to enjoy the best possible television experience. The Blu-ray Disc format offers consumers the ability to record their High Definition television broadcasts in their original quality for the first time, preserving the pure picture and audio level as offered by the broadcaster. As such it will become the next level in home entertainment, offering an unsurpased user experience. And since the Blu-ray Disc format incorporates the strongest copy protection algorithms of any format or proposal to date, the format allows for recording of digital broadcasts while meeting the content protection demands of the broadcast industry.

High Definition Video DistributionDue to its enormous data capacity of 25 to 50 GB per (single-sided) disc, the Blu-ray Disc format can store High Definition video in the highest possible quality. Because of the huge capacity of the disc, there is no need to compromise on picture quality. Depending on the encoding method, there is room for more than seven hours of the highest HD-quality video. There is even room for additional content such as special features and other bonus material to accompany the High Definition movie. Furthermore, the Blu-ray Disc movie format greatly expands on traditional DVD capabilities, by incorporating many new interactive features allowing content providers to offer an even more incredible experience to consumers. An Internet connection may even be used to unlock additional material that is stored on the disc, as there is enough room on the disc to include premium material as well.

High Definition Camcorder ArchivingAs the market penetration of High Definition TV sets continues to grow, so does the demand of consumers to create their own HD recordings. With the advent of the first HD camcorders, consumers can now for the first time record their own home movies in a quality level unlike any before. As these camcorders are tape-based, consumers cannot benefit from the convenience and direct access features they are used to from DVD players and recorders. Now, the Blu-ray Disc format, with its unprecedented storage capacity, allows for the HD video recorded with an HD camcorder to be converted and recorded on a Blu-ray Disc. When the HD content is stored on a Blu-ray Disc, it can be randomly accessed in a way comparable to DVD. Furthermore, the disc can be safely stored for many years, without the risk of tape wear.

Mass Data StorageIn its day, CD-R/RW meant a huge increase in storage capacity compared to traditional storage media with its 650 MB. Then DVD surpassed this amount by offering 4.7 to 8.5 GB of storage, an impressive 5-10 x increase. Now consumers demand an even bigger storage capacity. The growing number of broadband connections allowing consumers to download vast amounts of data, as well as the ever increasing audio, video and photo capabilities of personal computers have led to yet another level in data storage requirements. In addition, commercial storage requirements are growing exponentially due to the proliferation of e-mail and the migration to paperless processes. The Blu-ray Disc format again offers 5-10 x as much capacity as traditional DVD resulting in 25 to 50 GB of data to be stored on a single rewritable or recordable disc. As Blu-ray Disc uses the same form factor as CD and DVD, this allows for Blu-ray Disc drives that can still read and write to CD and DVD media as well.

Digital Asset Management and Professional StorageDue to its high capacity, low cost per GB and extremely versatile ways of transferring data from one device to another (because of Blu-ray Disc's extremely wide adoption across the industry), the format is optimized for Digital Asset Management and other professional applications that require vast amounts of storage space. Think of medical archives that may contain numerous diagnostic scans in the highest resolution, or catalogs of audiovisual assets that need to be instantly retrieved in a random manner, without the need to "restore" data from a storage carrier. One Blu-ray Disc may replace many backup tapes, CDs, DVDs or other less common or proprietary storage media. And contrary to network solutions, the discs can be physically stored in a different location for backup and safekeeping.

BD Key Characteristics- Broadest Industry Support- Lifespan- Content Protection- Cost- Capacity- Robustness of Disc

Broadest Industry SupportHistory has shown that unified industry support for a particular format is most likely to lead to success. Therefore, the participation of the world's most renowned consumer electronics manufacturers and IT companies are leading in the success of the best standard for next-generation storage: Blu-ray Disc. Blu-ray Disc is supported by leading hardware manufacturers across the CE and IT fields from the U.S., Europe, Japan and Korea, including Dell, HP, Hitachi, LG Electronics, Matsushita (Panasonic), Mitsubishi, Pioneer, Philips, Samsung, Sharp, Sony and Thomson/RCA. Finally, major blank media manufacturers including TDK are supporting the Blu-ray Disc format as the successor of DVD. This broad industry support will lead to a broad selection of Blu-ray Disc products, including home video decks, PC drives, PCs line-fitted with Blu-ray Disc drives and blank media, to be available when the format is launched in the various regions in the world.

LifespanThe Blu-ray Disc format is designed to stay relevant for at least 10 to 15 years. Its high storage capacity of 25 to 50 GB allows for the best-possible High Definition video quality and satisfies even the most demanding data storage needs. As we have seen with DVD in the past, most premium titles require two discs. This is why Blu-ray Disc incorporates the additional storage space that is required for a High Definition feature film including bonus bonus material in the new standard from the beginning. Formats with a lesser capacity are only suitable as interim solutions, requiring them to be replaced much sooner than a format that takes tomorrow's data storage needs into account from day one. This will of course require multiple investments in production equipment, and will lead to increased consumer confusion.

Content ProtectionBlu-ray Disc provides some of the strongest copy protection methods ever developed for any consumer format. It makes Blu-ray Disc the best choice for any content publisher wanting assurance that their valuable assets are protected from piracy. Based on feedback from the content industry and taking a cue from the lessons learned by other formats, the Blu-ray Disc format incorporates a robust copy protection mechanism, which not only relies on implementation at the playback device, but which also includes precautions at replicator level, which will be strictly controlled. Unlike the voluntary implementation of CSS protection in DVD, the copy protection mechanism for Blu-ray Disc is mandatory and will be governed by strict licensing procedures.

CostBlu-ray Disc is developed to offer the best long-term profitability model for content providers. Although it might require a nominal investment in advance, it provides greater and longer-term profit potential. This is because the format is designed to last for a period of at least 10 to 15 years. Due to its enormous storage capacity, short-erm replacement of the technology is unnecessary, unlike other format proposals that might require less investment in advance, but higher investments in the long term due to the replacement of the technology when it becomes outdated. At comparable volumes, Blu-ray Disc production costs are within 10% of DVD production costs, although a Blu-ray Disc offers 5-10 x the capacity. It is by far the cheapest format measured in cost per GB. Since Blu-ray Disc requires fewer slots in a replication line compared to other formats, it will bring costs on par with DVD, or even cheaper, much sooner. Production facilities can produce many more Blu-ray Discs in the same time period as DVDs. Also, contrary to some rumors circulating, Blu-ray Discs do not require cartridges for any of the format variations (BD ROM, BD RE, and BD R).

CapacityThe Blu-ray Disc format offers the highest capacity of any consumer media format to date, also greatly surpassing the capacity of other format proposals. Blu-ray Disc's huge capacity allows not only for the highest quality High Definition video to be recorded at large bitrates (thereby eliminating the need for tight compression that could affect picture quality), it also opens the doors to new and existing applications. Think of extra sessions on a disc that could be unlocked when a user's Blu-ray Disc player connects to the Internet to validate authorization. Or what about bonus material and special features that will eventually also be recorded in High Definition quality? With Blu-ray Disc's large capacity, these extras can be included in high quality on the same disc, so there is no need for separate bonus discs to accompany the movie title. Only Blu-ray Disc will be able to offer these value-added options.

Robustness of DiscAs the result of recent breakthroughs in the development of hard coating for Blu-ray Disc, the discs offer much stronger resistance to scratches and fingerprints than other existing and proposed formats. Hard-coated Blu-ray Discs do not require a cartridge and can be used as a bare disc, similar to DVD and CD. This avoids extra production costs, and allows for small form factor applications, such as the implementation of Blu-ray Disc drives in a notebook computer. The hard-coating technology is used for BD ROM discs, giving them the same bare disc look and feel consumers know from DVD, and it can be applied to rewritable and recordable Blu-ray Discs as well.

Benefits for the Industry

Consumer ElectronicsThe consumer electronics industry is rapidly migrating toward High Definition. In the U.S., over seven million digital televisions (DTVs) have already been sold. Demand for HD programming is rapidly growing. Digital TV is currently established in the U.S., with 85% household penetration by 2010.

In this light, consumers will demand playback and recording equipment giving them the most benefits from their High Definition television sets. Blu-ray Disc offers the best of both worlds. It is the ideal content delivery system for pre-recorded media, while, at the same time, featuring the most advanced recording capability for owners of HDTVs. In fact, only Blu-ray Disc, due to its enormous storage capacity, allows consumers to record large amounts of High Definition broadcasts in the absolute best quality, retaining all the details from the original.

Likewise, increased popularity of Blu-ray Disc will likely stimulate awareness and sales of High Definition TV sets. As more and more enriched content or special editions become available exclusively on Blu-ray Disc, this will drive consumers toward the adoption of HDTV, even in areas with little High Definition broadcasts, such as Europe.

Blu-ray Disc is supported by all major consumer electronics companies, including Hitachi, LG Electronics, Matsushita (Panasonic), Mitsubishi, Pioneer, Philips, Samsung, Sharp and Sony. This will help accelerate the widespread adoption of the format in the consumer electronics world.

 PC and PC Peripheral

Due to ever-increasing file sizes and the constantly growing adoption of such applications as digital music, photo and video storage on PCs, consumers constantly demand larger storage capacities. Broadband connections allow for the downloading of vast amounts of data, and increasing developments in digital photo and video equipment not only raises quality requirements, but also the amount of memory that is necessary to store content. In addition, commercial storage requirements are increasing exponentially with the proliferation of e-mail, the migration to paperless processes, and the movement to archive documents. Backing up all of this data is becoming a costly necessity in business.

Although CDs and DVDs can fill a substantial portion of the market for data storage, consumers and businesses are demanding larger capacities without the need to abandon their existing collection of pre-recorded and home-recorded media or backup data storage. Blu-ray Disc is the solution to this growing need. Offering 25 to 50 GB of data on a single-sided disc, and boasting a physical size identical to that of today's DVD, storage needs will be solved for many years to come. A Blu-ray Disc is also a very economical storage medium, offering the lowest cost per GB. What's more: a Blu-ray Disc drive in a PC is also likely to allow reading from and recording to CD and DVD media, making Blu-ray Disc the ideal upgrade.

PC drive vendors will be able to sell Blu-ray Disc drives in the after-market, enabling consumers to upgrade their existing PCs to take advantage of the larger storage capacity offered by the format. Likewise, PC vendors can equip their PCs with a Blu-ray Disc drive as a line-fitted solution, to make their products stand out of the crowd.

The world's largest computer companies, including Apple, HP and Dell, have adopted the Blu-ray Disc format, offering it as a line-fitted option in their top-of-the-line models. As line-fitted drives in PCs currently make up for about 75% of DVD drives sold, such a widespread adoption in the IT industry will boost acceptance of the Blu-ray Disc format.

DiscsMedia manufacturers, both active in blank media (recordable and rewritable) and pre-recorded media (used to distribute software, movies and other content on ROM discs) will enjoy great benefits by taking up production of

Blu-ray Disc media. As the Blu-ray Disc format is supported by almost all consumer electronics manufacturers in the world, as well as the world's two largest IT companies, there will be an enormous boost of Blu-ray Disc players, recorders and drives, greatly accelerating demand for Blu-ray Disc media.

At the same output levels, production costs for Blu-ray Discs are about 10% higher than DVD per disc. A production line can produce more Blu-ray Discs per hour, due to the curing times required for the dyes. This results in increased productivity and lower costs per disc. Investment costs to convert a DVD production line to Blu-ray Disc are expected to be comparable to the change from CD to DVD.

One of the major design goals of Blu-ray Disc is that the format should be viable for at least 10 to 15 years. For this, a major leap in storage capacity was needed, and this has been achieved in the form of 25 to 50 GB disc capacity. Other formats might have required less initial investment fees due to their similarities with DVD (although similarity with DVD also would have involved extra costs due the bonding process), but they were not likely to last as long as Blu-ray Disc. This would have required an additional change in the production lines as soon as the format becomes outdated and newer formats appear, overall resulting in much higher investment costs.

Content ProvidersWith the emerging trend of HDTVs becoming more common, and consumers getting used to High Definition quality and expecting the same from their pre-recorded media, the logical next step is the distribution of packaged media in HD format. Blu-ray Disc is the ideal format to suit this need.

Due to its huge capacity, the Blu-ray Disc format not only allows for a movie to be stored in the absolute best High Definition quality on a disc, but it also offers room for additional extras such as making of and special features. Blu-ray Disc even offers the room to store these extras in HD quality as well. There's no need to pack an additional disc to store these bonus materials, thereby eliminating costs for content providers and simplifying the end-user experience.

The large capacity of Blu-ray Disc may also be utilized to create discs with large amounts of material in standard definition quality, such as TV shows. Where a typical season of a TV show required multiple disc sets, entire series can now be stored on one Blu-ray Disc (for example: Seinfeld - Seasons 1-3, 40 episodes requires eight DVD's. This could fit on one or two BDs depending upon bonus content).

What's more, the capacity and the enhanced interactive and network features of the Blu-ray Disc for Movie Distribution format also offers new and unprecedented features. For example, additional video material may be stored on the disc in a "locked" way, only accessible to users who have been authorized to do so after an online payment procedure. Additionally, users might be able to make direct purchases of merchandise related to the disc's content, such as sequel discs or theater tickets. The interactive features of Blu-ray Disc go way beyond those offered by DVD-Video, adding additional value to a title release on Blu-ray Disc.

Lastly, although the Blu-ray Disc format does incorporate advanced codecs, it provides enough room to use the MPEG-2 format for encoding of High Definition images at a high bitrate. As MPEG-2 is the de facto industry standard used for DVD, digital broadcasting, HDTV and most other industry areas involving digital video, there is very broad industry support for MPEG-2 authoring equipment.

Advantages of Wi-Fi

Allows LANs to be deployed without cabling, typically reducing the costs of network deployment and expansion. Spaces where cables cannot be run, such as outdoor areas and historical buildings, can host wireless LANs.

Wi-Fi silicon pricing continues to come down, making Wi-Fi a very economical networking option and driving inclusion of Wi-Fi in an ever-widening array of devices.

Wi-Fi products are widely available in the market. Different brands of access points and client network interfaces are interoperable at a basic level of service. Products designated as Wi-Fi CERTIFIED by the Wi-Fi Alliance are interoperable and include WPA2 security.

Wi-Fi networks support roaming, in which a mobile client station such as a laptop computer can move from one access point to another as the user moves around a building or area.

Wi-Fi is a global set of standards. Unlike cellular carriers, the same Wi-Fi client works in different countries around the world.

Widely available in more than 250,000 public hot spots and millions of homes and corporate and university campuses worldwide.

As of 2006, WPA and WPA2 encryption are not easily crackable if strong passwords are used

New protocols for Quality of Service (WMM) and power saving mechanisms (WMM Power Save) make Wi-Fi even more suitable for latency-sensitive applications (such as voice and video) and small form-factor devices.

Disadvantages of Wi-Fi

Spectrum assignments and operational limitations are not consistent worldwide; most of Europe allows for an additional 2 channels beyond those permitted in the US; Japan has one more on top of that - and some countries, like Spain, prohibit use of the lower-numbered channels. Furthermore some countries, such as Italy, used to require a 'general authorization' for any Wi-Fi used outside an operator's own premises, or require something akin to an operator registration. For Europe; consult http://www.ero.dk for an annual report on the additional restrictions each European country imposes.

EIRP in the EU is limited to 20dbm.

Power consumption is fairly high compared to some other standards, making battery life and heat a concern.

The most common wireless encryption standard, Wired Equivalent Privacy or WEP, has been shown to be breakable even when correctly configured.

Wi-Fi Access Points typically default to an open (encryption-free) mode. Novice users benefit from a zero configuration device that works out of the box but might not intend to provide open wireless access to their LAN. WPA Wi-Fi Protected Access which began shipping in 2003 aims to solve these problems and is now generally available, but adoption rates remain low.

Many 2.4 GHz 802.11b and 802.11g Access points default to the same channel, contributing to congestion on certain channels.

Wi-Fi networks have limited range. A typical Wi-Fi home router using 802.11b or 802.11g with a stock antenna might have a range of 45 m (150 ft) indoors and 90 m (300 ft) outdoors. Range also varies with frequency band, as Wi-Fi is no exception to the physics of radio wave propagation. Wi-Fi in the 2.4 GHz frequency block has better range than Wi-Fi in the 5 GHz frequency block, and less range than the oldest Wi-Fi (and pre-Wi-Fi) 900 MHz block. Outdoor range with improved antennas can be several kilometres or more with line-of-sight.

Wi-Fi pollution, meaning interference of a closed or encrypted access point with other open access points in the area, especially on the same or neighboring channel, can prevent access and interfere with the use of other open access points by others caused by overlapping channels in the 802.11g/b spectrum as well as

with decreased signal-to-noise ratio (SNR) between access points. This is a widespread problem in high-density areas such as large apartment complexes or office buildings with many Wi-Fi access points.

It is also an issue when municipalities or other large entities such as universities seek to provide large area coverage. Everyone is considered equal when they use the band (except for amateur radio operators who are the primary licensee); often this causes contention when one user seeks to claim priority in this unlicensed band. This openness is also important to the success and widespread use of Wi-Fi, but makes Part 15 (US) unsuitable for "must have" public service functions.

Wi-Fi networks can be monitored and used to read and copy data (including personal information) transmitted over the network when no encryption such as VPN is used.

Interoperability issues between brands or deviations from the standard can disrupt connections or lower throughput speeds on other user's devices within range. Wi-Fi Alliance programs test devices for interoperability and designate devices which pass testing as Wi-Fi CERTIFIED.