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  Wikipedia: IEEE 802.11

Wikipedia: IEEE 802.11
IEEE 802.11
From Wikipedia, the free encyclopedia.

IEEE 802.11 or Wi-Fi denotes a set of Wireless LAN standards developed by working group 11 of IEEE 802. The term is also used specifically for the original version; to avoid confusion that is sometimes called "802.11legacy".

The 802.11 family currently includes three separate protocols that focus on encoding (a, b, g); other standards in the family (c-f, h-j, n) are service enhancement and extensions, or corrections to previous specifications. 802.11b was the first widely accepted wireless networking standard, followed, paradoxically, by 802.11a and 802.11g.

The frequencies used are in the microwave range.

Protocols

802.11legacy

The original version of the standard IEEE 802.11 released in 1997 and sometimes called "802.11legacy" specifies two data rates of 1 and 2 Megabits per second (Mbit/s) to be transmitted via infrared (IR) signals or in the ISM band at 2.4 GHz. IR has been dropped from later revisions of the standard, because it couldn't succeed against the well established IrDA protocol and had no known implementations. Legacy 802.11 was rapidly succeeded by 802.11b.

802.11b

802.11b has a range of about 50 metres with the low-gain omnidirectional antennas typically used in 802.11b devices. 802.11b has a maximum throughput of 11 Mbit/s, however a significant percentage of this bandwidth is used for communications overhead; in practice the maximum throughput is about 5.5 Mbit/s. Metal, water, and particularly thick walls absorb 802.11b signals and decrease the range drastically. 802.11 runs in the 2.4 GHz spectrum and uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) as its media access method.

With high-gain external antennas, the protocol can also be used in fixed point-to-point scenarios (8 kilometres), reports of up to 80-120 km line of sight) to replace costly leased lines, or in place of very cumbersome microwave communications gear. Current cards can operate at 11 Mbit/s, but will scale back to 5.5, then 2, then 1, if signal strength is an issue. 802.11b divides spectrum in 14 overlapping, staggered channels of 22 megahertz (MHz) each. Different channels or ranges are legal in different countries. Three or four channels may be used simultaneously in the same area with little or no overlap, typically 1, 6, and 11.

Extensions have been made to the 802.11b protocol in order to increase speed to 22, 33, and 44 Mbit/s, but the extensions are proprietary and not endorsed by the IEEE. Many companies call enhanced versions "802.11b+".

802.11a

In 2001 a faster relative started shipping, 802.11a, even though the standard was ratified in 1999. The 802.11a standard uses the 5 GHz band, and operates at a raw speed of 54 Mbit/s, and more realistic speeds in the mid-20 Mbit/s. The speed is reduced to 48, 36, 34, 18, 12, 9 then 6 Mbit/s if required. 802.11a has 12 nonoverlapping channels, 8 dedicated to indoor and 4 to point to point. Different countries have different ideas about support, although a 2003 World Radiotelecommunciations Conference made it easier for use worldwide. A mid-2003 FCC decision may open more spectrum to 802.11a channels as well.

802.11a has not seen wide adoption because of the high adoption rate of 802.11b, and concerns about range: at 5 GHz, 802.11a cannot reach as far with the same power limitations, and may be absorbed more readily. Most manufacturers of 802.11a equipment countered the lack of market success by releasing dual-band/dual-mode or tri-mode cards that can automatically handle 802.11a and b or a, b and g as available, or access points which can support all standards simultaneously.

802.11g

In June 2003, a third standard for encoding was ratified: 802.11g. This flavor works in the 2.4 GHz band like 802.11b, but operates at 54 Mbit/s raw or about 24.7 Mbit/s net throughput like 802.11a. It is fully backwards compatible with b, and details of making b and g work together well occupied much of the lingering technical process.

The 802.11g standard swept the consumer world of early adopters starting in January 2003, well before ratification. The corporate users held back and Cisco and other big equipment makers waited until ratification. By summer 2003, announcements were flourishing. Most of the dual-band 802.11a/b products became dual-band/tri-mode, supporting a, b, and g in a single card or access point.

Overview

StandardTransfer MethodFrequency BandData Rates [Mbit/s]
802.11 legacyFHSS, DSSS, IR2.4 GHz, IR 1, 2
802.11bDSSS, HR-DSSS2.4 GHz1, 2, 5.5, 11
"802.11b+" non-standardDSSS, HR-DSSS (PBCC)2.4 GHz1, 2, 5.5, 11, 22, 33, 44
802.11aOFDM 5.2, 5.5 GHz6, 9, 12, 18, 24, 36, 48, 54
802.11gDSSS, HR-DSSS, OFDM2.4 GHz1, 2, 5.5, 11; 6, 9, 12, 18, 24, 36, 48, 54
whis is

To be merged:

  • IEEE 802.11b enjoys international acceptance, as the 2.4-GHz radio frequency band is almost universally available. 802.11b hardware can transmit data at speeds of up to 11 megabits per second (Mbit/s). The first widespread commercial use of the 802.11b standard for networking was made by Apple Computer under the trademark AirPort.
  • IEEE 802.11g operates in the same frequency band as 802.11b, and is therefore backwards compatible with certain older Wi-Fi hardware. 802.11g hardware can transfer data at up to 54 Mbit/s, or at 11Mbit/s if operating with 802.11b devices. The first major manufacturer to use of 802.11g was again Apple, under the trademark AirPort Extreme.
  • IEEE 802.11a, which operates around the 5 GHz band, enjoys relatively clear-channel operation in the United States and Japan. In other areas, such as the EU, 802.11a is not yet approved for operation in the 5 GHz band, and European regulators are still considering the use of the European HIPERLAN standard. 802.11a also provides for up to 54 Mbit/s operation, but is not interoperable with 802.11b.

Certification

Because the IEEE only sets specifications but doesn't test them, a trade group called the Wi-Fi Alliance runs a certification program that members pay to participate in. Virtually all companies selling 802.11 equipment are members. The Wi-Fi trademark, owned by the group, guarantees interoperability. Currently, Wi-Fi can mean any of 802.11a, b, or g; by fall, Wi-Fi also includes the security standard Wi-Fi Protected Access or WPA. Products that say (they are) Wi-Fi are supposed to also indicate the band in which they operate in, 2.4 or 5 GHz.

Standards

The following standards and task groups exist with the working group:

Community networks

With the proliferation of cable modems and
DSL, there is an ever-increasing market of people who wish to establish small networks in their homes to share their high speed Internet connection. Wireless office networks are often not protected and let "people on the street" connect to the internet. There are also efforts by volunteer groups to establish wireless community networks to provide free wireless connectivity to the public.

Security

In 2001, a group from the University of California at Berkeley presented a paper describing a weakness in 802.11b described by Fluhrer, Mantin, and Shamir entitled "Weaknesses in the Key Scheduling Algorithm of RC4". This presentation was soon followed by Adam Stubblefield and AT&T publicly announcing the first verification of the attack. In the attack they were able to intercept transmissions and gain unauthorized access to wireless networks.

The IEEE set up a dedicated task group to create a replacement security solution, 802.11i (previously this work was handled as part of a broader 802.11e effort to enhance the MAC layer). While 802.11i is still a work in progress, and is not expected to be completed until late 2003, the Wi-Fi Alliance has announced an interim specification called Wireless Protected Access (WPA) based on a subset of the current IEEE draft. These started to appear in products in mid-2003, and implementation will be mandatory by fall 2003 in order to display the Wi-Fi logo.

See also

  • Bluetooth, another wireless protocol primarily designed for shorter range applications.
  • Apple Airport, with implementations of 802.11b and 802.11g.

External Links


  

From Wikipedia, the free encyclopedia. 
Modified by Geona