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IEEE
802.11
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.
The standard has been proprietary extended to support 22, 33 and
44 Mbit/s and is then promoted as "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
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Standard
|
Transfer
Method
|
Frequency
Band
|
Data
Rates [Mbit/s]
|
|
802.11
legacy
|
FHSS,
DSSS,
IR
|
2.4
GHz, IR
|
1,
2
|
|
802.11b
|
DSSS,
HR-DSSS
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2.4
GHz
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1,
2, 5.5, 11
|
|
"802.11b+"
non-standard
|
DSSS,
HR-DSSS (PBCC)
|
2.4
GHz
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1,
2, 5.5, 11, 22, 33, 44
|
|
802.11a
|
OFDM
|
5.2,
5.5 GHz
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6,
9, 12, 18, 24, 36, 48, 54
|
|
802.11g
|
DSSS,
HR-DSSS, OFDM
|
2.4
GHz
|
1,
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 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:
-
IEEE
802.11 - The original 2 Mbit/s, 2.4 GHz standard
-
IEEE
802.11a - 54 Mbit/s, 5 GHz standard (1999, shipping products
in 2001)
-
IEEE
802.11b - Enhancements to 802.11 to support 5.5 and 11 Mbit/s
(1999)
-
IEEE
802.11d - New countries
-
IEEE
802.11e - Enhancements: QoS,
including packet bursting
-
IEEE
802.11f - Inter-Access
Point Protocol (IAPP)
-
IEEE
802.11g - 54 Mbit/s, 2.4 GHz standard (backwards compatible
with b) (2003)
-
IEEE
802.11h - 5 GHz spectrum, Dynamic
Channel/Frequency
Selection (DCS/DFS) and Transmit
Power Control (TPC) for European compatibility
-
IEEE
802.11i - Enhanced security
-
IEEE
802.11j - Extensions for Japan
-
IEEE
802.11n - Higher throughput improvements
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 an 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
This
content from Wikipedia
is licensed under the GNU
Free Documentation License.
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