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
Standard Transfer Method Frequency Data Rates [Mbit/s]
Band
802.11 legacy FHSS, DSSS, IR 2.4 GHz, IR 1, 2
802.11b DSSS, HR-DSSS 2.4 GHz 1, 2, 5.5, 11
"802.11b+" DSSS, HR-DSSS 1, 2, 5.5, 11, 22, 33,
non-standard (PBCC) 2.4 GHz 44
802.11a OFDM 5.2, 5.5 GHz 6, 9, 12, 18, 24, 36,
48, 54
802.11g DSSS, HR-DSSS, 2.4 GHz 1, 2, 5.5, 11; 6, 9,
OFDM 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.
This content from Wikipedia is licensed under the GNU Free Documentation License.
|
