From Wikipedia, the free encyclopediaIEEE
a set of media
access control (MAC)
specifications for implementing wireless
local area network (WLAN)
computer communication in the 900 MHz and 2.4, 3.6,
5, and 60 GHz frequency
bands. They are created and maintained by the Institute
of Electrical and Electronics Engineers (IEEE) LAN/MAN Standards
The base version of the standard was released in 1997,
and has had subsequent amendments. The standard and
amendments provide the basis for wireless network
products using the Wi-Fi brand.
While each amendment is officially revoked when it is
incorporated in the latest version of the standard, the
corporate world tends to market to the revisions because
they concisely denote capabilities of their products. As
a result, in the marketplace, each revision tends to
become its own standard.
The 802.11 family consists of a series of half-duplex over-the-air modulation techniques
that use the same basic protocol. 802.11-1997 was the
first wireless networking standard in the family, but
802.11b was the first widely accepted one, followed by
802.11a, 802.11g, 802.11n, and 802.11ac. Other standards
in the family (c–f, h, j) are service amendments that
are used to extend the current scope of the existing
standard, which may also include corrections to a
802.11b and 802.11g use the 2.4 GHz ISM
band, operating in the United States under Part
15 of the
Communications Commission Rules
and Regulations. Because of this choice of frequency
band, 802.11b and g equipment may occasionally suffer interference from microwave
telephones, and Bluetooth devices.
802.11b and 802.11g control their interference and
susceptibility to interference by using direct-sequence
spread spectrum (DSSS)
frequency-division multiplexing (OFDM)
signaling methods, respectively. 802.11a uses the 5 GHz
U-NII band, which, for much of the world, offers at
least 23 non-overlapping channels rather than the
2.4 GHz ISM frequency band offering only three
non-overlapping channels, where other adjacent channels
of WLAN channels. Better or worse performance with
higher or lower frequencies (channels) may be realized,
depending on the environment. 802.11n can use either the
2.4 GHz or the 5 GHz band; 802.11ac uses only the 5 GHz
The segment of the radio
used by 802.11 varies between countries. In the US,
802.11a and 802.11g devices may be operated without a
license, as allowed in Part 15 of the FCC Rules and
Regulations. Frequencies used by channels one through
six of 802.11b and 802.11g fall within the 2.4 GHz amateur
Licensed amateur radio operators may operate 802.11b/g
devices under Part
97 of the
FCC Rules and Regulations, allowing increased power
output but not commercial content or encryption.
802.11 technology has its origins in a 1985 ruling by
the U.S. Federal Communications Commission that released
In 1991 NCR
Corporation/AT&T (now Nokia
Labs and LSI
Corporation) invented a precursor to 802.11 in Nieuwegein,
The Netherlands. The inventors initially intended to use
the technology for cashier systems. The first wireless
products were brought to the market under the name WaveLAN with
raw data rates of 1 Mbit/s and 2 Mbit/s.
Vic Hayes, who held the chair of IEEE 802.11 for 10
years, and has been called the "father of Wi-Fi", was
involved in designing the initial 802.11b and 802.11a
standards within the IEEE.
In 1999, the Wi-Fi
formed as a trade association to hold the Wi-Fi trademark
under which most products are sold.
network PHY standards
6, 9, 12, 18, 24, 36, 48, 54
1, 2, 5.5, 11
6, 9, 12, 18, 24, 36, 48, 54
400 ns GI : 7.2, 14.4, 21.7, 28.9, 43.3, 57.8,
65, 72.2 [B]
800 ns GI : 6.5, 13, 19.5, 26, 39, 52, 58.5, 65 [C]
400 ns GI : 15, 30, 45, 60, 90, 120, 135, 150 [B]
800 ns GI : 13.5, 27, 40.5, 54, 81, 108, 121.5,
400 ns GI : 7.2, 14.4, 21.7, 28.9, 43.3, 57.8,
65, 72.2, 86.7, 96.3 [B]
800 ns GI : 6.5, 13, 19.5, 26, 39, 52, 58.5, 65,
78, 86.7 [C]
400 ns GI : 15, 30, 45, 60, 90, 120, 135, 150,
180, 200 [B]
800 ns GI : 13.5, 27, 40.5, 54, 81, 108, 121.5,
135, 162, 180 [C]
400 ns GI : 32.5, 65, 97.5, 130, 195, 260,
292.5, 325, 390, 433.3 [B]
800 ns GI : 29.2, 58.5, 87.8, 117, 175.5, 234,
263.2, 292.5, 351, 390[C]
400 ns GI : 65, 130, 195, 260, 390, 520, 585,
650, 780, 866.7 [B]
800 ns GI : 58.5, 117, 175.5, 234, 351, 468,
702, 780 [C]
Up to 6,912 (6.75 Gbit/s) 
OFDM, single carrier,
low-power single carrier
Up to 100,000 (100 Gbit/s)
OFDM, single carrier,
The original version of the standard IEEE 802.11 was
released in 1997 and clarified in 1999, but is now
obsolete. It specified two net
bit rates of
1 or 2 megabits
per second (Mbit/s),
error correction code.
It specified three alternative physical
diffuse infrared operating
at 1 Mbit/s; frequency-hopping spread
spectrum operating at 1 Mbit/s or 2 Mbit/s; and direct-sequence spread
spectrum operating at 1 Mbit/s or 2 Mbit/s. The latter
two radio technologies used microwave transmission
over the Industrial
Scientific Medical frequency band at
2.4 GHz. Some earlier WLAN technologies used lower
frequencies, such as the U.S. 900 MHz
Legacy 802.11 with direct-sequence spread spectrum was
rapidly supplanted and popularized by 802.11b.
Originally described as clause 17 of the 1999
specification, the OFDM waveform at 5.8 GHz is now
defined in clause 18 of the 2012 specification, and
provides protocols that allow transmission and reception
of data at rates of 1.5 to 54 Mbit/s. It has seen
widespread worldwide implementation, particularly within
the corporate workspace. While the original amendment is
no longer valid, the term 802.11a is
still used by wireless access point (cards and routers)
manufacturers to describe interoperability of their
systems at 5.8 GHz, 54 Mbit/s.
The 802.11a standard uses the same data link layer
protocol and frame format as the original standard, but
an OFDM based
air interface (physical layer). It operates in the 5 GHz
band with a maximum net data rate of 54 Mbit/s, plus
error correction code, which yields realistic net
achievable throughput in the mid-20 Mbit/s.
Since the 2.4 GHz band is heavily used to the point of
being crowded, using the relatively unused 5 GHz band
gives 802.11a a significant advantage. However, this
brings a disadvantage: the effective overall range of
802.11a is less than that of 802.11b/g. In theory,
802.11a signals are absorbed more readily by walls and
other solid objects in their path due to their smaller
wavelength, and, as a result, cannot penetrate as far as
those of 802.11b. In practice, 802.11b typically has a
higher range at low speeds (802.11b will reduce speed to
5.5 Mbit/s or even 1 Mbit/s at low signal strengths).
802.11a also suffers from interference, but
locally there may be fewer signals to interfere with,
resulting in less interference and better throughput.
The 802.11b standard has a maximum raw data rate of 11 Mbit/s,
and uses the same media access method defined in the
original standard. 802.11b products appeared on the
market in early 2000, since 802.11b is a direct
extension of the modulation technique defined in the
original standard. The dramatic increase in throughput
of 802.11b (compared to the original standard) along
with simultaneous substantial price reductions led to
the rapid acceptance of 802.11b as the definitive
wireless LAN technology.
Devices using 802.11b experience interference from other
products operating in the 2.4 GHz band. Devices
operating in the 2.4 GHz range include microwave ovens,
Bluetooth devices, baby monitors, cordless telephones,
and some amateur radio equipment.
In June 2003, a third modulation standard was ratified:
802.11g. This works in the 2.4 GHz band (like 802.11b),
but uses the same OFDM based
transmission scheme as 802.11a. It operates at a maximum
physical layer bit rate of 54 Mbit/s exclusive of
forward error correction codes, or about 22 Mbit/s
average throughput. 802.11g
hardware is fully backward compatible with 802.11b
hardware, and therefore is encumbered with legacy issues
that reduce throughput by ~21% when compared to 802.11a.
The then-proposed 802.11g standard was rapidly adopted
in the market starting in January 2003, well before
ratification, due to the desire for higher data rates as
well as to reductions in manufacturing costs. By summer
2003, most dual-band 802.11a/b products became
dual-band/tri-mode, supporting a and b/g in a single
access point. Details of making b and g work well
together occupied much of the lingering technical
process; in an 802.11g network, however, activity of an
802.11b participant will reduce the data rate of the
overall 802.11g network.
Like 802.11b, 802.11g devices suffer interference from
other products operating in the 2.4 GHz band, for
example wireless keyboards.
In 2003, task group TGma was authorized to "roll up"
many of the amendments to the 1999 version of the 802.11
standard. REVma or 802.11ma, as it was called, created a
single document that merged 8 amendments (802.11a, b, d, e, g, h, i, j)
with the base standard. Upon approval on March 8, 2007,
802.11REVma was renamed to the then-current base
802.11n is an amendment that improves upon the previous
802.11 standards by adding multiple-input
(MIMO). 802.11n operates on both the 2.4 GHz and the
5 GHz bands. Support for 5 GHz bands is optional. It
operates at a maximum net data rate from 54 Mbit/s to
600 Mbit/s. The IEEE has approved the amendment, and it
was published in October 2009. Prior
to the final ratification, enterprises were already
migrating to 802.11n networks based on the Wi-Fi
Alliance's certification of products conforming to a
2007 draft of the 802.11n proposal.
In 2007, task group TGmb was authorized to "roll up"
many of the amendments to the 2007 version of the 802.11
standard. REVmb or 802.11mb, as it was called, created a
single document that merged ten amendments (802.11k, r, y, n, w, p, z, v, u, s)
with the 2007 base standard. In addition much cleanup
was done, including a reordering of many of the clauses. Upon
publication on March 29, 2012, the new standard was
referred to as IEEE
IEEE 802.11ac-2013 is an amendment to IEEE 802.11,
published in December 2013, that builds on 802.11n. Changes
compared to 802.11n include wider channels (80 or
160 MHz versus 40 MHz) in the 5 GHz band, more spatial
streams (up to eight versus four), higher-order
modulation (up to 256-QAM vs.
64-QAM), and the addition of Multi-user
As of October 2013, high-end implementations support
80 MHz channels, three spatial streams, and 256-QAM,
yielding a data rate of up to 433.3 Mbit/s per spatial
stream, 1300 Mbit/s total, in 80 MHz channels in the
5 GHz band. Vendors
have announced plans to release so-called "Wave 2"
devices with support for 160 MHz channels, four spatial
streams, and MU-MIMO in 2014 and 2015.
This section needs
to be updated. (November
IEEE 802.11ad is an amendment that defines a new physical
802.11 networks to operate in the 60 GHz millimeter
This frequency band has significantly different
propagation characteristics than the 2.4 GHz and 5 GHz
bands where Wi-Fi networks
operate. Products implementing the 802.11ad standard
are being brought to market under the WiGig brand name.
The certification program is now being developed by the Wi-Fi
of the now defunct WiGig Alliance. The
peak transmission rate of 802.11ad is 7 Gbit/s.
TP-Link announced the world's first 802.11ad router in
IEEE 802.11af, also referred to as "White-Fi" and "Super
an amendment, approved in February 2014, that allows
WLAN operation in TV white
space spectrum in
the VHF and UHF bands
between 54 and 790 MHz. It
to transmit on unused TV channels, with the standard
taking measures to limit interference for primary users,
such as analog TV, digital TV, and wireless microphones. Access
points and stations determine their position using a
satellite positioning system such as GPS,
and use the Internet to query a geolocation database
(GDB) provided by a regional regulatory agency to
discover what frequency channels are available for use
at a given time and position.The
physical layer uses OFDM and is based on 802.11ac. The
propagation path loss as well as the attenuation by
materials such as brick and concrete is lower in the UHF
and VHF bands than in the 2.4 and 5 GHz bands, which
increases the possible range. The
frequency channels are 6 to 8 MHz wide, depending on the
regulatory domain. Up
to four channels may be bonded in either one or two
contiguous blocks. MIMO
operation is possible with up to four streams used for
block code (STBC)
or multi-user (MU) operation. The
achievable data rate per spatial stream is 26.7 Mbit/s
for 6 and 7 MHz channels, and 35.6 Mbit/s for 8 MHz
four spatial streams and four bonded channels, the
maximum data rate is 426.7 Mbit/s for 6 and 7 MHz
channels and 568.9 Mbit/s for 8 MHz channels.
IEEE 802.11ah defines a WLAN system operating at
sub-1 GHz license-exempt bands, with final approval
slated for September 2016. Due
to the favorable propagation characteristics of the low
frequency spectra, 802.11ah can provide improved
transmission range compared with the conventional 802.11
WLANs operating in the 2.4 GHz and 5 GHz bands. 802.11ah
can be used for various purposes including large scale
sensor networks, extended
range hotspot, and outdoor Wi-Fi for cellular traffic
offloading, whereas the available bandwidth is
relatively narrow. The protocol intends consumption to
be competitive with low power Bluetooth,
at a much wider range.
IEEE 802.11ai is an amendment to the 802.11 standard
that will add new mechanisms for a faster initial link
IEEE 802.11aj is a rebanding of 802.11ad for use in the
45 GHz unlicensed spectrum available in some regions of
the world (specifically China).
IEEE 802.11aq is an amendment to the 802.11 standard
that will enable pre-association discovery of services.
This extends some of the mechanisms in 802.11u that
enabled device discovery to further discover the
services running on a device, or provided by a network.
IEEE 802.11ax is the successor to 802.11ac, and will
increase the efficiency of WLAN networks. Currently at a
very early stage of development this project has the
goal of providing 4x the throughput of 802.11ac.
This section needs
to be updated. (March
IEEE 802.11ay is a standard that is being developed. It
is an amendment that defines a new physical
802.11 networks to operate in the 60 GHz millimeter
It will be an extension of the existing 11ad, aimed to
extend the throughput, range and use-cases. The main
use-cases include: indoor operation, out-door back-haul
and short range communications. The peak transmission
rate of 802.11ay is 20 Gbit/s. The
main extensions include: channel bonding (2, 3 and 4),
MIMO and higher modulation schemes.
Across all variations of 802.11, maximum achievable
throughputs are given either based on measurements under
ideal conditions or in the layer-2 data rates. This,
however, does not apply to typical deployments in which
data is being transferred between two endpoints, of
which at least one is typically connected to a wired
infrastructure and the other endpoint is connected to an
infrastructure via a wireless link.
This means that, typically, data frames pass an 802.11
(WLAN) medium, and are being converted to 802.3 (Ethernet)
or vice versa. Due to the difference in the frame
(header) lengths of these two media, the application's
packet size determines the speed of the data transfer.
This means applications that use small packets (e.g.,
VoIP) create dataflows with high-overhead traffic (i.e.,
a low goodput).
Other factors that contribute to the overall application
data rate are the speed with which the application
transmits the packets (i.e., the data rate) and, of
course, the energy with which the wireless signal is
received. The latter is determined by distance and by
the configured output power of the communicating
The same references apply to the attached graphs that
show measurements of UDP throughput.
Each represents an average (UDP) throughput (please note
that the error bars are there, but barely visible due to
the small variation) of 25 measurements. Each is with a
specific packet size (small or large) and with a
specific data rate (10 kbit/s – 100 Mbit/s). Markers for
traffic profiles of common applications are included as
well. These figures assume there are no packet errors,
which if occurring will lower transmission rate further.
802.11b, 802.11g, and 802.11n-2.4 utilize the 2.400–2.500
one of the ISM
bands. 802.11a and 802.11n use the more heavily
These are commonly referred to as the "2.4 GHz and 5 GHz
bands" in most sales literature. Each spectrum is
sub-divided into channels with
a center frequency and bandwidth, analogous to the way
radio and TV broadcast bands are sub-divided.
The 2.4 GHz band is divided into 14 channels spaced
5 MHz apart, beginning with channel 1, which is centered
on 2.412 GHz. The latter channels have additional
restrictions or are unavailable for use in some
The channel numbering of the 5.725–5.875
is less intuitive due to the differences in regulations
between countries. These are discussed in greater detail
on the list
of WLAN channels.
In addition to specifying the channel center frequency,
802.11 also specifies (in Clause 17) a spectral
the permitted power distribution across each channel.
The mask requires the signal be attenuated a
minimum of 20 dB from
its peak amplitude at ±11 MHz from the centre frequency,
the point at which a channel is effectively 22 MHz wide.
One consequence is that stations can use only every
fourth or fifth channel without overlap.
Availability of channels is regulated by country,
constrained in part by how each country allocates
radio spectrum to
various services. At one extreme, Japan permits the use
of all 14 channels for 802.11b, and 1–13 for
802.11g/n-2.4. Other countries such as Spain initially
allowed only channels 10 and 11, and France allowed only
10, 11, 12, and 13; however, they now allow channels 1
through 13. North
America and some Central and South American countries
allow only 1
Since the spectral mask defines only power output
restrictions up to ±11 MHz from the center frequency to
be attenuated by −50 dBr, it is often assumed that the
energy of the channel extends no further than these
limits. It is more correct to say that, given the
separation between channels, the overlapping signal on
any channel should be sufficiently attenuated to
minimally interfere with a transmitter on any other
channel. Due to the near-far
transmitter can impact (desense) a receiver on a
"non-overlapping" channel, but only if it is close to
the victim receiver (within a meter) or operating above
allowed power levels.
Confusion often arises over the amount of channel
separation required between transmitting devices.
802.11b was based on DSSS modulation and utilized a
channel bandwidth of 22 MHz, resulting in three "non-overlapping"
channels (1, 6, and 11). 802.11g was based on OFDM
modulation and utilized a channel bandwidth of 20 MHz.
This occasionally leads to the belief that four "non-overlapping"
channels (1, 5, 9, and 13) exist under 802.11g, although
this is not the case as per 188.8.131.52 Channel Numbering
of operating channels of the IEEE Std 802.11 (2012),
which states "In a multiple cell network topology,
overlapping and/or adjacent cells using different
channels can operate simultaneously without interference
if the distance between the center frequencies is at
least 25 MHz." and
section 184.108.40.206 and Figure 18-13.
This does not mean that the technical overlap of the
channels recommends the non-use of overlapping channels.
The amount of interference seen on a configuration using
channels 1, 5, 9, and 13 can have very small difference
from a three-channel configuration, and
in the paper entitled "Effect of adjacent-channel
interference in IEEE 802.11 WLANs" by Villegas this is
Although the statement that channels 1, 5, 9, and 13 are
"non-overlapping" is limited to spacing or product
density, the concept has some merit in limited
circumstances. Special care must be taken to adequately
space AP cells, since overlap between the channels may
cause unacceptable degradation of signal quality and
more advanced equipment such as spectral
available, overlapping channels may be used under
certain circumstances. This way, more channels are
IEEE uses the phrase regdomain to
refer to a legal regulatory region. Different countries
define different levels of allowable transmitter power,
time that a channel can be occupied, and different
available channels. Domain
codes are specified for the United States, Canada, ETSI
(Europe), Spain, France, Japan,
default to regdomain 0,
which means least
common denominator settings,
i.e., the device will not transmit at a power above the
allowable power in any nation, nor will it use
frequencies that are not permitted in any nation.
The regdomain setting
is often made difficult or impossible to change so that
the end users do not conflict with local regulatory
agencies such as the United
The datagrams are
Current 802.11 standards specify frame types for use in
transmission of data as well as management and control
of wireless links.
Frames are divided into very specific and standardized
sections. Each frame consists of a MAC
check sequence (FCS).
Some frames may not have a payload.
The first two bytes of the MAC header form a frame
control field specifying the form and function of the
frame. This frame control field is subdivided into the
bits representing the protocol version. Currently
used protocol version is zero. Other values are
reserved for future use.
- Type: Two
bits identifying the type of WLAN frame. Control,
Data, and Management are various frame types defined
in IEEE 802.11.
- Subtype: Four
bits providing additional discrimination between
frames. Type and Subtype are used together to
identify the exact frame.
- ToDS and FromDS: Each
is one bit in size. They indicate whether a data
frame is headed for a distribution system. Control
and management frames set these values to zero. All
the data frames will have one of these bits set.
However communication within an Independent
Basic Service Set (IBSS)
network always set these bits to zero.
- More Fragments: The
More Fragments bit is set when a packet is divided
into multiple frames for transmission. Every frame
except the last frame of a packet will have this bit
- Retry: Sometimes
frames require retransmission, and for this there is
a Retry bit that is set to one when a frame is
resent. This aids in the elimination of duplicate
bit indicates the power management state of the
sender after the completion of a frame exchange.
Access points are required to manage the connection,
and will never set the power-saver bit.
- More Data: The
More Data bit is used to buffer frames received in a
distributed system. The access point uses this bit
to facilitate stations in power-saver mode. It
indicates that at least one frame is available, and
addresses all stations connected.
Protected Frame bit is set to one if the frame body
is encrypted by a protection mechanism such as Wired
Equivalent Privacy (WEP), Wi-Fi
Protected Access (WPA),
Protected Access II (WPA2).
- Order: This
bit is set only when the "strict ordering" delivery
method is employed. Frames and fragments are not
always sent in order as it causes a transmission
The next two bytes are reserved for the Duration ID
field. This field can take one of three forms: Duration,
Contention-Free Period (CFP), and Association ID (AID).
An 802.11 frame can have up to four address fields. Each
field can carry a MAC
address. Address 1 is the receiver, Address 2 is the
transmitter, Address 3 is used for filtering purposes by
The remaining fields of the header are:
- The Sequence
Control field is a two-byte section used for
identifying message order as well as eliminating
duplicate frames. The first 4 bits are used for the
fragmentation number, and the last 12 bits are the
- An optional
two-byte Quality of Service control field that was
added with 802.11e.
The payload or frame body field is variable in size,
from 0 to 2304 bytes plus any overhead from security
encapsulation, and contains information from higher
The Frame Check Sequence (FCS) is the last four bytes in
the standard 802.11 frame. Often referred to as the
Cyclic Redundancy Check (CRC), it allows for integrity
check of retrieved frames. As frames are about to be
sent, the FCS is calculated and appended. When a station
receives a frame, it can calculate the FCS of the frame
and compare it to the one received. If they match, it is
assumed that the frame was not distorted during
Management frames allow for the maintenance of
communication. Some common 802.11 subtypes include:
frame: 802.11 authentication begins with the Wireless
network interface card (WNIC)
sending an authentication frame to the access point
containing its identity. With an open system
authentication, the WNIC sends only a single
authentication frame, and the access point responds
with an authentication frame of its own indicating
acceptance or rejection. With shared key
authentication, after the WNIC sends its initial
authentication request it will receive an
authentication frame from the access point
containing challenge text. The WNIC sends an
authentication frame containing the encrypted
version of the challenge text to the access point.
The access point ensures the text was encrypted with
the correct key by decrypting it with its own key.
The result of this process determines the WNIC's
request frame: Sent from a station it enables the
access point to allocate resources and synchronize.
The frame carries information about the WNIC,
including supported data rates and the SSID of
the network the station wishes to associate with. If
the request is accepted, the access point reserves
memory and establishes an association ID for the
response frame: Sent from an access point to a
station containing the acceptance or rejection to an
association request. If it is an acceptance, the
frame will contain information such an association
ID and supported data rates.
Beacon frame: Sent periodically from an access
point to announce its presence and provide the SSID,
and other parameters for WNICs within range.
frame: Sent from a station wishing to
terminate connection from another station.
frame: Sent from a station wishing to terminate
connection. It's an elegant way to allow the access
point to relinquish memory allocation and remove the
WNIC from the association table.
- Probe request
frame: Sent from a station when it requires
information from another station.
- Probe response
frame: Sent from an access point containing
capability information, supported data rates, etc.,
after receiving a probe request frame.
request frame: A WNIC sends a reassociation request
when it drops from range of the currently associated
access point and finds another access point with a
stronger signal. The new access point coordinates
the forwarding of any information that may still be
contained in the buffer of the previous access
response frame: Sent from an access point containing
the acceptance or rejection to a WNIC reassociation
request frame. The frame includes information
required for association such as the association ID
and supported data rates.
2. In terms of ICT,
is a part of management frames in the IEEE 802.11
wireless LAN protocol. IEs are a device's way to
transfer descriptive information about itself inside
management frames. There are usually several IEs inside
each such frame, and each is built of TLVs mostly
defined outside the basic IEEE 802.11 specification.
The common structure of an IE is as follows:
← 1 → ← 1 → ← 3 → ← 1-252 →
| Type |Length| OUI | Data |
Whereas the OUI (organizationally
unique identifier) is used only when necessary to
the protocol being used, and the data field holds the TLVs relevant
to that IE.
Control frames facilitate in the exchange of data frames
between stations. Some common 802.11 control frames
- Acknowledgement (ACK)
frame: After receiving a data frame, the receiving
station will send an ACK frame to the sending
station if no errors are found. If the sending
station doesn't receive an ACK frame within a
predetermined period of time, the sending station
will resend the frame.
- Request to Send (RTS)
frame: The RTS
and CTS frames provide
an optional collision reduction scheme for access
points with hidden stations. A station sends a RTS
frame as the first step in a two-way handshake
required before sending data frames.
- Clear to Send
(CTS) frame: A station responds to an RTS frame with
a CTS frame. It provides clearance for the
requesting station to send a data frame. The CTS
provides collision control management by including a
time value for which all other stations are to hold
off transmission while the requesting station
Data frames carry packets from web pages, files, etc.
within the body. The
body begins with an IEEE
with the Destination Service
Access Point (DSAP)
specifying the protocol; however, if the DSAP is hex AA,
the 802.2 header is followed by a Subnetwork
Access Protocol (SNAP)
header, with the Organizationally
Unique Identifier (OUI)
and protocol ID (PID) fields specifying the protocol. If
the OUI is all zeroes, the protocol ID field is an EtherType value. Almost
all 802.11 data frames use 802.2 and SNAP headers, and
most use an OUI of 00:00:00 and an EtherType value.
Similar to TCP
congestion control on
the internet, frame loss is built into the operation of
802.11. To select the correct transmission speed or Modulation
and Coding Scheme, a rate control algorithm may test
different speeds. The actual packet loss rate of an
Access points vary widely for different link conditions.
There are variations in the loss rate experienced on
production Access points, between 10% and 80%, with 30%
being a common average. It
is important to be aware that the link layer should
recover these lost frames. If the sender does not
receive an Acknowledgement (ACK) frame, then it will be
Within the IEEE 802.11 Working Group, the
Standards Association Standard
and Amendments exist:
IEEE 802.11-1997: The WLAN standard was
originally 1 Mbit/s and 2 Mbit/s, 2.4 GHz RF and infrared (IR)
standard (1997), all the others listed below are
Amendments to this standard, except for Recommended
Practices 802.11F and 802.11T.
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 Mbit/s and 11 Mbit/s (1999)
IEEE 802.11c: Bridge operation procedures;
included in the IEEE
IEEE 802.11d: International (country-to-country)
roaming extensions (2001)
IEEE 802.11e: Enhancements: QoS,
including packet bursting (2005)
IEEE 802.11F: Inter-Access
Point Protocol (2003) Withdrawn
IEEE 802.11g: 54 Mbit/s, 2.4 GHz standard
(backwards compatible with b) (2003)
IEEE 802.11h: Spectrum Managed 802.11a (5 GHz)
for European compatibility (2004)
IEEE 802.11i: Enhanced security (2004)
IEEE 802.11j: Extensions for Japan (2004)
- IEEE 802.11-2007:
A new release of the standard that includes
amendments a, b, d, e, g, h, i, and j. (July 2007)
IEEE 802.11k: Radio resource measurement
IEEE 802.11n: Higher-throughput improvements
using MIMO (multiple-input, multiple-output
antennas) (September 2009)
IEEE 802.11p: WAVE—Wireless Access for the
Vehicular Environment (such as ambulances and
passenger cars) (July 2010)
IEEE 802.11r: Fast BSS transition (FT) (2008)
IEEE 802.11s: Mesh Networking, Extended
Service Set (ESS)
- IEEE 802.11T:
Wireless Performance Prediction (WPP)—test methods
and metrics Recommendation cancelled
IEEE 802.11u: Improvements related to HotSpots
and 3rd-party authorization of clients, e.g.,
cellular network offload (February 2011)
IEEE 802.11v: Wireless network
IEEE 802.11w: Protected Management Frames
IEEE 802.11y: 3650–3700 MHz Operation in the
- IEEE 802.11z:
Extensions to Direct Link Setup (DLS) (September
- IEEE 802.11-2012:
A new release of the standard that includes
amendments k, n, p, r, s, u, v, w, y, and z (March
- IEEE 802.11aa:
Robust streaming of Audio Video Transport Streams
IEEE 802.11ac: Very High Throughput <6 GHz; potential
improvements over 802.11n: better modulation scheme
(expected ~10% throughput increase), wider channels
(estimate in future time 80 to 160 MHz), multi user
IEEE 802.11ad: Very High Throughput 60 GHz
(December 2012) — see WiGig
- IEEE 802.11ae:
Prioritization of Management Frames (March 2012)
IEEE 802.11af: TV Whitespace (February 2014)
- IEEE 802.11mc:
Roll-up of 802.11-2012 with the aa, ac, ad, ae & af
amendments to be published as 802.11-2016 (~
IEEE 802.11ah: Sub-1 GHz license exempt
operation (e.g., sensor network, smart metering) (~
IEEE 802.11ai: Fast Initial Link Setup (~
IEEE 802.11aj: China Millimeter Wave (~
- IEEE 802.11ak:
General Links (~
- IEEE 802.11aq:
Pre-association Discovery (~
- IEEE 802.11ax:
High Efficiency WLAN (~
- IEEE 802.11ay:
Enhancements for Ultra High Throughput in and around
the 60 GHz Band (~
- IEEE 802.11az:
Next Generation Positioning (~
802.11F and 802.11T are recommended practices rather
than standards, and are capitalized as such.
802.11m is used for standard maintenance. 802.11ma was
completed for 802.11-2007, 802.11mb was completed for
802.11-2012, and 802.11mc is working towards publishing
Both the terms "standard" and "amendment" are used when
referring to the different variants of IEEE standards.
As far as the IEEE Standards Association is concerned,
there is only one current standard; it is denoted by
IEEE 802.11 followed by the date that it was published.
IEEE 802.11-2012 is the only version currently in
publication. The standard is updated by means of
amendments. Amendments are created by task groups (TG).
Both the task group and their finished document are
denoted by 802.11 followed by a non-capitalized letter,
for example, IEEE
802.11a and IEEE
802.11b. Updating 802.11 is the responsibility of
task group m. In order to create a new version, TGm
combines the previous version of the standard and all
published amendments. TGm also provides clarification
and interpretation to industry on published documents.
New versions of the IEEE
published in 1999, 2007, and 2012. The next is expected
Various terms in 802.11 are used to specify aspects of
wireless local-area networking operation, and may be
unfamiliar to some readers.
For example, Time
abbreviated TU) is used to indicate a unit of time equal
to 1024 microseconds. Numerous time constants are
defined in terms of TU (rather than the nearly equal
Also the term "Portal" is used to describe an entity
that is similar to an 802.1H bridge.
A Portal provides access to the WLAN by non-802.11 LAN
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
broadband Internet connection.
Many hotspot or free networks frequently allow anyone
within range, including passersby outside, to connect to
the Internet. There are also efforts by volunteer groups
to establish wireless
community networks to
provide free wireless connectivity to the public.
In 2001, a group from the University
of California, Berkeley presented
a paper describing weaknesses in the 802.11 Wired
Equivalent Privacy (WEP)
security mechanism defined in the original standard;
they were followed by Fluhrer,
Mantin, and Shamir's paper titled "Weaknesses in the
Key Scheduling Algorithm of RC4".
Not long after, Adam Stubblefield and AT&T publicly
announced 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).
an interim specification called Wi-Fi
Protected Access (WPA)
based on a subset of the then current IEEE 802.11i
draft. These started to appear in products in mid-2003. IEEE
known as WPA2)
itself was ratified in June 2004, and uses the Advanced
Encryption Standard AES,
instead of RC4,
which was used in WEP. The modern recommended encryption
for the home/consumer space is WPA2 (AES Pre-Shared
Key), and for the enterprise space is WPA2 along with a RADIUS authentication
server (or another type of authentication server) and a
strong authentication method such as EAP-TLS.
In January 2005, the IEEE set up yet another task
group "w" to
protect management and broadcast frames, which
previously were sent unsecured. Its standard was
published in 2009.
In December 2011, a security flaw was revealed that
affects some wireless routers with a specific
implementation of the optional Wi-Fi
Protected Setup (WPS)
feature. While WPS is not a part of 802.11, the flaw
allows an attacker within the range of the wireless
router to recover the WPS PIN and, with it, the router's
802.11i password in a few hours.
In late 2014, Apple announced
that its iOS 8
mobile operating system would scramble MAC addresses during
the pre-association stage to thwart retail
footfall tracking made
possible by the regular transmission of uniquely
identifiable probe requests.
Many companies implement wireless networking equipment
with non-IEEE standard 802.11 extensions either by
implementing proprietary or draft features. These
changes may lead to incompatibilities between these
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^ http://www.kb.cert.org/vuls/id/723755 US
8 strikes an unexpected blow against location