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`IEEE 802.11 - Wikipedia, the free encyclopedia
`
`IEEE 802.11
`From Wikipedia, the free encyclopedia
`
`IEEE 802.11 is a set of media access control (MAC) and physical layer (PHY) 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 Committee (IEEE 802). 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 market place, each revision tends to become its own standard.
`
`Contents
`
`1 General description
`
`2 History
`
`I
`
`'
`
`3 Protocol
`I
`
`3.1 802.11-1997 (802.11 legacy)
`
`3.2 802.11a (OFDM waveform)
`
`3.3 802.11b
`
`3.4 802.11g
`
`3.5 802.11-2007
`
`3.6 802.lln
`
`3.7 802.11-2012
`
`3.8 802.11ac
`
`3.9 802.11ad
`
`3.10 802.11af
`
`3.11 802.11ah
`
`3.12 802.1lai
`
`3.13 802.11aj
`
`3.14 802.1laq
`
`3.15 802.11ax
`
`3.16 802.11ay
`
`4 Common misunderstandings about achievable throughput
`
`5 Channels and frequencies
`
`5.1 Channel spacing within the 2.4 GHz band
`
`5.2 Regulatory domains and legal compliance
`
`6 Layer 2 — Datagrams
`
`6.1 Management frames
`I 6.1.1 Information Elements
`
`.
`
`6.2 Control frames
`
`https://en.wikipedia.org/wiki/|EEE_802.11
`
`
`
`The Linksys WRT54G contains a
`router with an 802.11b/g radio
`(common in the early 2000s) and
`two antennas
`
`WVR 2007
`Volkswagen v. WVR
`Volkswagen v. WVR
`IPR2016-00177
`H)R2016_00177
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`6.3 Data frames
`
`7 Standards and amendments
`
`7.1 In process
`
`I 7.2 Standard vs. amendment
`
`8 Nomenclature
`
`9 Community networks
`
`10 Security
`
`11 Non-standard 802.11 extensions and equipment
`
`12 See also
`
`13 References
`
`14 Footnotes
`
`15 External links
`
`General description
`
`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 802.1 lb was the first widely accepted one, followed by 802.1 la, 802.1 lg, 802.1 1n, and 802.1 lac.
`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 previous specif1cation.[1]
`
`802.1 lb and 802.1 lg use the 2.4 GHZ ISM band, operating in the United States under Part 15 of the U.S. Federal Communications Commission
`Rules and Regulations. Because of this choice of frequency band, 802.1 lb and g equipment may occasionally suffer interference from microwave
`ovens, cordless telephones, and Bluetooth devices. 802.l1b and 802.1 lg control their interference and susceptibility to interference by using direct-
`sequence spread spectrum (DSSS) and orthogonal frequency-division multiplexing (OFDM) signaling methods, respectively. 802.l1a 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 overlap—see list of WLAN channels. Better or worse performance with higher or
`lower frequencies (channels) may be realized, depending on the environment.
`
`The segment of the radio frequency spectrum used by 802.11 varies between countries. In the US, 802.1la and 802.1 lg 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.l1b and 802.1 lg fall
`within the 2.4 GHz amateur radio band. Licensed amateur radio operators may operate 802.l1b/g devices under Part 97 of the FCC Rules and
`
`Regulations, allowing increased power output but not commercial content or encryption.[2]
`
`History
`
`802.11 technology has its origins in a 1985 ruling by the U.S. Federal Communications Commission that released the ISM band for unlicensed use.[3]
`
`In 1991 NCR Corporation/AT&T (now Alcatel-Lucent 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 802.1 lb
`and 802.1 la standards within the IEEE.[4]
`
`In 1999, the Wi-Fi Alliance was formed as a trade association to hold the Wi-Fi trademark under which most products are sold.[5]
`
`Protocol
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`802.11 network PHY standards
`
`protocol date[6] 1-3-§
` Band-
`
`-
`1:‘
`re
`quency
`
`width
`
`Hz)
`
`2.4
`
`22
`
`Stream data rate[7]
`
`(Mbit/s)
`
`1, 2
`
`Approximate
`range
`
`(111)
`
`(ft)
`
`Outdoor
`(111)
`(ft)
`
`20
`
`66
`
`100
`
`330
`
`
`
`802,11 Release
`
`OFDM
`
`390
`HE 16,0001“
`
`N/A
`
`OFDM
`
`38
`
`125
`
`140
`
`460
`
`4
`
`70
`
`230
`
`250
`
`70
`
`230
`
`250
`
`[81
`
`820
`
`820[,,]
`
`5
`
`37W
`
`2.4
`
`20
`
`22
`
`6, 9, 12, 18,24, 36,48, 54
`
`1, 2, 5.5, 11
`
`0
`
`0
`
`2 2
`
`4
`
`7.2, 14.4, 21.7, 28.9, 43.3, 57.8, 65, 72.2 [B1 N'4:5?UI H-
`
`6, 9, 12, 18, 24, 36, 48, 54
`
`(6.5, 13, 19.5, 26, 39, 52, 58.5, 65) [C1
`
`15,30,45,60,90,120,135,150[B1
`(13.5, 27, 40.5, 54, 81, 108, 121.5, 135) [C1
`
`2013
`
`20
`
`40
`
`7.2, 14.4, 21.7, 28.9, 43.3, 57.8, 65, 72.2, 86.7, 96.3 [B1
`(6.5, 13, 19.5, 26, 39, 52, 58.5, 65, 78, 86.7) [C1
`15,30,45,60,90,120,135,150,180,200[B1
`(13.5, 27, 40.5, 54, 81, 108, 121.5, 135, 162, 180) [C1
`
`32.5, 65, 97.5, 130, 195, 260, 292.5, 325, 390, 433.3 [31
`(29.2, 58.5, 87.8, 117, 175.5, 234, 263.2, 292.5, 351, 390)
`[C]
`
`160
`
`65, 130, 195,260, 390, 520, 585, 650, 780, 866.7 [31
`(58.5, 117, 175.5, 234, 351,468, 702, 780) [C1
`
`Dec
`2012
`
`2,160
`
`.
`Up to 6,912 (6.75 Gbit/s)
`
`[10]
`
`U(Do
`
`35 1159]
`
`351159]
`
`35 115191
`
`200
`
`100
`
`300
`
`OFDM
`
`OFDM,
`single
`carrier,
`10W_p0Wer
`single
`carrier
`
`
`
`2017
`
`60
`
`8000
`
`Up to 100,000 (100 Gbit/s)
`
`4
`
`MIMO-
`OFDM
`
`OFDM,
`single
`carrier,
`
`60
`
`200
`
`1000
`
`3000
`
`ac
`
`ad
`
`ah
`
`aj
`
`ax
`
`ay
`
`I A1 A2 IEEE 802.11y-2008 extended operation of 802.1 1a to the licensed 3.7 GHz band. Increased power limits allow a range up to 5,000 In. As
`of 2009, it is only being licensed in the United States by the FCC.
`I B1 B2 B3 B4 B5 B6 Assumes short guard interval (SGI) enabled.
`I C1 C2 C3 C4 C5 C6 Assumes short guard interval (SGI) disabled.
`
`802.11-1997 (802.11 legacy)
`
`The original version of the standard IEEE 802.11 was released in 1997 and clarified in 1999, but is today obsolete. It specified two net bit rates of l
`or 2 megabits per second (Mbit/s), plus forward error correction code. It specified three alternative physical layer technologies: 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 ISM band.
`
`Legacy 802.11 with direct-sequence spread spectrum was rapidly supplanted and popularized by 802.1 lb.
`
`802.11a (OFDM waveform)
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`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. Ila 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 802.1 la 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.[“1
`
`Since the 2.4 GHz band is heavily used to the point of being crowded, using the relatively unused 5 GHz band gives 802.1 la a significant advantage.
`However, this high carrier frequency also brings a disadvantage: the effective overall range of 802.1 la is less than that of 802.11b/g. In theory,
`802.1 la 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 802.1 lb. In practice, 802.1 lb typically has a higher range at low speeds (802.1 lb will reduce speed to 5.5 Mbit/s or even
`
`1 Mbit/s at low signal strengths). 802.1 la also suffers from interference,[12] but locally there may be fewer signals to interfere with, resulting in less
`interference and better throughput.
`
`802.11b
`
`The 802.1 lb standard has a maximum raw data rate of 11 Mbit/s, and uses the same media access method defined in the original standard. 802.1 lb
`products appeared on the market in early 2000, since 802.1 lb is a direct extension of the modulation technique defined in the original standard. The
`dramatic increase in throughput of 802.1 lb (compared to the original standard) along with simultaneous substantial price reductions led to the rapid
`acceptance of 802. 1 lb as the definitive wireless LAN technology.
`
`Devices using 802.1 lb 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.
`
`802.11g
`
`In June 2003, a third modulation standard was ratified: 802.1 lg. This works in the 2.4 GHz band (like 802.11b), but uses the same OFDM based
`transmission scheme as 802.1 la. 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.[13] 802.1 lg hardware is fully backward compatible with 802.1 lb hardware, and therefore is encumbered with legacy
`issues that reduce throughput by ~2l% when compared to 802.1 la.
`
`The then-proposed 802.1 lg 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 802.1 la/b products became dual-band/tri-mode,
`supporting a and b/g in a single mobile adapter card or access point. Details of making b and g work well together occupied much of the lingering
`technical process; in an 802.1 lg network, however, activity of an 802.1 lb participant will reduce the data rate of the overall 802.1 lg network.
`
`Like 802.11b, 802.1 lg devices suffer interference from other products operating in the 2.4 GHz band, for example wireless keyboards.
`
`802.11—2007
`
`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 (802.1la, b, d, e, g, h, i, j) with the base standard. Upon approval on March 8,
`
`2007, 802.1 1REVma was renamed to the then-current base standard IEEE 802.11-2007.[14]
`
`802.11n
`
`802.11n is an amendment that improves upon the previous 802.11 standards by adding multiple-input multiple-output antennas (MIMO). 802.11n
`operates on both the 2.4 GHz and the lesser-used 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.[15][16] 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
`802.1 ln proposal.
`
`802.11—2012
`
`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 802.1 lmb, as
`it was called, created a single document that merged ten amendments (802.l1k, r, y, n, w, p, 2, v, u, s) with the 2007 base standard. In addition much
`
`cleanup was done, including a reordering of many of the clauses.[17] Upon publication on March 29, 2012, the new standard was referred to as IEEE
`802.1 1-2012.
`
`802.11ac
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`IEEE 802.1lac-2013 is an amendment to IEEE 802.11, published in December 2013, that builds on 802.11n.[18] 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 MIMO (MU-MIMO). As of October 2013, high-end implementations support 80 MHz
`charmels, 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.[19] 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 2o15.[2°1l211[221
`
`802.11ad
`
`IEEE 802.1 lad is an amendment that defines a new physical layer for 802.11 networks to operate in the 60 GHz millimeter wave spectrum. 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 802.1 lad standard are being brought to market under the WiGig brand name. The certification program is now being developed by
`
`the Wi-Fi Alliance instead of the now defunct WiGig Alliance.[23] The peak transmission rate of 802.1 lad is 7 Gbit/s.[24]
`
`The first product is set to be released in 2016.05]
`
`802.11af
`
`IEEE 802.11af, also referred to as "White-Fi" and "Super Wi-Fi",[26] is 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.[27][28] It uses cognitive radio technology to transmit on unused TV
`charmels, with the standard taking measures to limit interference for primary users, such as analog TV, digital TV, and wireless microphones.[28]
`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. [28]
`The physical layer uses OFDM and is based on 802.11ac.[29] 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.[28] The frequency channels are 6 to
`8 MHz wide, depending on the regulatory domain.[28] Up to four channels may be bonded in either one or two contiguous blocks.[28] MIMO
`operation is possible with up to four streams used for either space—tirne block code (STBC) or multi-user (MU) operation. [28] 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 channels.[30] With 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.[3°]
`
`802.11ah
`
`IEEE 802.11ah defines a WLAN system operating at sub 1 GHz license-exempt bands, with final approval slated for March 20l6.[27][31] 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,[32] extended range hotspot, and outdoor Wi-Fi for cellular traffic offloading, whereas the available bandwidth is relatively narrow.
`
`802.11ai
`
`IEEE 802.1lai is an amendment to the 802.11 standard that will add new mechanisms for a faster initial link setup time.[33]
`
`802.11aj
`
`IEEE 802.1laj is a rebanding of 802.1 lad for use in the 45 GHz unlicensed spectrum available in some regions of the world (specifically China).[33]
`
`802.1 1 aq
`
`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.[33]
`
`802.11ax
`
`IEEE 802.1 lax is the successor to 802.1 lac, 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.[34]
`
`802.11ay
`
`IEEE 802.1 lay is a standard that is being developed. It is an amendment that defines a new physical layer for 802.11 networks to operate in the
`60 GHz millimeter wave spectrum. It will be an extension of the existing 1 lad, 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 802.1 lay is 20 Gbit/s.[35] The
`main extensions include: channel bonding (2, 3 and 4), MIMO and higher modulation schemes.
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`Common misunderstandings about achievable throughput
`
`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
`deViCeS_[36][37]
`
`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. Please note, this
`text and measurements do not cover packet errors, but information about this can be found at
`the references above.
`
`Channels and frequencies
`
`802.1 lb, 802.] lg, and 802.1 ln-2.4 utilize the 2.400—2.500 GHz spectrum, one of the ISM
`bands. 802.1 la and 802.1 ln use the more heavily regulated 4.9l5—5.825 GHz band. 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.
`
`[l|rUu1.;|ILIlI:rI'\':.'IL1p-t' \\'llI13[i|3.l lg
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`‘‘”‘''*'d 1”“ l"‘F‘|‘“""“i““' “|‘‘'‘'‘“*' ‘J''““' "'““l I-‘*"‘F‘-“'
`Graphical representation of Wi-Fi application specific
`(UDP) P6Tf0m1aI1C6 CHVCIOP6 34 GHZ banda With
`802.11g
`
`5
`
`.x""'""""‘
`
`W”
`
`ll:rulIg|IL|l I:lI\'t.'lL1pIL' will; 302.1 In L-H]!-pill.-. E"l1a1ni|L'lwidll'L3
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`:IITI.'rL'Ll. mu Lilp|1]i('.'1li(HII !«'||L'('lf_lL'1J¢'1l'.I ru[L']|I\-1I1pe:|
`Graphiea1 representation of wi_1:i application specific
`(UDP) performance envelope 2_4 GHZ band, with
`g()2_11n with 40MHz
`
`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 regulatory domains.
`
`14 Channel
`‘I3
`12
`11
`10
`9
`B
`T
`6
`5
`4
`3
`2
`1
`2.431 Center Frequency
`2.472
`2.-1-4? 2.452 2.45? 24$ 2.467
`2.432 2.43? 2.442
`2.42?
`2.422
`2.41?
`2.412
`
`
`
`
`22?».-'iHz
`
`Graphical representation of Wi-Fi channels in the 2.4 GHz band
`
`The channel numbering of the 5.725—5.875 GHz spectrum is less intuitive due to the differences in regulations between countries. These are
`discussed in greater detail on the list of WLAN channels.
`
`Channel spacing within the 2.4 GHz band
`
`In addition to specifying the channel centre frequency, 802.11 also specifies (in Clause 17) a spectral mask defining the permitted power distribution
`across each channel. The mask requires the signal be attenuated a minimum of 20 dB from its peak amplitude at ill 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 802.1 lb, and l—l3 for 802.11g/n-2.4. Other countries such as Spain initially allowed only
`
`channels 10 and 11, and France allowed only 10, ll, 12, and 13; however, they now allow channels 1 through l3.[38][39] North America and some
`Central and South American countries allow only 1 through ll.
`
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`Spectral masks for 802.1 lg channels l—14 in the 2.4 GHz band
`
`Since the spectral mask defines only power output restrictions up to 2:11 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
`problem a 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. 802.1 lb was based on DSSS modulation and
`utilized a channel bandwidth of 22 MHz, resulting in three "non-overlapping" channels (1, 6, and 11). 802.1 lg was based on OFDM modulation and
`utilized a channel bandwidth of 20 MHz. This occasionally leads to the belief thatfour "non-overlapping" channels (1, 5, 9, and 13) exist under
`802.11g, although this is not the case as per 17.4.6.3 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."[40] and section 18.3.9.3 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 configu.ration,[41] and in the paper entitled "Effect of
`adjacent-channel interference in IEEE 802.11 WLANs" by Villegas this is also demonstrated.[42]
`
`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
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`Regulatory domains and legal compliance
`
`IEEE uses the phrase regdomain to refer to a legal regulatory region. Different countries define different
`levels ofallowable transmitter power, time that a channel can be occupied, and different available
`channels.[44] Domain codes are specified for the United States, Canada, ETSI (Europe), Spain, France, Japan,
`and China.
`
`Most Wi-Fi certified devices 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.
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`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 States‘ Federal Communications Commission.
`
`Layer 2 — Datagrams
`
`The datagrams are calledframes. 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 header, payload, and frame 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 following sub-fields:
`
`Protocol Version: Two 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 Sub Type 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 set.
`I 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
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`elimination of duplicate frames.
`I Power Management: This 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.
`I 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.
`I Protected Frame: The 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), or Wi-FI Protected Access 11 (WPA2).
`I 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 performance penalty.
`
`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 receiver.
`
`The remaining fields of the header are:
`
`I 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 sequence number.
`I An optional two-byte Quality of Service control field that was added with 802.1 le.
`
`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 layers.
`
`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
`transmission. [45]
`
`Management frames
`
`Management frames allow for the maintenance of communication. Some common 802.11 subtypes include:
`
`I Authentication 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 authentication status.
`
`I Association 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 WNIC.
`I Association 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.
`I Beacon frame: Sent periodically from an access point to announce its presence and provide the SSID,
`and other parameters for WNICS within range.
`I Deauthentication frame: Sent from a station wishing to terminate connection from another station.
`I Disassociation 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.
`I Probe request frame: Sent from a station when it requires information from another station.
`I Probe response frame: Sent from an access point containing capability information, supported data rates, etc., after receiving a probe request
`frame.
`
`Illustrated a“tt1°“ti°ati°“ method
`
`I Reassociation request frame: A WNIC sends a reassociation request when it drops from range of the currently associated access point and fin