IEEE 802.11

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IEEE 802.11 is a set of standards carrying out wireless local area network (WLAN) computer communication in the 2.4, 3.6 and 5 GHz frequency bands. They are implemented by the IEEE LAN/MAN Standards Committee (IEEE 802).


[edit] General description

A Linksys Residential gateway with an 802.11b radio and a 4-port Ethernet switch
A Compaq 802.11b PCI card

The 802.11 family includes over-the-air modulation techniques that use the same basic protocol. The most popular are those defined by the 802.11b and 802.11g protocols, and are amendments to the original standard. 802.11-1997 was the first wireless networking standard, but 802.11b was the first widely accepted one, followed by 802.11g and 802.11n. Security was originally purposefully weak due to export requirements of some governments,[1] and was later enhanced via the 802.11i amendment after governmental and legislative changes. 802.11n is a new multi-streaming modulation technique that is still under draft development, but products based on its proprietary pre-draft versions are being sold. Other standards in the family (c–f, h, j) are service amendments and extensions or corrections to previous specifications.

802.11b and 802.11g use the 2.4 GHz ISM band, operating in the United States under Part 15 of the US Federal Communications Commission Rules and Regulations. Because of this choice of frequency band, 802.11b and g equipment may occasionally suffer interference from microwave ovens, cordless telephones and Bluetooth devices. Both 802.11 and Bluetooth control their interference and susceptibility to interference by using spread spectrum modulation. Bluetooth uses a frequency hopping spread spectrum signaling method (FHSS) while 802.11b/g use the direct sequence spread spectrum signaling (DSSS) and orthogonal frequency division multiplexing (OFDM) methods respectively. 802.11a uses the 5 GHz U-NII band, which, for much of the world, offers at least nineteen non-overlapping channels rather than the three offered in the 2.4 GHz ISM frequency band.[2] However propagation around objects such as walls and furniture tends to be better at higher frequencies[citation needed]. This is because higher frequencies scatter more which helps them get around objects[citation needed]. However penetration is better with lower frequencies. You may get better or worse performance with higher or lower frequencies (channels) depending on your environment. WiFi generally reflects around objects rather than going through them.

The other major factor in performance is absorption by water and moisture. 2.4GHz is very close to the O-H bond frequency. Water is full of O-H bonds so it tends to really absorb 2.4GHz WiFi signals. Higher and lower frequencies have less of a problem with this.

The segment of the radio frequency spectrum used 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 (802.11b) fall within the 2.4 GHz amateur radio band. 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.[3]

[edit] Protocols

[edit] 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 1 or 2 megabits per second (Mbit/s), plus forward error correction code. It specifed 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. Previous WLAN technologies used lower frequencies, such as the U.S. 900 MHz ISM band.

Legacy 802.11 with direct-sequence spread spectrum was rapidly supplemented and popularized by 802.11b.

[edit] 802.11a

Release date Op. Frequency Throughput (Typ) Net bit rate (Max) Gross bit rate (Max) Range (Indoor)
October 1999 5 GHz 27 Mbit/s[4] 54 Mbit/s 72 Mbit/s ~35 m[citation needed]

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[citation needed].

Since the 2.4 GHz band is heavily used to the point of being crowded, using the relatively un-used 5 GHz band gives 802.11a a significant advantage. However, this high carrier frequency also brings a disadvantage: The effective overall range of 802.11a is less than that of 802.11b/g; and in theory 802.11a signals cannot penetrate as far as those for 802.11b because they are absorbed more readily by walls and other solid objects in their path due to their smaller wavelength. However, in practice 802.11b typically has a higher distance range at low speeds (802.11b will reduce speed to 5 Mbit/s or even 1 Mbit/s at low signal strengths). However, at higher speeds, 802.11a typically has the same or higher range due to less interference.

[edit] 802.11b

Release date Frequency band Throughput (typical) Net bit rate Range (Indoor)
October 1999 2.4 GHz ~5 Mbit/s[4] 11 Mbit/s ~38 m[citation needed]

802.11b 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.

802.11b devices suffer 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 and cordless telephones.

[edit] 802.11g

Release date Op. Frequency Throughput (Typ) Net bit rate (Max) Gross bit rate (Max) Range (Indoor)
June 2003 2.4 GHz ~22 Mbit/s[4] 54 Mbit/s 128 Mbit/s ~up to 100 m[citation needed]

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 19 Mbit/s average throughput[citation needed]. 802.11g hardware is fully backwards compatible with 802.11b hardware and therefore is encumbered with legacy issues that reduce throughput when compared to 802.11a by ~21%.

The then-proposed 802.11g standard was rapidly adopted by consumers starting in January 2003, well before ratification, due to the desire for higher data rates, and 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 mobile adapter card or access point. Details of making b and g work well together occupied much of the lingering technical process; in an 802.11g network, however, activity by a 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.

[edit] 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 (802.11a,b,d,e,g,h,i,j) with the base standard. Upon approval on March 08, 2007, 802.11REVma was renamed to the current standard IEEE 802.11-2007.[5] This is the single most modern 802.11 document available that contains cumulative changes from multiple sub-letter task groups.

[edit] 802.11n

802.11n is a proposed amendment which improves upon the previous 802.11 standards by adding multiple-input multiple-output (MIMO) and many other newer features. The TGn workgroup is not expected to finalize the amendment until December 2009.[6] Enterprises, however, have already begun migrating to 802.11n networks based on Draft 2 of the 802.11n proposal. A common strategy for many businesses is to set up 802.11b and 802.11g client devices while gradually moving to 802.11n clients as part of new equipment purchases[7]

Release date Op. Frequency Throughput (Typ) Net bit rate (Max) Range (Indoor)
Jan 2010 (speculated)[6] 5 GHz and/or 2.4 GHz Unknown (unreleased) 600 Mbit/s[8] ~up to 300 m

[edit] Channels and international compatibility

Graphical representation of Wi-Fi channels in 2.4 GHz band

802.11 divides each of the above-described bands into channels, analogously to how radio and TV broadcast bands are carved up but with greater channel width and overlap. For example the 2.4000–2.4835 GHz band is divided into 13 channels each of width 22 MHz but spaced only 5 MHz apart, with channel 1 centred on 2412 MHz and 13 on 2472, to which Japan adds a 14th channel 12 MHz above channel 13.

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 (with the exclusion of 802.11g/n from channel 14), while at the other Spain allowed only channels 10 and 11 (later all of the 14 channels have been allowed[9] ), and France that allowed only 10, 11, 12 and 13 (now channels 1 to 13 are allowed[10]). Most other European countries are almost as liberal as Japan, disallowing only channel 14, while North America and some Central and South American countries further disallow 12 and 13. For more details on this topic, see List of WLAN channels.

Besides specifying the centre frequency of each channel, 802.11 also specifies (in Clause 17) a spectral mask defining the permitted distribution of power across each channel. The mask requires that the signal be attenuated by at least 30 dB from its peak energy at ± 11 MHz from the centre frequency, the sense in which channels are effectively 22 MHz wide. One consequence is that stations can only use every fourth or fifth channel without overlap, typically 1, 6 and 11 in the Americas, 1, 5, 9 and 13 in Europe, etc. Another is that channels 1-13 effectively require the band 2401–2483 MHz, the actual allocations being for example 2400–2483.5 in the UK, 2402–2483.5 in the US, etc.

Since the spectral mask only defines power output restrictions up to ± 22 MHz from the center frequency to be attenuated by 50 dB, 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 1, 6, and 11, the 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 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.

Although the statement that channels 1, 6, and 11 are "non-overlapping" is limited to spacing or product density, the 1–6–11 guideline has merit. If transmitters are closer together than channels 1, 6, and 11 (for example, 1, 4, 7, and 10), overlap between the channels may cause unacceptable degradation of signal quality and throughput [11]. However, overlapping channels may be used under certain circumstances. This way, more channels are available [12].

[edit] Frames

Current 802.11 standards define "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 has a 2-byte frame control field that provides detailed information on the wireless link. This field is segmented 11 ways and will be presented in order, with the first two bits reserved for identification of the protocol being used (e.g. 802.11g, 802.11b, etc.). These respectively two and four bit fields are used for identification of which frame type is used. The next two segment are reserved for type and subtype. The next two bits are the To DS and From DS fields. They indicate whether a frame is headed for a distributed system. All frames will have one of these bits set. The More Fragmentation bit is set most notable when higher level packets have been partitioned and will be set for all non-final sections. Some management frames may required partitioning as well. Sometimes frames require retransmission, and for this there is a Retry bit which is set to one when a frame is resent. This aids in the elimination of duplicate frames station side. The Power Management 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. 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. The WEP bit is modified after processing a frame. It is toggled to one after a frame has been decrypted or if no encryption is set it will have already been one. The last bit is the Order bit and is only set 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 take on one of three forms, Duration, contention-free period (CFP), and PS-Poll.

An 802.11 frame can contain up to four address fields. Six bytes are reserved for each address field. Each field is numbered are is used for different purposes. Address 1 is the receiver, Address 2 is the transmitter, Address 3 is used for filtering purposes by the receiver. As addresses are only 46 bits long and there are 48 bits reserved for each address, the first bit has a special function. A 0 indicates a single stations address (unicast), while a 1 represent a group of stations (multicast). If all the bits are 1's then the frame is broadcast to all station connected to an access point. 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. The Frame Body field is variably size, from 0 – 2132 bytes, an 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 appending. 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.[13]

Management Frames allow for the maintenance of communication. Some common 802.11 subtypes include:

  • Authentication frame: 802.11 authentication begins with the WNIC sending an authentication frame to the access point containing its identity. With an open system authentication the WNIC only sends 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.
  • 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 reserve memory and establishes and association ID for the WNIC.
  • 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.
  • Beacon frame: Sent periodically from an access point to announce its presence and provide the SSID, and other parameters for WNICs within range.
  • Deauthentication frame: Sent from a station wishing to terminate connection from another station.
  • 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.
  • Probe request frame: Sent from a station when it requires information from another station.
  • Probe response frame: Sent from a station containing capability information, supported data rates, etc., after receiving a probe request frame.
  • Reassociation 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 point.
  • Reassociation 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.

Control frames facility in the exchange of data frames between station. Some common 802.11 control frames include:

  • 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 point with hidden stations. A station sends a RTS frame to 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 stations transmits.

Data frames carry packets from web pages, files, etc. within the body.


[edit] Standard and amendments

Within the IEEE 802.11 Working Group,[6] the following IEEE Standards Association Standard and Amendments exist:

  • IEEE 802.11 - THE WLAN STANDARD was original 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 and 11 Mbit/s (1999)
  • IEEE 802.11c - Bridge operation procedures; included in the IEEE 802.1D standard (2001)
  • 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 February 2006
  • 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 & j. (July 2007)
  • IEEE 802.11k - Radio resource measurement enhancements (2008)
  • IEEE 802.11l - (reserved and will not be used)
  • IEEE 802.11m - Maintenance of the standard. Recent edits became 802.11-2007. (ongoing)
  • IEEE 802.11n - Higher throughput improvements using MIMO (multiple input, multiple output antennas) (November 2009)
  • IEEE 802.11o - (reserved and will not be used)
  • IEEE 802.11p - WAVE - Wireless Access for the Vehicular Environment (such as ambulances and passenger cars) (working - 2009?)
  • IEEE 802.11q - (reserved and will not be used, can be confused with 802.1Q VLAN tagging)
  • IEEE 802.11r - Fast roaming Working "Task Group r" - (2008)
  • IEEE 802.11s - Mesh Networking, Extended Service Set (ESS) (working - Jul 2010?)
  • IEEE 802.11T - Wireless Performance Prediction (WPP) - test methods and metrics Recommendation (2008)
  • IEEE 802.11u - Interworking with non-802 networks (for example, cellular) (proposal evaluation - Mar 2010?)
  • IEEE 802.11v - Wireless network management (early proposal stages - Sept 2010?)
  • IEEE 802.11w - Protected Management Frames (early proposal stages - 2009?)
  • IEEE 802.11x - (reserved and will not be used, can be confused with 802.1x Network Access Control)
  • IEEE 802.11y - 3650-3700 MHz Operation in the U.S. (2008)
  • IEEE 802.11z - Extensions to Direct Link Setup (DLS) (Aug 2007 - Dec 2011)
  • IEEE 802.11aa - Robust streaming of Audio Video Transport Streams (Mar 2008 - May 2011)
  • IEEE 802.11ac - Very High Throughput <6GHz (Sep 2008 - Dec 2012)
  • IEEE 802.11ad - Extremely High Throughput 60GHz (Dec 2008 - Dec 2012)

There is no standard or task group named "802.11x". Rather, this term is used informally to denote any current or future 802.11 amendment, in cases where further precision is not necessary. (The IEEE 802.1x standard for port-based network access control is often mistakenly called "802.11x" when used in the context of wireless networks.)

802.11F and 802.11T are recommended practices rather than standards, and are capitalized as such.

[edit] Standard or amendment?

Both the terms "standard" and "amendment" are used when referring to the different variants of IEEE 802.11.

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 802.11-2007 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 802.11 were published in 1999 and 2007.

The working title of 802.11-2007 was 802.11-REVma. This denotes a third type of document, a "revision". The complexity of combining 802.11-1999 with 8 amendments made it necessary to revise already agreed upon text. As a result, additional guidelines associated with a revision had to be followed.

[edit] Nomenclature

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 Unit (usually 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 millisecond).

Also the term "Portal" is used to describe an entity that is similar to an IEEE 802.1D bridge. A Portal provides access to the WLAN by non-802.11 LAN STAs.

[edit] 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 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.

[edit] Security

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 entitled "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). The Wi-Fi Alliance announced 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 802.11i (also known as WPA2) itself was ratified in June 2004, and uses government strength encryption in 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 PreShared Key) and for the Enterprise space is WPA2 along with a radius server the strongest is EAP-TLS.

In January 2005, IEEE set up yet another task group, TGw, to protect management and broadcast frames, which previously were sent unsecured. See IEEE 802.11w

[edit] Non-standard 802.11 extensions and equipment

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 extensions.[citation needed]

[edit] See also

[edit] References

  1. ^ Looking for 802.11g Wireless Internet Access information, definitions and technology descriptions?
  2. ^ List of WLAN Channels
  3. ^ "ARRLWeb: Part 97 - Amateur Radio Service". American Radio Relay League. 
  4. ^ a b c Wireless Networking in the Developing World: A practical guide to planning and building low-cost telecommunications infrastructure (2nd ed.). Hacker Friendly LLC. 2007. pp. 425.  page 14
  5. ^ IEEE. ISBN 0-7381-5656-9. 
  6. ^ a b c "Official IEEE 802.11 working group project timelines". 2007-11-15. Retrieved on 2007-11-18. 
  7. ^ "How to: Migrate to 802.11n in the Enterprise". Retrieved on 2008-10-08. 
  8. ^
  9. ^ "Cuadro nacional de Atribución de Frecuencias CNAF". Secretaría de Estado de Telecomunicaciones. Retrieved on 2008-03-05. 
  10. ^ "Evolution du régime d’autorisation pour les RLAN". French Telecommunications Regulation Authority (ART). Retrieved on 2008-10-26. 
  11. ^ "Channel Deployment Issues for 2.4 GHz 802.11 WLANs". Cisco Systems, Inc. Retrieved on 2007-02-07. 
  12. ^ Garcia Villegas, E.; et. al. (2007), "Effect of adjacent-channel interference in IEEE 802.11 WLANs", CrownCom 2007., ICST & IEEE 
  13. ^ "802.11 Technical Section". Retrieved on 2008-12-15. 
  14. ^ "Understanding 802.11 Frame Types". Retrieved on 2008-12-14. 

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