Online Encyclopedia
IEEE 802.11
IEEE 802.11 or Wi-Fi denotes a set of Wireless LAN standards developed by working group 11 of IEEE 802. The term is also used to refer to the original 802.11, which is now sometimes called "802.11legacy".
The 802.11 family currently includes six over-the-air modulation techniques that all use the same protocol, the most popular (and prolific) techniques are those defined by the a, b, and g amendments to the original standard; security was originally included, and was later enhanced via the 802.11i amendment. Other standards in the family (c–f, h–j, n) are service enhancement and extensions, or corrections to previous specifications. 802.11b was the first widely accepted wireless networking standard, followed (somewhat counterintuitively) by 802.11a and 802.11g.
802.11b and 802.11g standards use the unlicensed 2.4 GHz band. The 802.11a standard uses the 5 GHz band. Operating in an unregulated frequency band, 802.11b and 802.11g equipment can incur interference from microwave ovens, cordless phones, and other appliances using the same 2.4 GHz band.
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Protocols
802.11 legacy
The original version of the standard IEEE 802.11 released in 1997 specifies two raw data rates of 1 and 2 Megabits per second (Mbit/s) to be transmitted via infrared (IR) signals or in the Industrial Scientific Medical frequency band at 2.4 GHz. IR remains a part of the standard but has no actual implementations.
The original standard also specifies a MAC protocol incorporating Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), fragmentation, and variable interframe gaps derived from the Photonics Photolink II infrared LAN as the media access method. A significant percentage of the available raw channel capacity is sacrificed (via the MAC mechanisms) in order to improve the reliability of data transmissions under diverse and adverse environmental conditions.
At least five different, somewhat-interoperable, commercial products appeared using the original specification, from companies like Alvarion (PRO.11 and BreezeAccess-II), Netwave Technologies (AirSurfer Plus and AirSurfer Pro) and Proxim (OpenAir). A weakness of this original specification was that it offered so many choices that interoperability was sometimes challenging to realize. It is really more of a "meta-specification" than a rigid specification, allowing individual product vendors the flexibility to differentiate their products. Legacy 802.11 was rapidly supplemented (and popularized) by 802.11b.
802.11b
The 802.11b amendment to the original standard was ratified in 1999. 802.11b has a maximum raw data rate of 11 Mbit/s and uses the same CSMA/CA media access method defined in the original standard. Due to the CSMA/CA protocol overhead, in practice the maximum 802.11b throughput that an application can achieve is about 5.9 Mbit/s over TCP and 7.1 Mbit/s over UDP.
802.11b operates in the 2.4 GHz RF spectrum. Hence, metal, water, and thick walls absorb 802.11b signals and decrease the range drastically.
802.11b products appeared on the market very quickly, since 802.11b is a direct extension of the DSSS modulation technique defined in the original standard. Hence, chipsets and products were easily upgraded to support the 802.11b enhancements. The dramatic increase in throughput of 802.11b (compared to the original standard) along with substantial price reductions lead to the rapid acceptance of 802.11b as the definitive wireless LAN technology.
With high-gain external antennas, the protocol can also be used in fixed point-to-point arrangements, typically at ranges up to eight kilometers (although some report success at ranges up to 80–120 km where line of sight can be established). This is usually done to replace costly leased lines, or in place of very cumbersome microwave communications equipment. Current cards can operate at 11 Mbit/s, but will scale back to 5.5, then 2, then 1, if signal quality becomes an issue.
Extensions have been made to the 802.11b protocol (e.g., channel bonding and burst transmission techniques) in order to increase speed to 22, 33, and 44 Mbit/s, but the extensions are proprietary and have not been endorsed by the IEEE. Many companies call enhanced versions "802.11b+".
The first widespread commercial use of the 802.11b standard for networking was made by Apple Computer under the trademark AirPort. On the non-Apple market, Linksys could be considered the current leader.
Channels and international compatibility
802.11b and 802.11g divide the spectrum into 14 overlapping, staggered channels of 22 megahertz (MHz) each. Channels 1, 6 and 11 (and in some geographic areas channel 14) do not overlap and those channels (or other sets with similar gaps) can be used such that multiple networks can operate in close proximity without interfering with each other.
Channels 10 and 11 are the only channels which work in all parts of the world, because Spain hasn't licensed channels 1 to 9 for 802.11b operation. The full frequency list from IEEE STD 802.11b-1999/Cor 1-2001 is:
| Channel | MHz | US X10 | Canada X20 | Europe ETSI X30 | Spain X31 | France X32 | Japan X40 | Japan X41 |
|---|---|---|---|---|---|---|---|---|
| 1 | 2412 | x | x | x | x | x | ||
| 2 | 2417 | x | x | x | x | x | ||
| 3 | 2422 | x | x | x | x | x | ||
| 4 | 2427 | x | x | x | x | x | ||
| 5 | 2432 | x | x | x | x | x | ||
| 6 | 2437 | x | x | x | x | x | ||
| 7 | 2442 | x | x | x | x | x | ||
| 8 | 2447 | x | x | x | x | x | ||
| 9 | 2452 | x | x | x | x | x | ||
| 10 | 2457 | x | x | x | x | x | x | x |
| 11 | 2462 | x | x | x | x | x | x | x |
| 12 | 2467 | x | x | x | ||||
| 13 | 2472 | x | x | x | ||||
| 14 | 2484 | x |
802.11a
The 802.11a amendment to the original standard was ratified in 1999. The 802.11a standard uses the same core protocol as the original standard, operates in 5 GHz band, and uses a new modulation technique with a maximum raw data rate of 54 Mbit/s, which yields realistic net achievable throughput in the mid-20 Mbit/s. The data rate is reduced to 48, 36, 34, 18, 12, 9 then 6 Mbit/s if required. 802.11a has 12 non-overlapping channels, 8 dedicated to indoor and 4 to point to point. Different countries have different ideas about regulatory support, although a 2003 World Radiotelecommunciations Conference made it easier for use worldwide. A mid-2003 FCC decision may open more spectrum to 802.11a channels as well.
802.11a products starting shipping in 2001, lagging 802.11b products due to the slow availability of the 5 GHz components needed to implement products. 802.11a has not seen wide adoption because of the high adoption rate of 802.11b, and because of concerns about range: at 5 GHz, 802.11a cannot reach as far as 802.11b, other things (such as same power limitations) being equal; it is also absorbed more readily. Those concerns were in part caused by poor initial product implementations of 802.11a. Current generation 802.11a technology has range characteristics much closer to 802.11b. Most manufacturers of 802.11a equipment countered the lack of market success by releasing dual-band or dual-mode/tri-mode cards that can automatically handle 802.11a and b or a, b and g as available. Mobile adapter and access point equipment which can support all these standards simultaneously is also available.
802.11g
In June 2003, a third modulation standard was ratified: 802.11g. This flavor works in the 2.4 GHz band (like 802.11b) but operates at a maximum raw data rate of 54 Mbit/s, or about 24.7 Mbit/s net throughput like 802.11a. It is fully backwards compatible with b and uses the same frequencies. Details of making b and g work well together occupied much of the lingering technical process. In older networks, however, the presence of an 802.11b participant significantly reduces the speed of an 802.11g network.
The 802.11g standard swept the consumer world of early adopters starting in January 2003, well before ratification. The corporate users held back and Cisco and other big equipment makers waited until ratification. By summer 2003, announcements were flourishing. Most of the dual-band 802.11a/b products became dual-band/tri-mode, supporting a, b, and g in a single mobile adaptor card or access point. While 802.11g held the promise of higher throughput, actual results were mitigated by a number of factors: conflict with 802.11b-only devices (see above), exposure to the same interference sources as 802.11b, limited channelization (only 3 fully non-overlapping channels like 802.11b) and the fact that the higher data rates of 802.11g are often more susceptible to interference that 802.11b, causing the 802.11g device to reduce the data rate to effectively the same rates used by 802.11b. The move to dual-mode/tri-mode products also carries with it economies of scale (e.g. single chip manufacturing). The use of dual-band/tri-mode products ensures the best possible throughput in any given environment.
A new proprietary feature called Super G is now integrated in certain access points. These can boost network speeds up to 108 Mbit/s by using channel bonding. This feature may interfere with other networks and may not support all b and g client cards. In addition, packet bursting techniques are also available in some chipsets and products which will also considerably increase speeds. Again, they may not be compatible with some equipment.
The first major manufacturer to use 802.11g was Apple, under the trademark AirPort Extreme. Cisco joined the game by buying up Linksys, an early adopter, and also offers its own wireless mobile adaptors under the name Aironet.
802.11n
In January 2004 IEEE announced that it had formed a new 802.11 Task Group (TGn) to develop a new amendment to the 802.11 standard for local-area wireless networks. The real data throughput will be at least 100 Mbit/s (which may require an even higher raw data rate at the PHY level), and so up to 4–5 times faster than 802.11a or 802.11g, and perhaps 20 times faster than 802.11b. As projected, 802.11n will also offer a better operating distance than current networks. The standardization process is expected to be completed by the end of 2006.
802.11n builds upon previous 802.11 standards by adding MIMO (multiple-input multiple-output). The additional transmitter and receiver antennas allow for increased data throughput through spatial multiplexing and increased range by exploiting the spatial diversity, perhaps through coding schemes like Alamouti coding.
Overview
| Standard | Modulation Method | Frequencies | Data Rates Supported (Mbit/s) |
|---|---|---|---|
| 802.11 legacy | FHSS, DSSS, infrared | 2.4 GHz, IR | 1, 2 |
| 802.11b | DSSS, HR-DSSS | 2.4 GHz | 1, 2, 5.5, 11 |
| "802.11b+" non-standard | DSSS, HR-DSSS (PBCC) | 2.4 GHz | 1, 2, 5.5, 11, 22, 33, 44 |
| 802.11a | OFDM | 5.2, 5.8 GHz | 6, 9, 12, 18, 24, 36, 48, 54 |
| 802.11g | DSSS, HR-DSSS, OFDM | 2.4 GHz | 1, 2, 5.5, 11; 6, 9, 12, 18, 24, 36, 48, 54 |
To be merged:
- IEEE 802.11a, which operates around the 5 GHz band, enjoys relatively clear-channel operation in the United States and Japan. In other areas, such as the EU, 802.11a had a longer wait for approval, and European regulators were considering the use of the European HIPERLAN standard. 802.11a was cleared for use in Europe around mid 2002. 802.11a also provides for up to 54 Mbit/s operation, but is not interoperable with 802.11b, except in the case of equipment implementing both standards.
Certification
Because the IEEE only sets specifications but does not test equipment for compliance with them, a trade group called the Wi-Fi Alliance runs a certification program that members pay to participate in. Virtually all companies selling 802.11 equipment are members. The Wi-Fi trademark, owned by the group and usable only on compliant equipment, is intended to guarantee interoperability. Currently, "Wi-Fi" can mean any of 802.11a, b, or g. As of fall 2003, Wi-Fi also includes the security standard Wi-Fi Protected Access or WPA. Eventually "Wi-Fi" will also mean equipment which implements the 802.11i security standard (aka WPA2). Products that say they are Wi-Fi are supposed to also indicate the frequency band in which they operate, 2.4 or 5 GHz.
Standards
The following standards and task groups exist within the working group:
- IEEE 802.11 - The original 1 Mbit/s and 2 Mbit/s, 2.4 GHz RF and IR standard
- IEEE 802.11a - 54 Mbit/s, 5 GHz standard (1999, shipping products in 2001)
- IEEE 802.11b - Enhancements to 802.11 to support 5.5 and 11 Mbit/s (1999)
- IEEE 802.11d -– international (country-to-country) roaming extensionsNew countries
- IEEE 802.11e - Enhancements: QoS, including packet bursting
- IEEE 802.11F - Inter-Access Point Protocol (IAPP)
- IEEE 802.11g - 54 Mbit/s, 2.4 GHz standard (backwards compatible with b) (2003)
- IEEE 802.11h - 5 GHz spectrum, Dynamic Channel /Frequency Selection (DCS/DFS) and Transmit Power Control (TPC) for European compatibility
- IEEE 802.11i (ratified 24 June 2004) - Enhanced security
- IEEE 802.11j - Extensions for Japan
- IEEE 802.11k - Radio resource measurements
- IEEE 802.11n - Higher throughput improvements
- IEEE 802.11p - WAVE - Wireless Access for the Ability in Vehicular Environments (such as ambulances and passenger cars)
- IEEE 802.11r - Fast roaming
- IEEE 802.11s - Wireless mesh networking
- IEEE 802.11T - Wireless Performance Prediction (WPP) - Ttest methods and metrics
- IEEE 802.11u - Interworking with non-802 networks (e.g., cellular)
- IEEE 802.11v - Wireless network management
Community networks
With the proliferation of cable modems and DSL, there is an ever-increasing market of people who wish to establish small networks in their homes to share their high speed Internet connection.
Wireless office networks are often unsecured or secured with WEP, which is said to be easily broken, although a substantial amount of data has to be collected before it can be cracked successfully. These networks frequently allow "people on the street" 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.
Security
In 2001, a group from the University of California at Berkeley presented a paper describing weaknesses in the 802.11 WEP (wired equivalent privacy) 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 announcing the first verification of the attack. In the attack they were able to intercept transmissions and gain unauthorized access to wireless networks.
The IEEE set up a dedicated task group to create a replacement security solution, 802.11i (previously this work was handled as part of a broader 802.11e effort to enhance the MAC layer). 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. 802.11i (aka WPA2) itself was ratified in June 2004, and uses the Advanced Encryption Standard, instead of RC4, which was used in WEP and WPA.
See also
- Bluetooth, another wireless protocol primarily designed for shorter range applications.
- Apple Airport, with implementations of 802.11b and 802.11g.
- Linksys, current market-leader in 802.11b and 802.11g on the Wintel platform.
- Wimax (aka 802.16), another wireless protocol designed for MANs.
External links
- the working group's own website, http://www.ieee802.org/11/
- where to download the 802.11 protocol reference: http://standards.ieee.org/getieee802/802.11.html
- Wi-Fi Alliance website, http://www.wi-fi.org/
- "Weaknesses in the Key Scheduling Algorithm of RC4", Scott Fluhrer, Itsik Mantin, Adi Shamir, http://citeseer.nj.nec.com/fluhrer01weaknesses.html
- Attack verification by Stubblefield, (original gone, wep_attack paper mirrored here) http://ftp.die.net/mirror/papers/802.11/
- Wireless LAN Security whitepapers & howto's http://802.11-security.com/security/links
- WLAN in Debian http://wiki.debian.net/index.cgi?Wi-fi .
- Software-based positioning system for standard IEEE 802.11 networks, Ekahau http://www.ekahau.com