Friday, March 11, 2005

IEEE 802.11

IEEE 802.11 or Wi-Fi denotes a set of Wireless LAN standards developed by working group 11 of the IEEE LAN/MAN Standards Committee (IEEE 802). The term is also used to refer to the original 802.11, which is now sometimes called "802.11legacy".

A D-Link 802.11b Wireless router for SOHO use.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 GigaHertz (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.

Protocols
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 defines Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) as the media access method. A significant percentage of the available raw channel capacity is sacrificed (via the CSMA/CA 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 (km) although some report success at ranges up to 80–120 km where line of sight can be established. This is usually done in place of costly leased lines or 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.

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 52-subcarrier OFDM (Orthogonal Frequency Division Multiplexing) 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. Is not interoperable with 802.11b, except if equipment that implements both standards.

Since the 2.4 GHz band is heavily used, using the 5 GHz band gives 802.11a the advantage of less interference. However, this high carrier frequency also brings disadvantages. It restricts the use of 802.11a to almost line of sight, necessitating the use of more access points; it also means that 802.11a cannot penetrate as far as 802.11b, since and it is also absorbed more readily, other things (such as power) being equal.

Different countries have different regulatory support, although a 2003 World Radiotelecommunciations Conference made it easier for use worldwide. 802.11a is now approved by regulations in the United States and Japan, but in other areas, such as the European Union, it had to wait longer for approval. European regulators were considering the use of the European HIPERLAN standard, but in mid-2002 cleared 802.11a for use in Europe. In the US, a mid-2003 FCC decision may open more spectrum to 802.11a channels.

Of the 52 OFDM subcarriers, 48 are for data and 4 are pilot subcarriers with a carrier separation of 0.3125 MHz (20 MHz/64). Each of these subcarriers can be a BPSK, QPSK, 16-QAM or 64-QAM. The total bandwidth is 20 MHz with an occupied bandwidth of 16.6 MHz. Symbol duration is 4 microseconds with a guard interval of 0.8 microseconds. The actual genration and decoding of orthogonal components is done in baseband using DSP which is then upconverted to 5 GHz at the transmitter. Each of the subcarriers could be represented as a complex number. The time domain signal is generated by taking an Inverse Fast Fourier Transform (IFFT). Correspondingly the receiver downconverts, samples at 20 MHz and does an FFT to retrieve the original coefficients. The advantages of using OFDM include reduced multipath effects in reception and increased spectral efficiency.

802.11a products started shipping in 2001, lagging 802.11b products due to the slow availability of the 5 GHz components needed to implement products. 802.11a was not widely adopted overall because 802.11b was already widely adoped, because of 802.11a's disadvantages, because of poor initial product implementations, making its range even shorter, and because of regulations. Manufacturers of 802.11a equipment responded to the lack of market success by improving the implementations (current-generation 802.11a technology has range characteristics much closer to those of 802.11b), and by making technology that can use more than one 802.11 standard. There are dual-band, or dual-mode or tri-mode cards that can automatically handle 802.11a and b, or a, b and g, as available. Similarly, there are mobile adapters and access points which can support all these standards simultaneously.

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 physical layer), and so up to 4–5 times faster than 802.11a or 802.11g, and perhaps 20 times faster than 802.11b. It is projected that 802.11n will also offer a better operating distance than current networks. There are two competing variants of the 802.11n standard; WWiSE (backed by companies including Broadcom) and TGn Sync (backed by Intel and Philips). 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.

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 Vehicular Environment (such as ambulances and passenger cars)
  • IEEE 802.11r - Fast roaming
  • IEEE 802.11s - Wireless mesh networking
  • IEEE 802.11T - Wireless Performance Prediction (WPP) - test 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.

1 Comments:

At 5:20 AM, Blogger Sara said...

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