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802.11N: Wireless Wave of The Future?

Hausarbeit 2006 10 Seiten

Technik

Leseprobe

802.11N: Wireless Wave of The Future?

Wireless Internet is changing our lives almost as much as the advent of the Internet itself. No longer are we tied to a desk to browse online. With wireless we are able to browse the Internet anywhere we are able to get a signal: this could be a park, in the privacy of your bathroom, almost any location imaginable as long as there is a sufficient signal. As our needs change the face of wireless Internet must change to meet them.

The IEEE under 802.11 approves the standards governing the transmission of a wireless signal. The latest standard to be considered is 802.11N: this new specification seems destined to supercede the current standards. 802.11B and 802.11G are the two prevalent standards in use today. The 802.11B standard provides for transmission at up to 11 MBPS. 802.11G is very similar to the earlier B standard except that the transmission rate was improved to a possible 54 MBPS. Most all equipment that is rated for the G standard is backward compatible with the B standard. Both standards operate at 2.4GHz. There is another standard known as 802.11A, which came out (oddly) after the release of 802.11B. This is a modification of the original standard 802.11 and also operates at up to 54MBPS: it is not widely used in North America.

All three of the currently -in- use standards (A, B, G) have weaknesses from a transmission standpoint. Due to the CSMA/CA protocol overhead, in actuality the maximum 802.11B throughput that an application can achieve is about 5.9 MBPS over TCP. Both B and G standards provide for 14 usable bands for transmission 20 MHz wide: only three (1, 6,11) do not in any way overlap. While 802.11G is rated to 54MBPS, actual speeds are degraded by a number of factors such as conflicts with 802.11B-only devices, exposure to the same interference sources as 802.11B, only 3 fully non-overlapping channels, and the fact that the higher data rates of 802.11G are often more susceptible to interference than 802.11B. In many cases G transmission rates are little higher than B because the transmission device lowers the rate due to interference.

Finally, 802.11A transmits at 5GHz instead of 2.4 GHz. While this wavelength has far less interference, it is more easily absorbed by structure and effective distance is often no more than line-of-sight. In all protocols, bandwidth is shared between users.

With all these weaknesses, pressure from numerous manufacturers, and the fact that wireless Internet (also known as WIFI) is growing at a phenomenal rate, the IEEE decided that a better standard for transmission needed to be developed and in June 2004 the 802.11N Task Group was organized. The proposed standard has a theoretical maximum transmission rate of 540 MBPS. This is about 40 times as fast as the B standard and 10 times as fast as the G or A standard. Industry experts agree that an actual transmission rate of between 100 and 200 MBPS is more likely initially.

The proposed 802.11N is to operate using a technology known as “MIMO”, which is an acronym for multiple-input/ multiple-output. MIMO uses multiple transmitter and receiver antennas to allow for increased data throughput through spatial multiplexing and increased range. While the B and G standards operate with 20MHz wide bands, the proposed N standard will have a 40 MHz band, which in essence will double capacity in itself.

Spatial multiplexing is an important part of the new specification, and was originally discovered in the 1940’s, but was buried in obscurity for years. The concept was “rediscovered” in 1984 by Jack Winters of Bell Labs, and was originally referred to as BLAST (Bell Labs Layered Space- Time). This multiplexing scheme using multiple antennas for transmission and reception is capable of producing a significant increase in capacity from single antenna systems, but only if the channels from a transmitter to a receiver follow independent paths. Any correlation between the separate streams of data makes stream separation and decoding more difficult. Extreme correlation of the data paths leads to transmission speeds often no greater than a single antenna configuration. However, given the proper conditions, the spatial multiplexing concept can produce an output equal to the square of the number of antennas, or put another way, two antennas given the proper conditions can produce up to four times the output of one antenna.

Just how the process works is fairly complicated, but to simply things a little having multiple receivers not only increases the receive power, but also reduces multipath problems by combining the received signals for each received frequency component separately. This is called subcarrier-based maximal receive combining. It significantly improves overall gain, especially in multipath environments. In such environments, signals pass through and reflect from various objects so that different signal characteristics reach the receiving antennas. Some frequencies may tend to be attenuated at one antenna but not the other. At frequencies where signals have similar strength, the receiver combines their signal strength, more than doubling the signal power. Using multiple power amplifiers and antennas completes the scenario. The transmitter uses phase-shifting algorithms to drive multiple antennas that focus toward the receiver. This can increase power by the square of the number of transmit antennas. There is also an array gain from focusing the delivered energy in the direction of the receiver, so that less is wasted. This is called beamforming. The combination of these two effects can increase realized power by a factor of four.

Given that multiple antennas are to be utilized in this configuration, antenna design becomes an issue. The best performance would be obtained from antennas spaced far apart (several times the bandwidth), however this is not possible with devices such as laptop computers. A solution to this problem is to use antennas with different polarizations and radiation patterns. By choosing the patterns and polarizations of the antennas carefully, it is possible to achieve good performance by orienting the antennas such that they have minimal overlap between their patterns. Dr. Robert Heath from the University of Texas at Austin reports that their research indicates that “a circular type patch array yields significant polarization/pattern diversity gains from conventional uniform linear array patterns while considerably reducing the physical size of the array.” While a circular patch pattern may be ideal, in all likelihood the first offerings of 802.11N devices will probably contain 2 to 4 antennas.

So far we have discussed spatial multiplexing and antenna characteristics: where will the proposed new N standard operate? Vendors such as Airgo and Belkin are producing wireless routers and network cards called “Super G”, Turbo G”, and “Pre-N”. These are all operating in the 2.4 GHz band, the same as the B and G standard. They are for the most part grabbing channels 5 and 6 for operation to produce a 40MHz wide path. While this can produce a substantial increase in throughput, the method has caused transmission problems throughout the whole 2.4GHz spectrum. All wireless routers shipped by Airgo In the last 2 years have had the turbo mode turned off, or are to be run in dynamic mode, which uses the additional bandwidth only if there is no other traffic/ usage detected. Where to place the operation of the new standard has been and still is a stumbling block for the approval of the proposed N standard.

Placing additional traffic on an already saturated frequency range in many metropolitan areas is less than palatable. Placement in the 5GHz band is not an option as the range is not sufficient, although this band has 23 currently operable channels and less interference: there far are fewer devices operating there.

Another stumbling block that has hampered the development of the proposed 802.11N standard is the fact that there were actually multiple proposals of this new standard. The first was WWiSE (World-Wide Spectrum Efficiency), backed by companies including Broadcom, and the second was TGn Sync backed by Intel and Philips. Still a third standard proposed was MITMOT ("Mac and mImo Technologies for MOre Throughput"): this proposal was backed by Motorola and Mitsubishi. The differing proposals were finally merged in July of 2005 and sent to the IEEE for consideration in September. The Enhanced Wireless Consortium (EWC) was formed to help accelerate the IEEE 802.11N development process and promote a technology specification for interoperability of next-generation wireless local area networking (WLAN) products. Many of the stakeholders from the former competing proposals are represented, although several notable companies, such as Motorola and Phillips, are absent.

The proposed new standard, 802.11N was approved as a draft proposal on January 24, 2006. Currently, this draft proposal is up for a vote over a 40-day period. During this period, criticisms and recommended changes will be solicited.

In the May meeting, those changes will be discussed, and some will be adopted and others may not. If all goes well, a re-ballot will happen following a similar course. In July, a final draft could win the day, which would then go on to a group of experts at a higher IEEE level who typically approve drafts—by the time they have reached this point, most technical and harmonization issues will have been settled. It is quite possible that the standard will be approved in final form by the end of 2006.

With all this discussion about a new WIFI standard with a proposed rate of as high as 540MBPS, where does the consumer fit into all this? As previously noted, there is what is called Pre-N products already on the market being produced by firms such as Belkin. Given that the standard is not officially released, and it is probable that some changes will yet be made, these Pre-N products may not be fully compliant without upgrades in software or hardware.

Even if a consumer purchases one of these devices and upgrades are available, will the end product be up to the quality of the post-approval products? From a conservative standpoint it might be better to wait until after the standard is finally released.

It is doubtful that many home-users could reap much of the benefits of such a high powered system, at least at the present time. Currently most homeowners are lucky to see 5MBPS through a fast broadband connection: a B or G compliant device could easily service this. However, and industries or institutions with a much larger “pipeline” might be able to reap the benefits, given that they may be servicing hundreds of clients or employees. Another instance of a user that could possibly reap the benefits of such a fast broadcast rate would be a wireless Internet provider. A faster connection may help them to compete with a “wired” broadband service, providing they properly secure their network to prevent hackers from pirating bandwidth, or without a doubt far more damaging exploits, such as data and /or identity theft. A larger pipeline does not mean a more secure pipeline unless proper security measures are taken. A point to remember is that wireless communication is inherently insecure: anyone with the proper equipment can easily intercept it. Proper security precautions must be taken and this proposed new standard is no exception.

From a security standpoint, the proposed standard is said to have no new major security innovations implemented in it, that is, no new security features have been devised specifically for 802.11N, although the new standard will support the features of the 802.11I standard, such as WPA/WPA2. This 802.11I standard specifically deals with security weaknesses discovered and documented associated with wireless Internet. A specific weakness identified with wireless Internet is the use of WEP (Wired Equivalent Privacy). Using tools readily available on the Internet, WEP can be cracked and should therefore be avoided where possible. WPA2 or WPA should be chosen because they are far more secure: the choice may depend on hardware, as WPA2 may not be fully compatible with certain older devices. WPA2 is far more desirable as WPA was introduced as an interim measure until WPA2 was released in 802.11I: WPA was designed to be compatible with WEP devices and uses some of the same features. If a device can be upgraded to WPA2 from WPA using a manufacturer software patch it should be done. Worthy of note is that all network devices produced after March 13, 2006 must be certified to WPA2 in order to be certified by the Wi-Fi Alliance. A commonsense precaution is the avoidance of default passwords and the use of strong passphrases.

In a nutshell, that is an overview of the new 802.11N wireless standard that is currently in draft form. We have seen a quick overview of the legacy protocols and the weaknesses with each that have brought about the need for this new and exciting concept. The document in place awaiting final approvals represents a compromise of three competing groups. The remaining issues with the implementation of the standard involve the exact placement within the 2.4GHz band. This new standard promises much faster transmission speeds than are currently available using new concepts such as MIMO and spatial multiplexing. Using proper modern security precautions the new standard should prove to be a valuable asset to businesses of all kinds and possibly to home users as larger and more bandwidth intensive applications come about.

Works Cited

Heath, Jr., Robert. “Miniaturized Antenna Design for MIMO Communication Systems: Exploiting Polarization and Pattern Diversity”. University of Texas at Austin. 1 April, 2006. http://www.ece.utexas.edu/~rheath/research/mimo/antenna/

“Getting the Most Out of MIMO: Boosting Wireless LAN Performance with Full Capability.” Whitepaper. Atheros Communications, Inc. June 2005.

Frederick, Jonathan D., Wang, Yuanxun, and Itoh, Tatsuo. Smart Antennas Based on Spatial Multiplexing of Local Elements (SMILE) for Mutual Coupling Reduction. IEEE Transactions on Antennas and Propagation, Vol. 52, No. 1, January 2004.

IEEE Std.802.11I. 23 July 2004. IEEE. New York, New York.

Fleishman, Glenn. “Brinksmanship in 802.11N”. 14 March 2006. Wi-Fi Net News. 26 March,2006. http://www.wifinetnews.com/archives/cat_80211n.html.

Akhtar, Jabran.“Spatial Multiplexing Over Correlated MIMO Channels with A Closed Form Precoder.” University of Oslo, Oslo Norway. 2003. www.eurecom.fr/~gesbert/papers/globecom03_jabran.pdf. 27 March 2006.

Wilson, James M. “The Next Generation of Wireless LAN Emerges with 802.11N”. 29 August 2004. http://www.deviceforge.com/articles/ AT5096801417.html. 27 March 2006.

“802.11I.” Wilkipedia. 27 March 2006. http://en.wikipedia.org/wiki/Wi-Fi_Protected_Access. 28 March 2006.

Details

Seiten
10
Jahr
2006
Dateigröße
510 KB
Sprache
Englisch
Katalognummer
v110035
Institution / Hochschule
East Carolina University
Note
Satisfact.
Schlagworte
Wireless Wave Future ICTN4040

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Titel: 802.11N: Wireless Wave of The Future?