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Sluggish WiFi Connections Hurt Everyone
MindNumbingOblivion writes "Wireless technology has revolutionized access to local area networks when one can't always be close to an ethernet jack. But a recent research paper from the French Centre Nationale de la Recherche Scientifique tells how one slow user accessing a hot point can hurt the whole group. Apparently the very nature of CSMA/CD guarantees such anomalies. Here's the story, and here's the release from CNRS (in French)."
There are definetly better ways to perform DOS attacks on a wireless network.
"Wlan Jack" can send fake disassociation packets, which disconnects clients from an AP. As long as its running, nobody would be able to lock onto an access point.
I also imagine someone could do something that would just flood noise into the spectrum that would kill the connection better than somebody just hogging it at 1 Mbps.
Ethernet uses CSMA/CD. 802.11b uses CSMA/CA (collision avoidance). After all, not every node in a WiFi network can see or hear ther nodes, so they may not be able to detect collisions (but your receiving node might).
Thus, each node must try to avoid causing the collision in the first place - hence such techniques as RTS/CTS protocol...
I think this guy was trying to be funny, not offtopic. Mod em up.
:-]
Thank you! The humor was playing off the French origins of the referenced article and was based upon the recent story of the French government banning the use of the term "email" in favor of "courriel". So....you...see......the humor...... references........Oh, if I have to explain it, I guess it's not that funny.
By the way, this should in no way be considered Francophobic. Vive la France! (especially the cuisine and the wine).
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For anyone who understands how these technologies work, this makes sense. It's one of the compromises that allows connectivity to happen over longer distances. As you might expect, signal degrades as you get furth away from the access point and rather than being unable from far away, it drops down to a lower speed. It just makes sense. Most people understand that wireless (802.11anything) technologies are shared bandwidth. People think "Oh it's 11mbit, that means when the access point is saturated it will be a total of 11mbit." Of course, this is only partly true. If everyone is really close and connecting at 11mbit then it will be 11mbit shared, but if everyone is connecting at 1mbit (far away) then it will 1mbit shared between everyone. This is nothing new, this is just how things work. This is why many access points have the option of only allowing 11mbit and 5mbit connections. This just seems like a bigger problem as peoples connection speeds are further apart in speed. (e.g. An 802.11g user humming along at 55mbit alone will loose approximately half his speed if a 1mbit user starts using half his bandwidth.) Don't expect to get the best of both worlds. People want a large range and high speed, it can't really happen, there are tradeoffs.
From the yahoo article: their research paper that anomalies in the IEEE 802.1x standard -- including 802.11a, 802.11g, and the most widely-used Wi-Fi protocol, 802.11b
I was under the impression that the IEEE 802.1x standard is solely a security standard and that it is not to be used as a generic term for the 802.11* wireless protocols. Anyone care to clear this up?
I had but a simple dream, to destroy all humans.
This is why years of language study is a useful thing :D I was a bit disappointed, though, because it doesn't give much information on exactly why this happens. Guess I'll have to go see how CSMA/CA is different from CSMA/CD on my own.
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A primary analysis of Wi-Fi network performance anomalies was done by four reserchers of the Institute for Information Technology and Applied Mathematics (IMAG)'s Software Systems Resources unit. Martin Heusse, Franck Rousseau, Gilles Berger-Sabbatel, and Andrzej Duda just published the surprising results of their study for the INFOCOM conference in San Francisco, on of the most prestigious in the domain of networks research. it reveals that in certain very --- circumstances, this type of wireless network produces a relatively penalizing slowdown: users with better connectivity, and thus with better data flow, are penalized by those with degraded connections.
Local wireless networks based on the "Wi-Fi" (IEEE 802.11b) standard are starting to be deployed in a relatively large number of locations, and many models of portable computers already come with a Wi-Fi network card. Attempts providing connectivity in public places, by way of what are called "hot spots", are becoming more common. The number of potential users are increasing rapidly, and the first hot spots are in wide use, but can Wi-Fi networks stand up to the needs of numerous users and increased bandwidth demands?
In their usual operating mode, Wi-Fi networks are built upon on a wired network infrastructure. Wireless access points rely on a local, high-bandwidth network, most commonly Ethernet, and create a link between wireless network equipment and the local wired network, as well as the internet. In practice, wireless network cards use four flow levels with different signal modulation techniques that can be selected according to the quality of the connection to the access point. More simply, a card close to an access point can get good bandwidth, nominally 11 megabits/second; as it gets farther away, the levels go to 5.5 Mbit/s, 2 Mbit/s, and finally 1 Mbit/s, as the signal gets weaker and degrades.
As Wi-Fi networks are created, some users get the best flow (11 Mbit/s) in the access point's coverage area because they're close to the access point. A user enters in this coverage area and, being relatively far away, is connected at 1 Mbit/s. When this user communicates over wireless channels, that is, when he transmits data, it causes a drop in bandwidth for all the others, leaving them at a bandwidth apparently identical to his, 1 Mbit/s. No matter which bandwidth levels the original users are connected at, the weakest will be observed by all hosts.
This anomaly, inherent in networks relying on CSMA/CA (Carrier Sense Multiple Access / Collision Avoidance) which is defined in the Wi-Fi standard and revealed by the CNRS research team, penalises the network users. Despite a good connection, their apparent performance can become strongly degraded in a completely unforseeable manner, due simply to the activity of a third party connected to the same wireless access point at a lower bandwidth level.
However, though it will be observed on any network of this type, the impact of this anomaly should be more or less moderate for two reasons. First, most equipment today connects to the network in a sporadic and non-continuous manner; periods of activity, like downloading a web page, are relatively short compared to the time spent reading it. Conversly, if a long communication takes place - downloading a large video, for example - it will continuously penalize all users. A second mitigating factor comes from higher-level protocols, especially TCP, which perform some sort of flow-control that creates an effect on apparent bandwidth.
The researchers are currently working on solutions to limit or suppress this anomaly, which could become extremely limiting with the development of new communication applications, notably audio and video over the Internet.
Configure the access point to only allow 11 mbit connections. If it's a Cisco AP, then that's relatively easy to do. I don't know about other APs.
- Give a man a fire and he's warm for a day, but set him on fire and he's warm for the rest of his life.
For anyone who understands how these technologies work, this makes sense. It's one of the compromises that allows connectivity to happen over longer distances. As you might expect, signal degrades as you get furth away from the access point and rather than being unable from far away, it drops down to a lower speed. It just makes sense. Most people understand that wireless (802.11anything) technologies are shared bandwidth. People think "Oh it's 11mbit, that means when the access point is saturated it will be a total of 11mbit." Of course, this is only partly true. If everyone is really close and connecting at 11mbit then it will be 11mbit shared, but if everyone is connecting at 1mbit (far away) then it will 1mbit shared between everyone. This is nothing new, this is just how things work. This is why many access points have the option of only allowing 11mbit and 5mbit connections. This just seems like a bigger problem as peoples connection speeds are further apart in speed. (e.g. An 802.11g user humming along at 55mbit alone will loose approximately half his speed if a 1mbit user starts using half his bandwidth.) Don't expect to get the best of both worlds. People want a large range and high speed, it can't really happen, there are tradeoffs.
Yep, check this out: Best Wi-Fi Ever: 802.11g.
Of three 802.11g AP's they tested, bandwidth was reduced by 15-50% for the entire wireless network when a single 802.11b card was present. It is also notable that WEP reduces bandwidth by ~30%.
Whenever the offence inspires less horror than the punishment, the rigour of penal law is obliged to give way...
For an excellent explanation of why this type of thing occurs, check out:
C ellWhitePaper/TurboCell%20White%20Paper.htm
http://www.karlnet.com/Documents/WhitePaper/Turbo
Even better is that it details a superior system (albeit pricey per node). It's based on military technology and military technology is light years ahead of what most of us are using on a daily basis.
Blue skies...
The answer is to use something like the "wireless switch" that Vivato developed, which uses phased-array antenna technology and can give the full 11 Mbps 802.11b bandwidth to each client.
Ouch! The truth hurts!
In the american context they might not be so vile,
In the Australian context where we already have a thriving independent cafe culture, Starbucks is offering an inferior product and using marketting size and brand recognition to crush smaller players (plus wireless access).
If their coffee was better than the alternatives I wouldn't get so riled up,
'There is a Light that never goes out.'
The problem will occur in any shared multiple-access radio network when users are at different distances from the base station. Those far away from the base station use spectrum less efficiently than those close to the base station because they're forced to put more RF energy into each data bit to close the link.
The same thing happens in 1xEV-DO. As in 802.11, a wide range of data rates is available to adapt to varying channel conditions, and the lower data rates use the channel less efficiently.
Digital radio designers work hard to make their modulation, coding and multiple access techniques as efficient and adaptive as possible. But at some point, you have no alternative but to add more base stations so that each need serve only a reasonable number of users.
I thought 802.11b and friends were not CSMA/CD but CSMA/CA. That's collision avoidance, not collision detection. CD is done electrically in ethernet.. the voltage on the line is wrong if there is a collision.. a transmitter can know immediately via feedback if it's caused a collision.
In wireless, we don't have this... instead you re-transmit packets that get lost, and you try to avoid collisions in the first place.
Basically, the way CSMA/CA works, when I want to transmit, I send a jamming signal. If I don't receive a jamming signal in some small amount of time, I can assume with some degree of safety that no-one else is trying to transmit at the same time.
/CD and not /CA).
Hmm? That's roughly how Ethernet works (and it's
With 802.11* communication, you typically transmit at ~20dBm and receive at ~-60dBm. The difference in signal strength is ~10^8, so it is pretty much impossible to detect someone else transmitting at the same time. Instead, 802.11* use Collission Avoidance. In short - listen before transmit, and explicit ACK.
The worst case scenario will end up with simple alternation: you send a packet, the other person sends a packet; you send a packet, the other person sends a packet; etc.
Yup. That's pretty much how the 802.11 MAC layer works if several wireless stations are trying to communicate at the same time. All stations have roughly the same chance of sending a packet, and the client @1Mbps will use 11 times more air time per packet than a client @11Mbps.
Anyway.. I don't quite understand why you have to be a researcher to 'discover' that a client that is associated at 1Mbps can drag an entire 802.11 segment down the drain. This has been known for a long time.
If J.K.R wrote Windows: Puteulanus fenestra mortalis!
If you would just get a ham liscense then you would have primary allocation for some of the 2.4ghz band and you could take steps to resolve the phone interference properly. I cant wait untill more hams start useing 802.11whatever and have a reason to do some 2.4ghz fox hunting. If you want more information on the Hinternet ham internet then check out www.arrl.org.
Faith_Healer -- The antethsis to almost everything, and the worlds worst speller.