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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)."
How long before this becomes an exploit in order to perform a DOS attack on wifi points?
I say about 1 month, maybe less. Any takers?
MMORPG fan-boy? Prove your worth
"What the -- we're down to 1 Mbps!"
"There he is! Get him!"
The coolest voice ever.
Sluggish WiFi Connections Hurt Everyone
Typing le courrier electronique or "courrier" instead of email also slows everyone down.
Visit Jonesblog and say hello.
It could even have a message that would pop up reading:
You are the slowest link. Goodbye!
If I have been able to see further than others, it is because I bought a pair of binoculars.
I could have had first post, but unfortunately, another user on this hotspot has caused the network to slow down for some reason.
If you have an 11b AP with a bunch of PCs with 11 meg cards and some kind of 2 meg hand held device then the 2 meg device is going to hog the connection until its done.
A solution would be to leave the 802.11b AP inplace and servicing the older 2 meg devices and put an 11a or 11g AP in next to it to service the faster devices. Alternitively you might be able to put the slower devices on channel 1 and put the faster one on 6 and 11 (they have to be 5 apart to avoid overlap, and in the US you only have a total of 11 unregulated channels)
Wireless is different than wired communication. People are just going to ahve to get used to it. More stuff to learn, more rules to follow, more work for people like me.
Its a good thing- especially in this economy.
"Well, serves you right for running a wireless hot-spot. You know those things are nothing more than anonymous leeching posts for music stealers anyhow." ;)
Honey, I shrunk the Cygwin
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...
If your in a public location using a hotspot the only transfers your most likely going to be doing are to/from the internet, not to/from your 1337 mp3 s3v3r box. Chances are the hot spots inet connection is ~1mbps anyway.
This is nothing new. This sort of flaw is inherent in CSMA/CD, the WiFi problem is just another example of it. Before CSMA/CD was even rolled out people knew about this situation. The pros just outweigh the cons, for what this network design philosophy is used for.
* LEET_POWERBOOK_1400 has joined server WIFI_POINT
<ibm_thinkpad> omfg lag
<LEET_POWERBOOK_1400> hi does this map have the bfg ?
<dell_dimension> boot the lpb
<iBook> boot leet_powerbook_1400
* ibm_thinkpad has initiated a vote to kick LEET_POWERBOOK_1400
* Vote to kick LEET_POWERBOOK_1400 was successful (6 for, 1 against, 1 abstain)
* LEET_POWERBOOK_1400 has been kicked from server WIFI_POINT
<dell_dimension> ah much better
* dell_dimension was gibbed by iBook's rocket
This isn't something that's likely to become a serious exploit, for the simple reason that the attacker would need to bring a compatible device within range of the access point. Unless someone has a serious grudge against the owner of the network, who'd want to spend the time? You'd either have to be present with a laptop/pda, or leave it behind. Remember, we are talking about radio waves here. Plenty of technology exists to track 802.11x signals, and all it takes is a well equipped sysadmin and a properly filed lawsuit to discourage the attacker.
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.
Its good to see this sort of stuff put to paper anyway. As a Free/Open software implementer working on developing cheap-as-possible wireless access points for rural area internet distribution, I can say this IS useful. Problems like this, and many others, creep out of nowhere and are very hard to track down without expensive equipment. This specific problem happened and was remedied after much head-scratching by dividing one site into multiple cells so slow users had more 'time' to get their data. just my 0.02
jeff
Who needs sophisticated DOS attacks when simple jamming would do the trick quite well.
I'm a firm believer in the philosophy of a ruling class. Especially since I rule. -Randal, Clerks
Taking into consideration that Milwaukee is a city of ~600,000 people, that not many are going to have wireless laptops and even fewer are gonna spend time in a park surfing the web, it still seems like 20-40 users clogging a public WI-FI is a bit odd.
I suppose they can't complain, though, since the city paid less than a couple of hundred dollars for the setup. Still, it just seems somewhat pointless if it's gonna be clogged all the time.
"This food is problematic."
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!
If I'm the only one on an 11mbps network and sitting right next to the station, I'd expect all the bandwidth. If somebody else joins the same network, in fairness my bandwidth may be halved. But do I really care whether the other guy is getting 1mbps or 11mbps during his timeslice? I'm still getting half the bandwidth as if I had it all to myself, right? I would only consider it strange if a single user joining with a weak connection cut my bandwith by > 1/n.
It sounds like it's time for some form of Token passing. That would prevent slow nodes from dominating the bandwidth, and would make sure everyone gets their fair share. Each node would have to register with the AP and then the AP would serve as the ring monitor.
--
Luck is just skill you didn't know you had.
All you need to shut down a wireless hotspot is a 2.4 GHz cordless phone. This will work for any of the 802.11X connections, lets give a hand to the FCC for this one.
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.
Huh? (blush) Oh yeah. They actually are. Excuse me for the lack of controversy in my opinions. Next time I just might offend you. ;-)