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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.
* 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
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.
--
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."
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!
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!
Huh? (blush) Oh yeah. They actually are. Excuse me for the lack of controversy in my opinions. Next time I just might offend you. ;-)