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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)."
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.
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...
Those with 802.11b cards that handle 11Mbps should not complain too loudly about those that connect at only 1Mbps. The 11Mbps connections slow faster wireless networks down to their speed.
802.11g is supposed to be both 54Mbps and backwards compatible to 802.11b. From my own experience with 802.11g, if there is even one person using an 11Mbps connection, the rest of the wireless nodes also slow down.
Just like driving on a one lane country highway behind a slow car doing only 11 mph in a 54 mph zone.
* 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 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.