INFOCOM'03 — Performance anomaly of Wi-Fi networks
Wireless local networks based on the IEEE 802.11b standard, better known as Wi-Fi or Airport, are starting to be deployed fairly widely, and many laptop models now ship with a card of this kind. Attempts to provide connectivity in public places through what are called hot spots are also multiplying: stations, cafés, hotels. With the number of potential users growing quickly and the first hot spot deployments well under way, it is worth asking how well this kind of network stands up to the load of many users and the throughput they ask for.
We present surprising results in a study published at the INFOCOM 2003 conference in San Francisco, CA. It turns out that under quite ordinary circumstances this kind of wireless network exhibits a damaging performance anomaly: users who could enjoy better connectivity, and so better throughput, are penalised by those in degraded conditions.
The results in more detail #
In their usual mode of operation, 802.11b networks rest on a fixed wired infrastructure. Wireless access points are attached to a high-speed local network, usually Ethernet, and bridge between wirelessly connected equipment and the wired network, and then the Internet.
Wireless cards support four different bit rates, corresponding to different signal modulation techniques selected according to the quality of the connection to the access point. Put simply, a device close to the access point gets good throughput, nominally 11 Mbit/s; as it moves away this drops to 5.5 Mbit/s, then 2 Mbit/s, and finally 1 Mbit/s as the signal weakens and degrades.
Here is what can happen. Suppose several users in an access point’s coverage area are close enough to get the best rate, 11 Mbit/s. Another user enters the area and, being relatively far away, connects at 1 Mbit/s. As soon as that user transmits data, throughput collapses for everyone else, bringing them down to the same apparent rate of 1 Mbit/s. The same holds for any of the four rates above: the lowest rate is the one every host ends up seeing.
This anomaly is inherent to the CSMA/CA medium access protocol defined in the IEEE 802.11 standard, and it penalises users of the network. Despite a good connection, their apparent performance can degrade sharply, without their knowing and quite unpredictably, simply because a third host with a worse connection to the same access point is active. That said, while the anomaly is plainly observable on any network of this kind, its impact should be qualified on two counts. First, most connected equipment today uses the network in bursts rather than continuously: periods of activity, such as loading a web page, are short compared with the time spent reading it. A long-running transfer — a download, an audio or video connection — will, by contrast, penalise other users continuously. Second, the higher-layer protocols, chiefly TCP, have internal mechanisms that regulate the throughputs observed. We are nevertheless working on ways to limit or remove this anomaly, which could prove very damaging as new audio and video communication applications develop over the Internet. Our study was validated experimentally using 802.11b; although they offer higher rates, products based on 802.11a and 802.11g are subject to the same kind of anomaly, since they use the same access technique.
- [INFOCOM 2003] Martin Heusse, Franck Rousseau, Gilles Berger-Sabbatel, and Andrzej Duda. Performance anomaly of 802.11b. In Proceedings of IEEE INFOCOM 2003, San Francisco, USA, 30 March – 3 April 2003.
- [IEEE Wireless] IEEE standards for wireless networks