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An initial analysis of performance anomalies on Wi-Fi(1)
networks was conducted by four researchers from the Logiciels Systèmes
Réseaux Unit of the CNRS Institut d'Informatique et de Mathématiques
Appliquées de Grenoble (IMAG). Martin Heusse, Franck Rousseau,
Gilles Berger-Sabbatel, and Andrzej Duda published the surprising results
of their study in time for the INFOCOM conference in San Francisco, one
of the most prestigious in the network research field. Their analysis
revealed that this type of wireless network manifests a relatively problematic
performance anomaly under some very ordinary circumstances: users with
the best connectivity and, as a result, higher transmission rates, are
penalized by those who work under poorer conditions.
Local wireless networks based on the WI-Fi standard (IEEE 802.11b) are
beginning to be relatively widely used and many laptop computer models
now come equipped with this type of network card. Experiments aimed at
providing connectivity in public places through what is known as hot spots(2)
are on the rise. The number of potential users is rapidly increasing and
the first hot spot experiments are well on their way. However, we must
ask ourselves if Wi-Fi networks will be able to withstand the pressure
of the number of users and the speed that is expected of them.
In their normal operating mode, Wi-Fi type networks rely on a fixed network
infrastructure. Wireless access points are connected to a local high-speed
network, most often of the Ethernet type, and provide the link between
equipment connected by wireless networks and the local fixed network,
and then Internet. Wireless network cards have four different throughput
levels that actually correspond to the different modulation techniques
of the signal that will be chosen according to the quality of the connection
at the access point. In other words, a computer near an access point will
have a good throughput level, say 11 megabits per second (Mbits/s). Then,
as it moves away from the source, the rate will drop to 5.5 Mbits/s, 2
Mbits/s, and, finally, to 1 Mbit/s, as the signal weakens and becomes
poorer.
Here is an illustration of what happens on Wi-Fi networks: several users
have the possibility of improved throughput in the area covered by an
access point as a result of their proximity, say 11 Mbits/s. A user enters
this area and, assuming that he is relatively far from the access point,
is therefore connected at a speed of 1 Mbit/s. When the user makes use
of the wireless channel to transmit data, this activity will result in
a drop in the apparent data transfer rate for all of the other users,
that is, 1 Mbit/s. This same type of observation is valid regardless of
which of the four throughput levels mentioned above is chosen. In other
words, all of the hosts will be reduced to the lowest transfer rate.
This anomaly, inherent in the CSMA/CA (Carrier Sense Multiple Access/Collision
Avoidance) access protocol defined in the Wi-Fi standard and demonstrated
by the CNRS research team, is detrimental for network users. Despite a
good connection, their apparent performances can by highly reduced without
their knowledge and in a totally unpredictable way, simply because of
the activity of a third party who has a poorer connection to the same
wireless access point.
Nevertheless, although this anomaly can be easily observed on any network
of this type, its impact should be somewhat lessened for the following
two reasons. First of all, most of the equipment connected to a network
today uses the network in a sporadic and non-continuous way: activity
periods such as downloading a WEB page are relatively short compared to
the time necessary to read it. On the other hand, if a lengthy data transfer
is in progress – downloading of an audio or video connection, for
example – users will be penalized on a more continuous basis.
A second mitigating factor is the result of protocols at higher levels
and especially the TCP(3) whose internal mechanisms have
a regulating effect on observed transmission rates.
Researchers are currently looking for solutions in order to limit or eliminate
this anomaly, which could prove to be very problematic, especially in
view of the development of new communication applications, particularly
audio and video on Internet.
(1)
Wi-Fi (Wireless Fidelity) is a new wireless local network technology.
(2) Places where wireless access is provided: train stations,
restaurants, hotels, etc.
(3) TCP: Transmission Control Protocol
Researcher
contact:
Andrzej Duda,
Professor at ENSIMAG (Institut National Polytechnique de Grenoble)
Laboratoire LSR-IMAG
Tel: +33 4 76 82 72 68
E-mail: Andrzej.Duda@imag.fr
Press contact:
Laetitia Louis,
Tel: +33 1 44 96 49 88
E-mail: Laetitia.Louis@cnrs-dir.fr
Communication and Information Science and Technology Department contact:
Armelle Toulemonde,
Tel: +33 1 44 96 53 88
E-mail: Armelle.Toulemonde@cnrs-dir.fr
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