[MLB-WIRELESS] Single slow link can bog network: New Scientist

[Clae]

http://www.newscientist.com/news/news.jsp?id=ns99994018

Single slow user can throttle wi-fi network
18:27 04 August 03
NewScientist.com news service

A single user with a slow connection to a wireless network can 
significantly degrade the overall service to everyone using that 
wi-fi access station, new research shows.

Researchers from the Institut d'Informatique et Mathématiques 
Appliquées de Grenoble, part of France's CNRS, studied the 
performance of networks using the popular wi-fi standard 802.11b.

They found that if a single user's connection is slowed, perhaps 
because they are far from the access station, every user can suffer 
reduced data transfer speeds. "That computer may degrade the nominal 
bit rate," Duda told New Scientist.

This is because of the way bandwidth is allocated to each user by the 
wi-fi standard's access protocol, called Carrier Sense Multiple 
Access/Collision Avoidance  (CSMA/CA).

If just one person is unable to connect at the optimal speed, the 
CSMA/CA protocol throttles back the maximum connection speed for all. 
This guarantees that any user, no matter what their access speed, can 
get stable access to the network.

If the faster users were not limited and the network approached its 
maximum bandwidth, the slower user's service would be the first to 
degrade to the point of uselessness. However, the speed limit could 
mean that the best connected users see their transfer speeds cut from 
11 Mb per second to about 1 Mb/s.

Duda says users may not notice a network snarl-up unless they are 
using a lot of bandwidth by, for example, downloading large files. 
And he adds that some manufacturers have already begun to address the 
issue with their latest wireless networking hardware.

But it is particularly important, he says, that devices using the 
newer and higher-bandwidth networking standards such as 802.11g do 
not suffer from the same problem.

"In general, designers should be aware of this problem," says Mingyan 
Liu, a wireless networking expert at the University of Michigan in 
the US. He says it is a question of balancing the stability of the 
network for all with the quality of service delivered to the majority 
of users.

"But it would be worthwhile for researchers and engineers to come up 
with mitigating methods," Lui told New Scientist.

Will Knight

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