Researchers see trouble ahead for WLAN performance

John Blau, IDG News Service   Today’s Top Stories   or  Other Mobile and Wireless Stories  

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July 31, 2003 (IDG News Service) -- French researchers said they have pinpointed a problem in wireless LAN technology that could severely impair the performance of newer high-speed networks.
Engineering experts at France's Centre Nationale de la Recherche Scientifique in Paris have demonstrated how a slow device connected to a WLAN hot spot can cause an entire network to drop to a slower speed, citing a common channel access method as the cause, said Andrzej Duda, one of the researchers, in an interview July 30.
The researchers have focused on performance anomalies found in products based on the IEEE 802.11b standard. They observed that when one wireless device, such as a notebook, connects to a WLAN at a lower bit rate than other devices -- because it is too far from the access point, for example -- performance of the other devices on the network becomes noticeably degraded.
The explanation has to do with the CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) channel-access method, according to Duda. This method, sometimes referred to as "listen before talk," guarantees equally long-term channel-access availability to all devices. In other words, when a device with a low bit rate captures the channel, it penalizes other devices using a higher rate by degrading the speed of their connections.
The CSMA/CA channel-access method is standardized in the 802.11a, b and g standards, Duda said.
"Although some manufacturers have introduced a method to prioritize speeds in 802.11b networks by essentially blocking devices that require bit rates below a certain speed, I'm not aware of any methods to correct this performance anomaly in the higher-speed 802.11a and 802.11g," Duda said. "And even if suppliers come up with their own methods to cope with this problem, they will go beyond standards. That means we could have suppliers delivering access points that operate as described by the standard or include technical features to solve the speed degradation problem and thus no longer conform to the standard."
A possible remedy, Duda said, is 802.11e, a proposed IEEE standard to define quality of service (QOS) mechanisms for wireless gear that gives support to bandwidth-sensitive applications such as voice and video.
"Intelligent access points that support QOS could be one possibility," he said. "The problem is, 802.11e is still in the workgroups, while operators are now in the starting blocks to introduce 802.11g access points. So during this transition phase, users of faster 802.11g wireless cards could be penalized by those with slower 802.11b ones."
In April, the group published initial findings from its research in a paper presented at the Infocom 2003 conference, held by Institute of Electrical and Electronic Engineers Inc. in San Francisco, Duda said.

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