Researchers see trouble ahead for WLAN performance

French researchers say 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 (CNRS) in Paris have demonstrated how a slow device connected to WLAN (wireless LAN) hotspot can cause an entire network to drop to a slower speed, citing a common channel access method as the cause, Andrzej Duda, one of the researchers, said Thursday in an interview.

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 Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) channel access method, according to Duda. This method, sometimes referred to as the “listen before talk,” guarantees equally long-term channel access probability 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 work groups, 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 their research in a paper presented at the Infocom 2003 conference, held by the Institute of Electrical and Electronic Engineers (IEEE) in San Francisco, Duda said.

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