R E L A T E D C O N T E N T

Congestion slows WLANs

Labs tests show that early 802.11g kit will not work well in the presence of existing 802.11b networks

Martin Courtney & Dave Bailey, IT Week 24 Mar 2003

Signal congestion in wireless LANs (WLANs) is set to get worse as more firms deploy Wi-Fi 802.11b and 802.11g equipment, leading to diminished data rates and potentially unusable network connections in some instances.

Recent reports predict that European spending on Wi-Fi kit will grow sharply, suggesting that many firms will deploy multiple access points.

This increases the chance of companies' WLANs being situated within a few feet of each other in multi-storey buildings, separated only by a wall or a ceiling through which radio signals can travel.

IT Week Labs tests and the experiences of users in environments with many WLAN access points show that signal congestion can reduce available bandwidth and in extreme cases can even bring networks to a halt.

Tests in our offices showed signal interference from 802.11b access points can cut 802.11g's measured data transfer rates almost in half. Our measured 802.11g data transfer rates fell from 19Mbit/s to 11Mbit/s where 802.11b interference was present. In one test, using a Fluke handheld network monitor, we identified interference from an access point 30ft away and operating on the floor below. The tests used kit based on a draft 802.11g specification, ahead of the final version due this summer.

"It's true that if an 802.11b and an 802.11g access point exist in the same environment, they can conflict, but only if they are within three channels of each other," said Craig Reid, technical sales engineer at WLAN vendor Buffalo Technology.

Our results reflect experiences at the Cebit show in Hanover, where an estimated 80 Wi-Fi access points created so much interference that data rates fell to unusable levels. "If there are too many access points, Wi-Fi behaves like the Hanover autobahn at rush hour - it stops," said Nick Hunn, managing director of wireless card maker TDK Systems. Hunn estimated that the worst affected areas of the exhibition centre saw throughput drop to just a few kilobits per second.

The news will worry firms trying to decide which WLAN kit to deploy. The safest option may be 802.11a. Both 802.11b and 802.11g run at the 2.4GHz frequency also used by short-range Bluetooth devices and other equipment.

In contrast, 802.11a uses the less congested 5.15GHz to 5.35GHz waveband. It supports more users across eight non-overlapping channels, compared with the 11 overlapping channels of 802.11b and 802.11g. It cannot connect to 802.11b/g clients, but dual-mode 802.11b/a access points can bridge the two types of WLAN.

Michael Wall of analyst Frost & Sullivan said firms should do a site survey before deployment: "Different buildings have different structures and that affects the extent to which interference can cause problems. Firms must get a site survey done before they discount 802.11b or g technology."

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