Network Management Strategies for the CIO
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The most common source is co-channel interference from neighboring WLANs (including competition between your own two routers). When you configure a Wi-Fi access point (or router) to use a specified channel, the channel is centered within a frequency range that's about 20 MHz wide. For example, when you select Channel 6 (2.437 GHz), your access point (AP) actually transmits between 2.427 GHz (the center of Channel 4) and 2.447 GHz (the center of Channel 8). To avoid co-channel interference, adjacent Wi-Fi APs must be configured to use non-overlapping channels. Most people use Channels 1, 6 and 11 because that results in the max three non-overlapping channels.
You can determine whether this is your problem by using a shareware tool like NetStumbler to detect nearby APs and the channels they are currently using. If your APs are not using two different channels in {1, 6 or 11} then reconfigure them to do so. If other nearby APs with strong signal are using one of these three channels, then avoid that channel. For example, if one of your routers is close to an AP on Channel 9, 10 or 11, then pick Channel 6 or 1 for that router. With only three non-overlapping channels, it can be hard to find "clean air" so just look for the best possible channels and of course avoid overlap between your own routers.
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If you don't spot any obvious sources, you can dig deeper using a wireless spectrum analyzer. To learn more about troubleshooting Wi-Fi interference using spectrum analysis, read this searchNetworking tip: WLAN troubleshooting with spectrum analyzers.
Most spectrum analyzers require buying a specialized wireless adapter. However, if you happen to have a relatively new laptop containing an Intel Centrino 2 wireless adapter, you might want to try AirMedic, currently available as a free beta trial.
This was first published in March 2009
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