Wireless LANs communicate over the same air medium, which results in congestion and collisions. Learn valuable tips on choosing the right RF channels for your access points that optimize performance.

With 802.11 wireless LANs, an access point is set to a specific RF channel. As part of the association process, each user’s radio card scans all channels, detects the access point, and then tunes to the channel of the access point to complete a connection. In order to deploy an effective wireless network, each adjacent access point within range of each other should be set to different, non-overlapping RF channels. This minimizes contention between users and the additional overhead of beacons coming from multiple access points, while improving performance. With 2.4GHz wireless LANs (802.11b and 802.11g), these channels are 1, 6, and 11. In the 5GHz band (802.11a), the channels don’t overlap, so you’re free to use any of them.

In terms of throughput, there’s not much of a problem with access points set to the same channel if the load on the network is light. The addition of beacons from multiple access points with common channel settings has insignificant impact. Nevertheless, keep in mind that if all of access points are set to the same channel, users may have troubles roaming from one access point to another. This situation makes it difficult for radio cards to distinguish the access points.

I’ve run tests that show significant decreases in throughput of a particular access point as you increase the load on an adjacent access point set to the same channel. For example in one of my workshops, I had two 802.11g access points (A and B) both set to channel 6 and separated by about 150 feet. With Airmagnet Analyzer, I ran throughput tests against access point A with access point B shut down. I then power up access point B and ran a throughput test before and after introducing a load on access point B. For the load, all of the students connected to access point B and continually pinged access point B with large packets.

After powering up access point B with no load (the access point was only emitting beacons), we found that the throughput on access point A was only about 5 percent lower. While applying the load on access point B, however, the throughput on access point A took a sharp nose dive. The addition of the heavy user traffic on access point B caused 70 percent throughput degradation on access point A!

Just to be complete, we then set access point A to channel 1 and left access point B on channel 6 (the preferred configurations) and reran the tests. As we’d expected, we found no drop in throughput on access point A. As a result, it’s certainly well worth configuring adjacent access points to non-overlapping channels, especially when heavier loads are present.

Some Tips to Ponder

If you’re installing a single access point in a home or small office, you might think that any channel will do. Be aware, however, that most access points are set in the factory to channel. In fact, a scan of typical neighborhoods shows that approximately 75% of the access points are set to channel 6. This means that if your neighbor has a wireless LAN, which is becoming much more probable, then you have a 75 percent chance of having significant interference on your network when your neighbor is actively using their network. If you do nothing else, at least set your access point to either channel 1 or 11 to steer away from what your neighbor is likely using.

For larger networks, perform a RF site survey. Consider using tools from companies such as AirMagnet, Helium Networks, and Ekahau. They have features, such as positioning, which helps you plot coverage patterns on electronic maps. For smaller wireless LANs, you can get by with NetStumbler, which you can readily download for free and install on a laptop. In fact, NetStumbler works pretty well to analyze the situation in your neighborhood to ensure that you’re choosing a non-overlapping channel.

Some enterprise access points have mechanisms that optionally and automatically set the RF channel in access points. This feature makes sense to activate in a highly dynamic environment, such as in highly congested areas, such as cities. Neighboring wireless LANs and sources of RF interference come and go almost daily, so setting specific static channels one day will likely not be best in the near future.

If your wireless LAN seems to have poor performance when your microwave oven is operating, such as email taking a long time to sync or web browsing is slow, then try setting the access point nearest to the microwave oven to a different channel. You’re probably experiencing microwave oven interference. The same goes with cordless phones.

Also, don’t be afraid to experiment a bit with RF channel settings. Try different configurations, and compare throughput of each one. You’ll quickly find the optimum settings that maximize performance!

Jim Geier is an independent consultant and founder of Wireless-Nets, Ltd (www.wireless-nets.com), a consulting firm assisting municipalities, enterprises, hospitals, airports, and equipment providers with the development and deployment of wireless networks.

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