Choosing Optimum RF Channels

By Jim Geier

October 24, 2005

All Wi-Fi networks communicate over the same air medium, which results in congestion and collisions. Here are valuable tips on choosing the right RF channels to optimize performance.

With 802.11 wireless LANs (WLANs), an AP (AP) is set to a specific radio frequency (RF) channel. As part of the association process, each user's wireless network interface card scans all channels, detects the AP, and then tunes to the channel of the AP to complete a connection. In order to deploy an effective wireless network, each adjacent AP within range of another should be set to different, non-overlapping RF channels. This minimizes contention between users and the additional overhead of beacons coming from multiple APs, 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 APs set to the same channel if the load on the network is light. The addition of beacons from multiple APs with common channel settings has insignificant impact. Nevertheless, keep in mind that if all of the APs are set to the same channel, users may have trouble roaming from one AP to another. This situation makes it difficult for radio cards to distinguish the APs.

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

After powering up AP B with no load (the AP was only emitting beacons), we found that the throughput on AP A was only about five percent lower. While applying the load traffic on AP B, however, the throughput on AP A took a sharp nosedive: heavy user traffic on AP B caused a 70 percent throughput degradation on AP A!

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

Some Tips to Ponder

If you're installing a single AP in a home or small office, you might think that any channel will do. Be aware, however, that most APs are set in the factory to channel 6. In fact, a scan of typical neighborhoods shows that approximately 75 percent of the APs are set to channel 6. This means that if your neighbor has a wireless LAN — which is highly 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 AP to either channel 1 or 11 to steer away from what your neighbor is likely using.

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

Some enterprise APs have mechanisms that optionally and automatically set the RF channel in APs. It makes sense to activate this feature in a highly dynamic environment, especially in highly congested areas such as cities. Neighboring WLANs and sources of RF interference come and go almost daily, so setting specific static channels may not be the best idea for the future.

If your WLAN seems to have poor performance when your microwave oven is operating, such as e-mail taking a long time to sync or Web browsing becoming slow, then try setting the AP nearest to the microwave oven to a different channel, since you're probably experiencing microwave oven interference. The same goes for 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 the principal of Wireless-Nets, Ltd., a consulting firm focusing on the implementation of wireless mobile solutions and training. He is the author of the books Deploying Voice over WLANs (Cisco Press), Wireless LANs (Sams) and Wireless Networks - First Step (Cisco Press).

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