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Archive for September 2011

And a one, and a four, and a eight, and a ‘leven…

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Channel choices can be a tricky thing, especially in the 2.4 GHz frequency band. I saw a network recently that had an unconventional channel design, but the network seemed to work pretty well.


Channel selection has long been a peculiar topic for 2.4 GHz WiFi networks. Per-channel frequency allocations in the band are 5 MHz wide (enough for a cordless phone or PowerPoint clicker, for example), but transmissions are much wider. The exact amount of bandwidth taken up by WiFi devices varies depending on the standards supported (802.11 b, g or n), the radio’s transmission power and possibly other mysterious factors as well. (Just try running a spectrum analyzer around gear that supports transmit beamforming (TxBF) and you’ll see what I mean.)

A seasoned rule of thumb has been to keep APs running on channels 1, 6 and 11 in an environment that supports ubiquitous coverage. The theory is that, at typical transmission/antenna configurations, WiFi devices will transmit over bandwidths that range up to about 22 MHz wide. Using the aforementioned channels gives one 25 MHz of space between channels (five channels at 5 MHz per channel), thereby limiting the likelihood of interference.

The counter to the 1-6-11 design is that a lot of channel space is being wasted. If WiFi devices operate O.K. with only a 20 MHz between APs (or even 18 or 15 MHz), then spacing the channels so far apart could be a waste.

Recently I worked in an office with a WiFi installation that was designed to avoid that type of waste. Channels 1, 4, 8 and 11 were used instead of 1, 6 and 11. I was unable to talk to the person who installed the wireless network, but I did get a chance to investigate a few things.

The first thing I looked at was the purely qualitative reaction of the people who use the network. Their reaction was one of unanimous approval. Now, that approval could have been due to the endpoint stations being used (just about all notebooks, iPads and smartphones) or due to the infrastructure vendor (Aruba, who I credit for turning the MGM Grand Garden Arena’s WiFi from the unusable mess it once was into one of the best sports arena WiFi networks I’ve seen) or due to the fact that it was a building that was far enough away from neighboring buildings that outside interference was a non-issue. Still, +1 for 1/4/8/11.

I also did a little semi-scientific stress test. I checked my channel (8, for the record), used WildPackets OmniPeek to see what other channels had strong signals in the room (only 4, though 1 and 11 could have been strong enough for a low rate connection) and then started watching NBA highlights on YouTube. The video played just fine, but I did notice something disconcerting. The Retry percentage on channel 8 jumped immediately. The number fluctuated, of course, but generally hovered somewhere between 8 and 12 percent.

Now, 8-12% Retrys can be just fine, but for this environment I’d say that is too high. I was on a notebook (easy), sitting still (easy), associated to an AP mounted in a hallway (hard) below the ceiling (could be easy or hard) in an isolated building (easy) and running a streaming web application (a little hard). All told, the environment leaned a bit towards an easier installation than a hard one.

It is always tough to get a feel for whether something could be improved without chatting with the wireless network administrator, but based on my quick sniff of the channel, it appeared to me that I probably would have gotten better results if APs were placed on channels 1, 6 and 11. (Though if they were, who knows if the VAR could have managed to sell an AP every 45 feet or so.)

I am curious to hear feedback from readers, too. If you have a WiFi network that has either channel design (or something altogether different than 1/6/11 or 1/4/8/11), drop me a line or post a comment. If you have the time to do a quick check of your Retry percentages, all the better. My guess is that 1/6/11 is going to give better results because of the 1/4 and 8/11 overlap, but I have been wrong before (as anyone who attended the 2006 CWNA train-the-trainer class and heard me repeatedly pronounce that 802.11n would never be widely adopted can attest).

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Written by sniffwifi

September 2, 2011 at 4:19 am

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