802.11 Wireless Channel Planning

Different Technology

I used to work with Motorola Canopy wireless gear back in the day. It was great ptmp wireless gear. The carrier to interference ratio on this gear was 3 dB, which meant that you could provide extremely reliable wireless links in high RF congested areas. Of course this was proprietary based equipment that didn’t come close to the contention based CDMA/CA 802.11 stuff. I was extremely spoiled when using the Motorola canopy line. It just worked.

802.11 Wireless Channel Planning

Why am I bringing up all this when talking about 802.11 wireless channel planning? Well it’s because of the preconceived notion that using all the 2.4Ghz 802.11 channels may be a good thing (including overlapping channels). At first, I though, just let the AP’s pick the best channel between 1-11 and that I would be good to go. Well that wasn’t the best solution as AP’s were selecting channels 1,3,4,7,10, etc. My thoughts were spurred on by this twitter discussion with wireless guru, Keith Parsons.

Javier S. “What’s better, SNR of 20db between two AP’s on channel 1 or SNR of 20db between AP on channel 1 and AP on channel 3?”
Keith P. “I’d go with the two on the same channel, given a choice. At least they’d ‘Play Nice’ with each other rather than ACI fighting.”
Javier S. “So is it CSMA/CA that works better at detection if interference is on same channel?”

Keith P. “It is the difference between how 802.11 deals with CCI vs ACI.”
Javier S. “So based on CCA, you would rather detect noise and back off rather than face possible data corruption from ACI.?”

The last question wasn’t answered. Basically I needed to do my homework and I would suggest you do the same. Check out the following link:

http://www.revolutionwifi.net/2011/03/understanding-wi-fi-carrier-sense.html

In the above article Andrew really dives into understanding how CDMA/CA works. What I gathered what that wifi can use carrier sense techniques in order to back off in order to avoid possible frame corruption. Keith’s statement of “AP’s on the same channel will likely play nice with each other” will allow carrier sense to do its job.

Research

Ok, so I started doing some more research which lead me to my safaribooks account and checking out a CWNA (certified wireless network administrator) study guide book. The book defines CCI as co-channel interference or “unnecessary medium contention overhead that occurs because all the AP’s are on the same channel.” (Coleman and Westcott) Basically you have wireless devices following the rules of CDMA/CA.

Now ACI is defined as adjacent channel interference and is what you get when you use channels that overlap with one another. The only non-overlapping channels in 2.4Ghz are 1,6,11. When Keith suggests that he would rather see two AP’s on the same channel instead of two using overlapping channels such as 1 and 3, it’s because the 1 and 3 will give you ACI. ACI will cause re-transmits due to corruption of frames. You will rather want CDMA/CA to work as it should instead of facing re-transmits due to corrupt frames.

Resolution

Where did this lead me? Well, I turned off the AP’s auto channel feature and went back to only using channels 1, 6, and 11. I hard set the channels myself and performed a site survey. My goal was to try to maximize the SNR between any two AP’s that my client could see that were on the same channel. At the end of the day, I’m seeing better performance especially since we just doubled the number of access points we used to have. I had to also play around with the minimum basic rate and power output levels as well in order to achieve maximum optimization. Hopefully this helps explain why proper channel planning is extremely important.

 

Here’s a busy, but healthy network using 1,6,11 (Shane, disregard my previous comments on your screenshot using only channels 1, 6, and 11) – Courtesy of Metageek Chanalyzer

Metageek wireless channel planning spectrum image

 

Healthy 2.4Ghz wireless channel planning. I’m working on 5Ghz as well. – Courtesy of Extreme Networks Oneview.

wireless channel planning 5Ghz building map

Wireless channel planning map

Wireless Channel Planning Resources

CWNA: Certified Wireless Network Administrator Official Study Guide: Exam PW0-105, 3rd Edition by David D. Coleman; David A. Westcott

 

2 comments

  • This is a VERY EASY thing to test… set up two AP’s connected to a switch and an FTP server – first put them on the same channel, and FTP Download/Upload from two client devices.

    Then put the AP’s on channels 1, and 2 – and re-run the tests. You’ll soon have your own empirical evidence showing how much worse ACI is than CCI.

    Keith

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