In my last post "High Channel Utilization-The LAB" I explained how external APs and even internal ones impact channel utilization, including bluetooh devices, speaking about 2.4 GHz. In this post I showed how you can lower channel utilization by disabling some of the lower data rates. Of course the results will vary, depending on how bad the utilization or your RF environment is, but the idea is to make cell smaller, and also make it faster by making the AP sent beacons and some other basic information at the lowest mandatory data rate, which in this case is going ot be 24 Mbps. Keep in mind that these settings will impact 802.11b only devices, that could not be there a lot them now days, but there are still some.
If you want to go to the details about packet analysis, take a look at the CWSP book.
The following pictures were taken from a real life scenario. The customer was complaining about slow performance in 2.4 GHz, APs were deployed in a open space area with a very crowded and diverse client device population. At the beginning I thought that the nature of the problem was completely related to the number of devices and other interference sources. The problem got worse when at midnight with almost no clients and interference sources, the channel utilization was still too high. After some packet captures and based in my theory in my last post, I decided to try two different scenarios.
With powers levels already set to achieve a good connection, at least 48 or 54 Mbps, I first disabled all lower data rates a left 24 Mbps as the lowest mandatory data rate, knowing that now all information sent to the farther device in the cell will have to be sent at 24 Mbps, making communications faster, making the medium rapidly available for the next device to transmit as well. All this was performed any time between 2 and 3 AM, so we don't have clients affecting our readings.
Excuse the maps, I still need to keep the customer privacy, even though it is kind of public and nice place.
This is with 11 Mbps as the lowest mandatory data rate, few or none clients. Channel utilization is really high, in some cases up to 75%.
This is with 24 Mbps as lowest mandatory data rate, anything below is disabled. As you can see utilization was decreased considerably.
I even tried 36 Mbps, it did improve utilization on some APs only, after that you don't really get a better performance.