One other best-practice that comes to mind if it might help with your fine tuning... As a rule of thumb you want to at least disable the lowest two data rates in each band. Selecting 12 or 18Mbps is a common practice. You can do this SSID-by-SSID on the Access Control page. This also has the (usually desirable) effect of disallowing very old legacy 802.11b devices from associating and avoids the protection mechanisms (and associated performance impacts) for backward compatibility. The default setting of 1Mbps creates the largest possible coverage circle, allowing clients to stay associated to APs very far away (despite other APs being closer) and staying connected with lower signal-to-noise ratios, therefore at lower data rates. Lower data rates will impact channel utilization since the data will be modulated onto the RF carrier that much slower. This causes a very high duty cycle (slower clients take longer to transmit the same amount of information) and impacts performance for every other nearby client. It's a half-duplex shared medium, channel utilization is a critical factor, and there is a large impact on the time it takes to modulate a signal into the air at 1Mbps versus 12 or 18Mbps. The faster a client can get on and off the air, the better. Using 12Mbps as a minimum bit rate improves channel utilization and encourages better roaming behavior. What we do NOT want to do is try to artificially force all clients to the higher data rates (like 48 or 54Mbps), as this can also create client problems. Remember, the CLIENTS, not the APs, ultimately make the roaming decisions, and we cannot control the clients, but we want to get them behaving well, by making it as easy as possible for devices to access the medium on a reasonable data rate with a good SNR. Thus the best practice is to size your coverage cells (resulting from a site survey) to allow the proper cell overlap (for example 15 to 20% overlap with -67dBm edges) to allow the elimination of lower data rates <12Mbps. If you find that you MUST support 802.11b data rates (those use cases still exist), consider using AP tags and SSID availability to only broadcast a legacy SSID at the 11Mbps rate from specific access points for example. Also, to reiterate what @Adam said, use automatic power reduction and don't run all APs at 100% transmit power. Since many of the APs will “hear” one another, especially in relatively open spaces, and in many designs you might plan for roughly 20% cell overlap, you can end up with lots of APs within range of one another. If all APs are set to use maximum transmit power, all the APs are just screaming at one another and you end up with far too much cell overlap, far too many APs within range of one another, and too much co-channel interference. Needless to say that will contribute to wacky roaming behavior. And remember that while APs might top out at 30dBm (1 full watt of Tx power, that's a lot) many client devices, especially handhelds, might only top out at 15dBm, because for them it's all about battery life. And those are dBm's so that 15dBm isn't half the power, it's more like 30 milliwatts for the client versus 1000 milliwatts for the AP. Using Automatic Power Reduction will allow all of the APs to not only properly balance their channel settings, but more importantly their transmit power settings. After Automatic Power Reduction has been running for some time, and perhaps after running an Update Auto Channels overnight, look back at the Transmit Power column on the Radio Settings page to make sure the way everything "settled" makes sense.
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