DFS Channels - lesson learned

[SofaKing]

My company has a handful of distribution centers around the country(US and a couple in Canada).  We recently did a much needed upgrade and removed all of the old workhorse Symbol Base Stations with Aruba 215 APs.  Ever since moving to the new APs we have had issues with the handheld scanners.  We have made several changes(reduce min/max tx/rx, enabled drop broadcast and unknown multicast, manually set the 802.11a/g to specific rates, and raised the SNR from 0 to 15) which helped but there were still some connectivity issues remaining. 

Earlier this week I was out at one of the distribution centers and was able to do some testing.  The first test I did was walk around one of the reported trouble areas with my laptop connected to the guest SSID and my phone connected to the handheld SSID.  This area consists of 3 floors with APs spread across each floor.  With my laptop I was able to track the APs my phone connected and was able to verify that my phone was establishing a connection with the correct AP.

I then used one of the handhelds and walked the same path using my laptop to track which APs the handheld connected to.  The handheld had a problem in two areas of the middle floor.  Instead of connecting to the AP it should have it was connecting to an AP either the floor above, the floor below, or to an AP on the same floor that was far away.  This occurred even when I was standing right below the AP.  Both my phone and the handheld always connected to 802.11a.

So from my first test I was able to tell that the coverage in this area was good.  My VDI connection never dropped or froze through out both tests.  The problem had to be with the handheld.  After doing some reading and reviewing the APs it seemed that enabling DFS channels on the APs may have been the problem.  The two APs the handhelds would not connect to were using DFS U-NII-2C channels while the other APs were using non-DFS channels.  The handhelds are a few years old and it turns out they do not support DFS U-NII-2C channels.  I then disabled all DFS channels on the APs and did another walk around.  The handhelds were now miraculously connecting to all desired APs and everything was good.

Lesson learned - know both the AP and the device connecting to the AP limitations.  This has been a problem going on for a few months.  Aruba TAC was involved and were able to remove the majority of the issues but this one problem still remained.  It was not until I could walk the area that it became clear what the issue was.  Don't let DFS kick your A$$ like it did mine 

[deanwebb]

Those old school devices that lack support for new school features can be a real pain point. ALWAYS check to see about compatibility with the features on your AP/WLC combo.

We got dinged hard on some Cisco wireless phones that didn't support certificates until the very latest firmware upgrade.

[wintermute000]

during our wireless designs we always assume DFS channels are unusable.

Even if they are usable now and confirmed with a survey, situation may change in future.

plenty of BW in 5Ghz to do a proper RF plan without overlap as long as you don't insist on maxing out channel width to get the paper throughput LOL