Evaluating High Collision Issues

[tandrews1]

I've looked through most of the available documentation I can find here including the Radio Frequency Interference Troubleshooting Guide (https://thehivecommunity.aerohive.com/s/article/Radio-Frequency-Interference-Troubleshooting?t=1545053626774). But am still perplexed by what I am seeing in some of our environments. I have policies set up that use the recommendations in the aforementioned guide, but I still see high collision issues across the enterprise.

I'm looking at a 250 in an elementary building right now. Summary State=High Collision. This AP was alerting earlier today and the teacher reported poor performance with a cart of Chromebooks. As this is a new building I had not made any adjustments yet, but in this area I applied radio policies to three APs in close proximity that have rectified similar issues in other buildings.

I also noted that one AP was definitely a bit too loud on 2.4 so I turned down the power. Now, looking at acsp neighbors the loudest I see is -67 on channel 6 on an AP two rooms away with that radio cranked down to 4. Nothing on 5ghz worse than -75.

Band steering is enabled, but all of these devices associate on 5ghz naturally. I see very little 2.4 activity.

So right now I have two clients associated. Both on wifi1.1. I cleared counters on wifi0 and wifi1 and let it go for 3 minutes. Then I show wifi0 _count and see:

0% rx retry rate

10833 rx CRC errors

85% rx CRC rate

And show int wifi1 _count:

3% rx retry rate

104 rx CRC errors

14% rx CRC rate

And now show int wifi0 and wifi1 show a summary state of "High Collision".

I'm baffled.

[samantha_lynn]

We do offer site surveys, which would help you adjust the network to the environment as much as possible. That is a paid service and you'd want to talk to our sales team if you're interested. If you'd like to send some pictures of a few typically placed APs, with as much of the surrounding environment shown as possible, to communityhelp@aerohive.com then I could let you know if I see any major problems and how we could work around them?

[tandrews1]

I have 265 access points spread across 15 buildings. There are definitely some RF challenges in some of them, but the building in this example is new construction. APs are all hanging from ceiling grid. Walls are mostly sheet rock on metal studs, but there are some block walls scattered around.

If environment is the issue, then I am probably going to have to live with this since I am hard-pressed to find an AP right now in this enterprise that isn't showing high collision on both interfaces.

I have read through the Chromebook suggestions and we disabled interstation traffic a while ago. We also disabled automatic updates or in some cases scattered update times for our chromebooks. We are avoiding DFS channels for now.

[sderikonja1]

Could you post a picture of the AP and surrounding?

[samantha_lynn]

The high CRC rate could be environmental, are there any sources of metal, glass, or water that the signal would need to pass through to reach the client devices?

Overall you are seeing that summary state because the CRC rate is over 10%. There are five possible states for the WiFI interfaces:  

Good: The CRC and Tx error rates are less than 5%, and channel utilization is less than 50%.  

Fair : The CRC and Tx error rates are less than 10%, and channel utilization is less than 65%.  

Channel RF Overcrowded: The channel utilization rate is 65% or greater.  

High Collision: The CRC error rate is 10% or greater.  

High Tx Error: The transmit error ration is more than 10%, which can occur when the RF environment is overcrowded and the collision rate is high.

Also you mentioned you have carts of Chromebooks in the network, have you read the guide regarding Chromebooks? They have a feature called peer to peer updating that can cause some significant broadcast increases, which might explain some of the packet damage you are seeing. The guide I'm referring to can be found here- https://thehivecommunity.aerohive.com/s/article/Chromebooks-and-WiFi