AP Reported channel utilization in Prime does not match reported channel utilization in Spectrum Expert

[patoberli]

Hello

I have a fairly dense Wireless network with an Accesspoint per room. The 3502 models are connected to WiSM with Software 7.0.240.0 and are managed with Prime Infrastructure 2.2 (the issue was also in older PI versions).

PI reports on the 802.11b/g/n interface a channel utilization of 32% with a RX and TX utilization of 0% (no clients are connected to this AP). If I open this AP now in Spectrum Expert (thanks to CleanAir), I have a channel utilization of less than 5%, no interference power at all. Also the Swept Spectrometer is only showing signal with less than -85dbi (with a few very rare higher powered specks).

So which value is now correct, or do they show something different?

Thanks,
Patrick

[George Stefanick]

Hello

 CLEANAIR looks at physical layer, layer 1. This is RF itself.

AP CHANNEL UTILIZATION looks at the data layer, layer 2. 

Why the difference. Layer 2 uses CSMA CA so the ap radio calculates how often the radio is busy and can't send. It then formulates this to a percentage you see. Clean Air is lookimg at raw energy. They are her ally always different. Remember when a radio send a packet all clients on channel need to list for it. 

Make sense ? 

[patoberli]

I think I understand what you wrote. Just to confirm, that means the AP hears other APs/Client communication on it's channel at 32% of the time and will not send/receive in that time?

Would that mean that the neighbor AP on this channel would need it's signal lowered so far, that the affected AP can't hear it anymore at all, or is there some signal strength after which it ignores the other talking AP?

[George Stefanick]

This is very important to understand. I should actually do a blog post on the subjey, it comes up often. 

Think like a wifi radio. You can only do 3 things: send, receive, listen. And you can only do one of these at a time. While in listen mode, you hear other traffic on your channel within range. In send mode you tx data on the channel. In receive mode you Rx data on the channel. Based on these functions. You as an AP can determine how often the medium (channel) is busy or not. 

Lets go a bit deeper. Before a frame is sent a preamble is triggered to sync all clients on the channel. These client do not know yet if the frame is for them or not. Then the phy header comes telling clients how lomg/time the frame will take to send. Then the actual frame gets punched and all the clients receive it. The client brimg it up from the PMD layer to the data layer. Look at the Mac address and if its for them they process the frame. If it's not for them they drop it. But before they drop it they peek at the Nav timer in the duration / ID field and back off again. This time for the SIFS and ACK. This busy time calculates the number you are seeing .. Better ? 

I normally dont don't worry about channel utilization till 40% under load. If you are at 40 without load I bet you habe 1 PHY mandatory .. Do you ? 

[patoberli]

Thanks for the explanation!

What do you mean with "you habe 1 PHY mandatory "

[George Stefanick]

Your PHY rates 1,3 5.5, 11 ect 

this is can be found under wireless tab and then wireless networks for 2.4 bgn .. Screen shoot that page and paste here 

[patoberli]

Ah no, those are disabled. I've tuned my network based on the Cisco High Density recommendations.

I've disabled 1, 2, 5.5, 11. First mandatory is 18 Mbit/s.

[George Stefanick · Accepted solution]

Pat,

Not being there and poking around its hard to say. But let me mention a few items that could get you started on your discovery. If the lowest is 18 PHY then co-channel utilization due to beacons can probably be dismissed. 

This makes me think:

1 - You have load on the ap when you seen the high utilization

2 - You have 2 or more access points on the same channel causing co-channel interference. Make sure you have -20 dBm different between same channel aps.

3 - You may have a bad radio (ap / client) causing interference

4 - You have clients sending long NAVs

Some next steps, packet captures on channel next to the ap and see whats in the air.

Hope this helps .. Thanks again for supporting the rating system.

[patoberli]

Ok, I guess then it's the co-channel interference (even though I tuned RRM a bit to make the cells smaller) and also probably loud on an other AP close by. Luckily I only have a few rare user complaints so far.

Also there is some Bluetooth from time to time, which might also cause a tad higher "channel utilization".

Also the 5 GHz seems to do absolutely fine without any complaints.

[George Stefanick]

Keep in mind ..

Interference on wifi has 2 meanings 

RAW energy interference meaning duty on a channel. WiFi clients will react to this by not sending frames because their radio will go into the busy state.

Channel Contention is another interference. Frankly I don't think it should be called interference, its contention. Your spectrum analyzer will not show this. In fact it may not show much of anything in some cases. Because the contention is happening on layer 2, meaning backoff timers ..

Make sense?

[patoberli]

Found one of the main reasons for it!

We recently got a new building which we supplied with access points (2702i). Some of the APs show a >90% channel utilization, although nobody and no computer is in the building. This was VERY weird :)

We also have an inhouse UMTS and LTE installation. As it happens, Swisscom (mobile provider) is using for UMTS a 2,1 GHz band. In some rooms, the ones with the highest channel utilization, the AP is mounted around 30 cm directly below the Swisscom antenna. A Swisscom mobile engineer confirmed to me, that this issue can happen if the AP is within 2 meters of the antenna, because the mobile signal spread is +- 400 MHz around the 2,1 GHz.

This means I will need to move my accesspoints by around 2 meters and the issue should be resolved.

This information is for others who might also register this problem.

[patoberli]

I found now one AP with a CU of currently 85%. It still had this much after a reload.

Then I went into that room with my Wi-Spy and Chanalyzer and let it run around a meter besides that AP. Chanalyzer showed a <2% CU. I tend to trust Wi-Spy, as there were no clients in the vicinity. It did look like there was a little bit of Bluetooth, but it was more than 20 dBm away and also only tiny little specks in Chanalyzer.

Might this AP be broken?

I will test it tomorrow with a replacement one and let you know.

[patoberli]

Finally found the time to sniff there. Indeed it looks really busy there, I simply haven't yet figured out the source and destination.

Wireshark shows some 3748 packets in 6 seconds on that channel.