Wifi Spectrum analisys

[GoncaloC]

Hi All,

i have some users on one of my company sites that are complaining a lot about the WiFi network. I have noticed on 1 of the floors that a specific access point is report extremely high latency, however in client health the DB signal is above the 32 DB. Wich according to Meraki documentation the signal should above 25 DB for applications that use Voice and thus, i conclude that a 32 DB is not excellent but good, which i understand its ok. This users complain about poor video calls, and since the dashboard reports a good signal reception the only thing that came into my mind is that there might be interference. I have run a couple of spectrum analisys and i wonder how can i actually detect the precense of interference.

On these sample i do think that there might be interference on ch 140 since i only have 1 AP on that channel and it looks like there is non stoping transmition on that channel. however on channel 36 i can observe some gaps between the horizontal lines and there is no interference on this channel, right?

On these other sample i see some vertical lines on ch 112, 64, 48 which i do not understand what they could possibly mean. Could some one help me to understand it?

Both access point are on the 20 channel width and i dont understand is why on the first sample i see less channels than on the second. 

Regards

[BrandonS · Accepted solution]

The Db measurement you see in the dashboard is actually signal to noise ratio (SNR), but you are correct about the recommendation of 25Db for voice and 32Db being fine, but not great.

I agree channel 140 looks busy and would try to avoid using it.

Are you sure both of the units you pulled these waterfall graphs from are 20MHz?  Because I suspect they would look different for different channel widths.  That, or maybe it decides to show the nearest channels as opposed to trying to fit the whole range?  Or are they different model MR units perhaps?

EDIT: I noticed there is a scroll bar under the graphs so you can scroll left or right to see the rest of the 5GHz channels 🙂

[GoncaloC]

Hi, thanks for your reply. They are indeed different AP models the first one is a MR32 whilst the second is a MR52, probably this is the reason why the waterfalls are different. Regarding the channel width i double check and we use 20 Mghz across all our organization. This is somewhat the recommendation since we have more channels and this way we can avoid interference, i guess .. 🙂

Regarding the second sample i do not understand what are those continuous vertical lines, could they be from a device that is 24/7 transmiting signals, since they dont go way never.

BTW: the scroll bar only appears on the MR52

Regards

[BrandonS]

Strane. I have the scroll bar for my MR32. When using 5 GHz, it is recommended to use at least 40 MHz channel width, as some client devices may not prefer 5 GHz unless it offers a greater channel width than 2.4 GHz.  I would try changing just that and see if performance improves.

[GoncaloC]

Let me know your results..

[GIdenJoe]

32dB not great, are you kidding?  32dB is roughly a -60 dBm RSSI upstream from a client.  I find that to be pretty great if all your clients are at that level.

[PhilipDAth]

Some tips that will probably sort it out:

• Use an RF profile and force the use of 20Mhz channels.
• Set the minimum bit rate to 12Mb/s to stop users talking to APs too far away, and disable the use of legacy protocols that consume a lot of airtime.
• If you can, disable 2.4Ghz.  You will get a bad experience because of the poor amount of bandwidth.

[GoncaloC]

Hi, thanks for the reply and for sharing those tips, which are what we are doing exactly and yet i have seen some macbook laptop connected to the farthest access point. I though of increasing the minimum bit rate but i have the feeling that by doing so i will end up causing more problems than solving. 

On these site in particular that as 4 floors with 4 APs on each floor, apart from the dashboard showing that i have 659 interfering APs when i look at the spectogram and see transmitions on every 5 ghz channel the only thing that comes into my mind is interference. 

Its 10 in the eve, where i am from and there is no one in the office, a part from a couple of IOT devices which use 5ghz and i have this beatifull spectogram show transmitions on every channel.

Isn't this most likely a sign of interference?

[Bruce]

That just looks like background noise. Anything coloured blue is very low level transmission. Off the top of my head it transmissions from blue though, yellows, red, and to white I believe. But that 5GHz spectrum looks clean.

[GIdenJoe]

While the graph is handy to have as a quick check, indeed you'll only have alot of interference if you see alot of red in there.  Green specs is usage but still low duty cycles.

I might wanna think about bringing someone in with an Ekahau sidekick and do some checks for retransmissions when your call quality comes down.  Also you have pinpointed the issue to a specific floor so maybe go check there to see how your signal fares there.

Also try to determine if it is one client type experiencing the issue or all of them.  There can be a huge difference between wifi client chips.

[GoncaloC]

Thank you very much for all your feedback, it is really valuable input for me.