Help with poor Wifi performance and possible Wifi interference

[shawn-ev]

My company has an R&D facility in one of the southern US "desert" states. The facility is 20 minutes outside of the nearest town and any significant Wifi interference sources, yet we started experiencing poor Wifi performance. I've been troubleshooting and cannot identify the root cause. Let me explain our configuration.

We have 2 FortiAP-432F outdoor AP's, the first is the ROOT AP, it is mounted to a post on the side of the office trailer and connected to the main switch. The other AP is the LEAF AP, and it is mounted on a telephone pole across the site near some other trailers. It is connected in a MESH configuration with the ROOT AP. I am using the 2.4 GHz channel as the backhaul, and the 5 GHz channel for the devices. We have three Wifi networks, corporate, guest, and the MESH network. the MESH network is assigned to the 2.4 GHz radio, and the corp and guest networks are assigned to the 5GHz radio. The channels and power are set to auto.

 

The firewall sees both APs and is reporting them as all green.

 

The users started reporting poor performance and getting kicked off Teams meetings and general latency. They are all connected to the ROOT AP and are in the same trailer that has the AP mounted to its side. The trailer is wide open on the inside, the signal only has to penetrate the roof. While 5GHz penetrates through objects less than 2.4GHz, I would not expect this level of degradation.

 

I've tried using the Spectrum Analyzer but had trouble interpreting the readings. The "interference" seems to accumulate the longer you let the SpecAn run. Here are two screenshots, the first was taken about 10 seconds after launching the SpecAn, the second was taken about 40 seconds later. If I let it go, it just gets worse until you see a giant blob of red. How am I supposed to interpret these results?

 

 

Any advice would be much appreciated. Thank you for your time.

[Toshi_Esumi]

You mentioned "started experiencing poor wifi perfomance". Does it mean it was working fine before then suddenly the current problem started?

First impression with those images is the signal reception at the root AP from those clients are lower than I would expect. How about the opposite direction, reception at the client side? Have you measured it with an wifi analyzer app on a laptop or a smart phone? Is it also showing -70 dBm range?

If that's heavy steel container trailer, you wouldn't be able to expect much signal penetration. I would consider adding an less expensive indoor AP inside.

Once both AP receptions and client receptions come to -60 dBm range, I wouldn't worry about those interference at -80 dBm range.

But is your R&D dept. dealing with some kind of RADIO devices or environment research? And, that's why it's in the desert? Then, you probably know much better than at least me about wireless signals.

Toshi

[shawn-ev · Accepted solution]

This problem started after the crew returned from their holiday break at the beginning of last week. Prior to that, signal strength inside the trailer was strong.

The trailer is a typical wood-framed office trailer. The exterior is a very thin aluminum cladding. It is not a conex box or a heavy steel trailer.

They are not using any radios that could interfere; it's not that kind of R&D. 

Unfortunately, I am not on site, and I do not have anyone on site who can help. I must rely on the reports of users. They are telling me their Wifi signal strength -- as measured by looking at the icon in the system tray -- drops to 2 and 1 bars and they start having problems with Teams and latency. Everything I see on my end points to a solid infrastructure, but the Spectrum Analyzer results were confusing to me. I'm used to using a real spectrum analyzer that shows RF signals in the moment rather than aggregated over time.

I swapped the 2.4GHz and 5GHz networks. I'm using the 2.4 for the users and the 5 for the backhaul. I had to put the 5 GHz transmit power to 100% on each antenna, but it works now. Thank you.

[adambomb1219]

I would highly recommend hiring your preferred VAR/partner choice to perform an RF survey of the facility.

[jiahoong112]

Performing a wireless site survey would be highly recommended. As your signal is going through metal objects, that can cause excessive signal multipath and reflection that can degrade performance and cause a lot of wireless retransmissions. Aside from trying to identify interferers on the 5ghz, interferers on the 2.4ghz band are more prominent, so the spec-an for the 2.4ghz should be checked as well. 

Looking at you 5ghz spec-an, it looks normal.

Other than that, it seems like your FortiAP is "seeing" your client devices with a relatively low RSSI. This can also be a sign of asymmetric power levels whereby the txpower of the wireless AP is significantly higher than the txpower of the client device leaving you in a situation where the client device is able to 'see' the AP well but not the other way round. Refer: https://docs.fortinet.com/document/fortiap/7.4.2/fortiwifi-and-fortiap-configuration-guide/68434/sig... 

Lastly, aside from Signal Strength seen on the client device, Data Rates should be checked as well as they are the determinant factor for the quality of the connection. Wifi devices does something called rate shifting whereby if a certain data rate has too many retransmissions, then it will downgrade to a lower data rate in an attempt to reduce the retransmissions. 

[shawn-ev]

This is great information. Thank you. I have the power set to AUTO. I thought the power output would be adjusted automatically by the controller/firewall. I'm reducing the top power level.

[adambomb1219]

No, not necessarily.  This is why a professional site survey is so important.  Your consultant provider of choice should provide you recommended power level ranges based on that survey.  

[Toshi_Esumi]

Just be aware that reducing the tx power at the FAP obviously reduces the coverage area outdoor in case that FAP takes outdoor clients. If the root FAP doesn't take outdoor clients but only connects to the leaf FAP on a different RADIO, it wouldn't be your concern though.

The bottom line is it's still important to measure the receiving signal level on the client side then compare with those numbers on the FAP side in order to grasp the entire picture around the problems.

Toshi

[shawn-ev]

Yes, thank you. I am aware reducing the power levels reduces coverage area. 95% of the user are inside that same trailer. The other 5% are in the neighboring trailer. The AP is mounted between the two trailers. We don't have a need for a wide coverage outdoors except for the area covered by the LEAF AP. I'm looking into getting a site survey. Unfortunately, there is not a lot of professional IT infrastructure talent in that area, and the nearest Fortinet partner is a 2.5-hour drive. 

[Toshi_Esumi]

To me it doesn't have to be professional. I would ask somebody to download an app to his/her smatephone then get some screenshots at some specific spots and send them to me. That would provide good information to analyze. Much better than no info.