We have a Zyxel Wifi with an NXC2500 and two WAC6503D-S access points in our office. Around 30 devices are connected. We have both a 2,4GHz and a 5GHz radio active, both on channels without known interference.

We often experience bad Wifi performance, which means less than 1MBit/s for some minutes, while the shown link speed is > 800MBit/s. Stations connected by cable don't have any problems.

It is a sporadic problem, therefore hard to debug. We already checked the signal strength of the stations, which is fine. We tried diffent channels without any impovement.

To trace the problem, I setup two other access points (both Netgear, different models) on free channels and connected them directly to our modem. Interestingly, also there the performance is not satisfying. On the 2,4GHz band (CH1), the throughput is only around 2MBit/s. On 5GHz it's 80MBit/s (which is fine) most of the time, but sometimes drops to 30MBit/s too, even if there shouldn't be any other device sending on this channel and the access point is located only 1m away.

I would be interested to know how I can get further here. Are there any devices which can detect interferences? Or programs which show if there are problems with the connection (like dropped packets etc)? Any other ideas?

Thanks for any help, I'm slowly out of ideas.

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4 Answers 4


I would definitely recommend doing a site survey. There are many resources to do signal strength and interference surveys, but they won't reveal any non-wifi sources of interference that may be around. To see non-wifi sources of interference, you'll need a spectrum analyzer, which can be pretty pricey. I think MetaGeek's Chanalyzer is the most cost effective option from a reputable source, but I have never used it myself.

There are free phone apps that allow you to see what other networks are around, and what their signal strength is. This is a very basic option which doesn't really provide a whole lot of insight, but is better than nothing, and can tell you what channels are already in use by neighboring offices.

General configuration recommendations:

  • Limit SSIDs to 3 or fewer: More SSIDs mean more broadcast beacons, which take away airtime for client use. Depending on how heavily you use your wifi, you can get away with more than 3 without any issues, but I generally recommend 3 or fewer if you can help it.
  • Utilize Band Steering: This encourages clients to use the 5GHz band instead of the over-crowded and slower 2.4GHz band. Keep in mind though, this only encourages it, it does not force it. So if a client is stubborn and wants to use 2.4GHz, or is only capable of 2.4GHz, then it will still use 2.4GHz.
  • Utilize 20MHz or 40MHz channels in the 5GHz band, and only 20MHz in the 2.4GHz band. Also ensure only channels 1, 6, or 11 are used in the 2.4GHz bnad: The larger the channel widths the more likely someone else will be using an interfering channel and cause issues for you. Only use larger channel widths in the 5 GHz band since their are more channels available there. The 2.4GHz band only has three non-overlapping channels (1, 6, and 11), so channel bonding (using more than 20MHz channel widths) should never be used.
  • Ensure your client devices support the channels you're using: Some clients won't support the DFS channels in the 5GHz band. If you don't know what channels your clients support, then make sure you're only using the UNII-1 and UNII-3 channels (36,40,44,48,149,153,157,161) If clients don't support the 5GHz channels you're using, then they're fall back to 2.4GHz.
  • Ensure your switches aren't operating in half-duplex or at lower bandwidths: This sometimes happens when switchports are set for auto-negotation, the auto-negotiation fails and the ports are operating at much slower speeds than they should. If they're Gigabit ports, ensure they operating at full duplex and at Gigabit speeds, or at the fastest speed that both the switchport and AP are capable of. If auto-negotiation is not negotiating correctly, first try to shutdown the port and bring it back up. If that doesn't work, then statically set the speed and/or duplex settings appropriately.

A few observations:

  1. Fifteen to twenty devices on an access point is near the practical upper limit. You might consider adding more APs.

  2. Have you adjusted your access points to limit low-speed clients? If one client is far away and connects at a low speed, it slows everyone else down too.

  3. To detect additional interference, you will need some more equipment. Wifi analyzers and/or spectrum analyzers can be had for a few $100s.


The basic tips are exactly what you need:

  • survey your wireless location for neighbor SSIDs, their channels and traffic
  • survey your location for other sources of EM radiation in the 2.4 GHz band (5 GHz seems to be fine) - 2.4 GHz is shared with various other technology including Bluetooth, see WP:ISM band, and it's also influenced by devices like microwave ovens(!)
  • survey signal strength and signal-to-noise ratio for the client locations, especially when performance is bad

The ISM band is free for unlicensed use, so there's no guarantee that anything works reliably. In extreme you need to migrate your clients to 5 GHz which is much less loaded (but may have to yield to weather radar), or consider a wired solution.


Thanks for all the suggestions. We had a professional network engineer check our network, do some packet tracing etc. He didn't find the root cause either, but recommended to replace our Zyxel Wifi infrastructure with Ubiquity. We did that and didn't have any problems since then.

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