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I am using a Netgear WNR3500L router to share my internet connection via WiFi to my home computers. My trouble is that I get periodic dips in wireless speeds every 120 seconds (almost exactly every 120 seconds, very regular). I have measured these dips on both a laptop running XP and a stationary computer running ubuntu 11.04. On the laptop (which seems to have a better wifi receiver and therefore better connection) the dips are very brief and not really noticeable unless you continually measure network speeds. On the stationary computer, the dips are much more noticeable, more or less dropping connectivity completely for 5-10 seconds. Plugging in a ethernet cable to the router removes the problem, so the culprit is definitely the wireless.

The router is flashed with Tomato Firmware v1.28.7476 MIPSR2-Toastman-RT K26 USB Ext which seems to work very well for everything else. Does anyone have any suggestions on specific settings to try with Tomato? I would rather not go through the hassle of going back to the original Netgear firmware or DD-WRT if this is some known solvable problem.

Update:
The problem persists even after flashing back to DD-WRT, so it is not Tomato specific.

Update 2:
I just changed my WiFi dongle to a newer and better one. The problem is still there, but now the dips are very shortlived, so they are not really noticeable unless you graph the bandwidth usage. Will try to do some Wireshark sniffing and see if I can find out anything else.

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I'm always suspicious of wireless interference. But I don't know of a good way to really really test that. –  Doc Jul 6 '11 at 19:03
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Try changing channels on the access point and dropping your client's roaming aggressiveness setting. My bet is that it's channel scanning, looking for a better access point. –  David Schwartz May 16 '12 at 10:39
    
Thanks for the suggestion! I've tried switching channels, but it didn't really change anything. Will look into the roaming settings. –  Leo May 16 '12 at 12:40

3 Answers 3

up vote 3 down vote accepted

Nothing is wrong with your router. Your laptop periodically scans for Wi-Fi networks. The process involves switching to all 11 channels and sending a probe request and waiting for any potential probe response. That's why it significantly drops the throughout. I have this problem in my wireless experiment. If you use Wireshark you will see several probe requests exactly at the time the performance drops.

I should add that I modified the driver to stop periodic scan. The result was constant throughput with no few-second performance degradation. I use ath9k driver in Ubuntu. I noticed this problem during our experiments in ideal environment where we except to see no interference; however almost every minute the throughput decreases by about 7 Mbps for 4 to 5 seconds.

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Thanks, this seems plausible. Will have to experiment some more with wireshark and see if I can see the probe requests. –  Leo May 16 '12 at 12:41

Regularly-spaced periodic dips in Wi-Fi throughput could be due to some periodic interferer, but are much more likely to be due to the client periodically going off-channel to scan.

Periodic scans can be done for several different reasons, including, but not limited to:

  1. Your client could be looking for a better AP to roam to. Clients are more likely to do this when the signal they're getting from the current AP is below a threshold. Moving your client and AP closer together could fix his, or adding more APs to your network so you have better coverage in the area with the client is.
  2. Your client could be running a Wi-Fi scanning tool like NetStumbler. Find that tool and quit it. If your OS has a window where is shows wireless networks in range, be sure not to leave that window open. Besides apps, don't forget widgets/gadgets (Dashboard, SideShow, Yahoo! Widgets, etc.). Maybe you'd downloaded and installed a Wi-Fi scanning widget a long time ago and didn't realize it's been running in the background.
  3. Your client could be doing Wi-Fi based geolocation. If you know of a process that could be constantly looking up geolocation data, tell it to stop.
  4. Your AP could be looking for a cleaner channel to switch to. Consider disabling that feature of your AP, if you're already pretty sure it's on a good enough channel.

I'd fire up Wireshark (on another machine so it doesn't change the behavior) and do an 802.11 monitor mode packet capture and see if I can see anything different going on at the times the dips happen. I'd especially look for bursts of Probe Requests and Probe Responses, on the theory that the client or the AP is doing a scan every 2 minutes.

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Thanks for the suggestions, will look into doing some packet capturing. The fact that the dips are present when using a second computer running windows and with another wifi card seems to suggest that the problem is with the AP and not the client, but I have always found wifi hard to debug. Will look into getting another usb wifi dongle for my ubuntu box in any case. –  Leo Jul 6 '11 at 21:53

Wireless devices run at 2.4Ghz in the frequency spectrum - which is pretty good for data transfer but prone to interference from devices that are legally licensed to use the same frequency allocation, including but not limited to DECT cordless phones, microwaves, video senders, car alarms and other wireless networks.

Wikipedia has a great article about it here: http://en.wikipedia.org/wiki/Electromagnetic_interference_at_2.4_GHz

You could experiment with trying other wireless channels to see if you can get a more stable performance from them.

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+1 My microwave used to kill my wireless, as well as my 2.4 GHz phone. I actually decided to "move" the phone, and bought a 5.8 Ghz model. It has been great since. The microwave was solved by moving and getting a better shielded microwave oven. –  KCotreau Jul 6 '11 at 19:26
    
Thanks for the suggestion. I cannot think of anything except other wifi networks present that could interfere with my network, and I have tried changing channels without any effect. So probably an AP or wifi card problem. –  Leo Jul 6 '11 at 21:55

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