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Interference from unknown source?

I'm trying to help a friend solve a strange wifi problem.

The target room for wifi usage is across a small corridor from the room where the internet connection & router resides. The wifi connection allegedly worked before installation of a cable TV system. Other electronics might have been introduced around the same time frame to cause wifi malfunction, but it's purely speculation on my part as I do not live there and my source is fuzzy.

In my short test, I placed a laptop right next to the router ("Cnet" brand, model number unknown) and tried to establish a wireless connection. Wireless connection was detected and then disappeared. It would be detected and then disappear repeatedly. Then I tried with my Asus pocket wifi router, which has been working fine in my own apt. Same setup, router placed right next to the laptop. But I could not establish any wireless connection to the router at all. Both routers worked fine with wired connection in this location. The laptops used in both tests were a Sony Vaio with buit-in wifi and a Dell with a PCMCIA Asus WL-100b card.

Do interference usually just 'wipe out' a wifi device entirely, like my Asus router in this occasion? Could the cause be something else all together? How do I go about diagnosing the source of interference? Any help or pointers will be greatly appreciated.

Reply to
BLT
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One wireless problem that crops up routinely in new connections is to have 802.11x authentication checked on the advanced tab. This will cause the connection to repeatedly connect and disconnect.

Q
Reply to
Quaoar

Might cause it to loose connection or drastically reduce performance.

The best tool to find the source of interference is a frequency spectrum analyser but they're not your typical thing to have lying around. Airmagnet software can help has it has a channel scan feature which displays bar graphs of signal strength and noise (amongst other things) across all the wifi channels. Nowhere near as good as a spectrum analyser but you can get a good idea. The other problem here is that Airmagnet is not cheap.

You could get a similar result by using Netstumbler and changing the channel on the AP starting at 1 and going all the way up and see what the noise is on each channel. The only problem here is that you need the right wireless card for netstumbler to report noise so you might be scuppered there too.

Typical sources of problems are 2.4GHz cordless phones, 2.4GHz video senders, wireless cameras etc. Microwave ovens too but i've never had a problem with one of those and from what you describe, your problem is permanent?

I have a video sender which I had to set to it's "channel 4" which puts it at the top end of the WiFi channel range and on my AP choose channel

  1. Pretty much anything else and the wireless network gets killed instantly to the extent that Airmagnet sees this as an "RF Jamming Attack". :)

Short of using a highly directional antenna and repeating the search for noise experiment, in a domestic environment, it's not that easy to always pinpoint the cause of the problem.

David.

Reply to
David Taylor

The problem is permanent. There is a surveillance system in the household with the surveillance monitor in the corridor mentioned in previous post. Have to check with my friend if they upgraded and put in new video senders/cameras.

So I can toy with switching the channel of the router to see if I can get out of the interfering spectrum? (Yes, I'm a wifi newbie.)

I will play with Netstumbler next time I go to my friend's place. It won't be soon cuz we live quite far apart. Won't be able to come right back to this thread with new findings... Just want to get some info to see how I can tackle this problem.

Thanks for the help. :-)

Reply to
BLT

Are any of these surveillance cameras wireless? I had to deal with an interference problem caused by a series of wireless TV cameras from a security services dealer. The cameras transmit continuously on 2.4GHz and created considerable interference.

Netstumbler will not show non-802.11 interference. You'll need some sort of a spectrum analyzer to do that. I have some recommended hardware to beg, buy, borrow, or build if you want to try this.

You might also want to verify that the router and wireless computer are still functional in an area where there's no apparent interference. Drag the router and computers off-site to a known clean location and verify that they can still communicate without dropouts.

I don't think that CATV leakage can cause much interference at 2.4GHz unless there's considerable harmonic content, far too much power delivered by the distribution amplifier, and unterminated connections. However, if it is leakage, you should be able to "see" it on a portable TV. Borrow an LCD TV and tune it to one of the higher cable (not broadcast) channels. Then walk around and see if there's any leakage. That won't prove that it's the source, but if the cable is leaking badly, it might be sufficient justification to get the installer of the CATV system involved in the troubleshooting.

Reply to
Jeff Liebermann

Yes you can, also play with the wireless video sender channels if they are in use. Like I said, you might just get choices of 4 channels which don't directly corrolate to the channels in use on the wifi network.

Just bear in mind what I said about the noise issue in Netstumbler. If your card is using the NDIS support in netstumbler then I think you'll find that you just get a flatline for the noise value as the NDIS drivers have a habit of not reporting noise to the higher layers.

David.

Reply to
David Taylor

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