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.

> Do interference usually just 'wipe out' a wifi device entirely, like my

Might cause it to loose connection or drastically reduce performance.

> Asus router in this occasion? Could the cause be something else all
> together? How do I go about diagnosing the source of interference? Any

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.

BLT wrote:
> 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.

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

David Taylor wrote:
> 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?
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.

>
> 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".
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. :-)

> 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.)

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.

> 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.

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.

On 27 Sep 2005 08:09:46 -0700, "BLT" <(E-Mail Removed)> wrote:

>David Taylor wrote:
>> 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?

>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.

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.

>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.

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.


--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558