Locating interference to WIFI network

Well, part of the thing is to be able to confirm that it IS something outside of his apartment interfering. If it happens to be something in his apartment, which we didn't think about, he could control it... Otherwise, I'll tell him to put up RF shielding wallpaper or paint :-) I don't think he'll go to 802.11a at this point at least.

Yeah, unfortunately, even if he knew what the problem was, hemiht not be able to fix it. Rather difficult to ask your neighbors not to use their microwave, cordless phones, etc, eh?

You could also have networks with SSID broadcast turned off interfering. I've seen them with the Auditor Security Collection, but if, as you say, it's intermittent, it may be hard to find.

Is his apartment small enough for him to convert to 802.11a? AFAICT that won't penetrate walls, so he shouldn't have any interference issues...

Could you offer some clue as to how often is "sometimes". Getting dropped a few times a day is normal in an RF congested environment. I know one network that goes down around lunch time due to the large number of leaky microwave ovens operating in a nearby cafeteria.

What channel did he pick? The non-overlapping channels are 1, 6, and

1. If you pick a channel in between such as 3, you will get interference from users on *BOTH* channels 1 and 6. Stay with 1, 6, and 11 even if occupied.

An AP on the same channel will not cause a connection to drop. It may slow down, or become erratic, but it will not disconnect. Wi-Fi and spread spectrum is fairly resistant to co-channel interference. Incidentally, 802.11g is far more resistant to disconnects than

802.11b.

Disconnects are usually caused by non-802.11 type of interference such as cordless phones, microwave ovents, 2.4GHz security cameras and other sources of 2.4Ghz junk that tend to be continuously transmitting. Bluetooth will cause disconnects, but only if located almost on top of the access point or client radio.

Ouch. See:

$1500 for the YellowJacket. Be sure to view the video clips at the bottom of:

see how it looks. What you'll find is that it's NOT easy to identify the exact source of interference with one of these.

Borrow or rent something. The Proxim 7400 based system is VERY slow and insensitive. I use several but have largely given up in favor of using an MMDS downconverter and a portable (borrowed) spectrum analyzer.

Not really. It's best to do this by example. Put the device near a known source of interference and see what the display looks like. Even so, there is a considerable learning curve and all too much guesswork. I can do it with a decent spectrum analyzer, but even with years of experience, I'm still guessing all too often. There's also the problem of locating the source of interference. Big directional dish antennas are available, but reflections make using them also an art. I was once trying to locate an 802.11 user that had an obvious high power amplifier that was causing considerable interference. The signal was so strong that I spent most of the day chaseing reflections instead of the source.

I'm not exactly sure... In the three or so hours people were in front of the computer, it happened twice.

Yah, I moved him from 11 (which had a a nearby AP, which was getting received as 85% signal strength) to channel 1, which had an AP which is only sometimes detectable, and comes in at around 60%.

Would I be better on a channel with a stronger competing signal which is always there -- or a weak signal which the PC only sometimes picks up? That is, does the WIFI 'learn' how to get around the stronger signal... but get 'interrupted' by a signal which comes and goes?

But, it would take out alot of the guesswork and the trial and error... But, I'm still not paying $5000, unless I'd be doing it professionally.

Yep, I saw those at Interop as well.... It's still not worth $1500 for this purpose...

Any idea where to go to rent something like that? Again, since it's has a large learning curve, it may not even be too useful for the amount of time I'd be renting it.

In the meantime, I ordered one of those FHSS cards, to see if it helps at all...

I've seen some presentations on the web, also... hopefully they'll be of some help.

I'm also an SWLer... and I know that there are hobbiests (probably mostly HAMs) who go on signal hunting (not WIFI signals) trips for fun...

Thanks again...

Ok. About once an hour or more. That's not unusual. I would guess(tm) a microwave oven. It's not another wireless LAN because they tend to get used for longer periods of time. I can deduce quite a bit from the interference pattern and timing. Anything else you can supply such as the duration of the outage, whether it constitutes a disconnect or a loss in download speed, and whether it coincides with commercial breaks on TV.

The problem is always the hidden access points that don't broadcast their SSID. Your access point search utility will not show these. I suggest you download and run one of the Linux LiveCD's that has Kismet on it and search for other sources of 802.11 interference.

won't show non-802.11 interference but at least you can do an effective job of selecting the channel.

Good questions. Spread spectrum shows up as valid data that can be demodulated. The listen before talk system will wait for an empty time slot and interleave the transmit signals. I've run about 12 access points simultaneously in the same room copying files furiously without disconnects. Traffic did slow down considerably but never stopped.

The only difference between interference from an on channel SS signal and one on the adjacent channel is the signal levels. The demodulation process is not synchronized with the carrier so it really doesn't matter what frequency (channel) the interference is on.

Therefore, the channel weakest interference source is the best. What most people don't bother to do is check the channels on either side of the selected channel. A strong signal one or two channels on either side of the selected channel can create almost as much interference as a signal on the selected channel. It's not enough to just check channels 1, 6, and 11. You also have to check the nearby channels.

You can sometimes "see" interference if your client radio has a "noise" level indicator. It won't tell you anything other than there's some source of noise nearby, but it does help select the channel. Pick the one with the lowest "noise" level.

Sorta. Recent tweaks to Part 15.247 allow limited adaptive frequency selection. Many new access points have an automatic setting instead of a fixed channel selection (Netgear AutoCell).

they find the "best" channel, usually by just looking for the lowest noise level and possibly other users. When they select a new channel, the client radios will follow. This feature sounds good, but I'm not convinced it does anything more than present a moving target for users trying to find an optimum channel selection. I haven't played with AutoCell as it requires an additional license.

Find someone that has one and borrow it. I've thought of buying one and renting it to the local hacks. There seems to be a market.

Nope. What country are you in? What city are you in? There should be a local test equipment rental house in all the major cities. The problem is they tend to charge 10-15% of the purchase price per month for rentals. I don't think you'll like the cost.

Find the local WISP (Wireless Internet Service Provider) and see if they have some test gear. Same with the local commercial 2-way radio shop. If the municipal radio shop is friendly, they might have something useful. I've had best luck with the few radio station engineers that actually work on the hardware. They always seem to have decent test equipment. They may not loan it to you, but you might be able to bribe them into doing the sniffing (site survey) for you. Last resort are the local hams:

me, they tend to have workable junk, some expertise, and a willingness to help.

Here's a PowerPoint presentation on the card from the North Texas Microwave Society:

$20%20WIFI%20SA.zipI don't know how they got the signal strength up that high as shown on the "snoop" program display. I have 6 cards and none of them ever show anything stronger than perhaps half scale. Maybe I have 6 dead cards?

I used to do that all the time on VHF long ago. We still have transmitter hunts locally, usually when someone has their transmitter stuck on the air and we have to go find the culprit. I've been (slowly) writing an article on how to do direction finding using the audio delay through a repeater.

Note that the Proxim 7400 driver does not work with XP, but works well with previous Windoze mutations.

Yah, I knew that some software displayed the SSID for those who don't broadcast it, I just didn't know which. (Which is the reason why they say that not broadcasting SSID, doesn't really help security much.)

The software I was using showed WIFI activity in all channels.

Hopefully I'll be able to google for one of those in the area...

My radio engineer contacts are about 4000 miles away :-) I'll try to contact some Ham/SWL people I deal with and see if they have any ideas (since none are local - even if they have the equipment. My relative is in Southern CT and I'm in the greater NYC area.

Yep, I know it's not XP compatible...

Thanks again! I'll try to check up on the possibilities you mentioned....

I forgot that I had made an AVI file of what mine looks like scanning for my access point about 10 feet away.

(2.5MB) My appologies for the otto focus, shakey picture (no tripod), and oversize video clip.

Well, I cheated a bit. I also have a Symbol PPT4340 which has essentially the same card inside and came with similar software. It too is deaf and slow. However, I added a 2.4Ghz RF amplifier and an antenna connector, making it somewhat more useful. What bothers me is that my signal levels never seem to go above half scale on all my cards and laptops that I've tried running it. Even with an RF amplifier and right next to the access point, it only goes half scale. Yet, the photos on various web pages show almost full scale. Either I have 5 broken cards, or I'm doing something wrong.

That depends on what you consider expensive. I've been tempted to buy something useful and rent it to my competitors. There's definitely a need for interference identification. What I've been doing is using a modified MMDS down-converter and a borrowed Tektronix low frequency spectrum analyzer. Some of the Pacific Monolithics and Conifer dish antennas came with MMDS down-converters built into the antenna feed.

of destroying the MMDS converter and replacing the feed as shown above, I rip out the 2.4GHz filter, supply power through the coax, and watch the spectra on a lower frequency spectrum analyzer. 2.400 to 2.485 Ghz, the IF appears from 122 to 207Mhz, which is easily viewable on a cheaper spectrum analyzer.

The MMDS downconverter is currently mounted behind a dish which often lives on top of a 30ft telescoping fiberglass pole used for site surveys. When I do another one, I'll try to remember to take photos before I get arrested for being a theat to homeland security by walking around with such a contrivance.

[Jeff wrote]

Yeah, fortunately, I discovered an old Win2K laptop when my card came in. Unfortunately, I discovered that Jeff's right (imagine my surprise!), and it's slow enough to be essentially unusable, and deaf enough to only see APs right next to it. As a spectrum analyzer to be used as an interference location tool it's not very useful.

A real spectrum analyzer with decent speeds and high sensitivity is, as we're seeing, a very expensive piece of electronics, and even in trained hands can only point out that there's interference. Doing something about it in the 2.4GHz band is another rathole...