Multipath WLAN problem?

Hello Newsgroup,

After moving the office one of the things that doesn't work reliably anymore (besides a dead printer and a dead Tektronix) is the WLAN. It's a Linksys BEFW11S4 router with the routing parts disabled, to act only as a wireless AP. Worked well for all those years.

Long story short I can achieve good to excellent signal strength in the living area. However, while the connection via this WLAN point always works just fine in the office it is very erratic in the living area of the house. Frequent Internet access time-outs despite great signal strength. We have insulation in all exterior and interior walls, unfortunately aluminum backed.

Tried a parabolic reflector on one of the diversity antennas. Even more signal, but same problem. Can't put it on both because that kills access to part of the office. Any ideas?

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Regards, Joerg 

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Joerg
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CAT-5 ?:-)

Actually sounds like multi-path.

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Jim Thompson

On a sunny day (Thu, 19 Feb 2009 12:04:31 -0800) it happened Joerg wrote in :

Just some idea, with al those things not working, did you ever check the mains voltage? And scope it?

Reply to
Jan Panteltje

  1. Could be configuration problem. Check the box "reconnect automatically" in the wireless network setup. Disable Wireless-N, stick with wireless-G.
  2. Interferrence with some other equipment at 2.4GHz. Cordless phones, wireless headsets, etc.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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Vladimir Vassilevsky

Yep, 121.6V, quite sinusoidal, plus it's all protected.

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Joerg

Possibility 1 - might be a noise problem. Do you have any other devices in the area using the 2.4-gig ISM band? Wireless phones or baby monitors, microwave ovens, that sort of thing?

Possibility 2 - might be a multipath problem. 802.11b is rather sensitive to multipath, and your aluminum-foil-backed insulation is probably creating a very reflection-rich environment. Simply moving your body around, or moving the laptop/PC an inch or so, or having someone walk around in the room, might be altering the pattern of cancellations and reinforcements enough to cause signal dropouts.

Solutions might be:

- Upgrade to an 802.11g AP or router - the OFDM modulation is reportedly less sensitive to multipath cancellation than the DSS modulation used in 802.11b.

- Run some wire, and either move you AP to a more central location or add a second on a non-overlapping channel.

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Dave Platt                                    AE6EO
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Dave Platt

It's very difficult to access this router because it sits behind another router. So dynamic IP :-(

Anyhow, it doesn't offer much in terms of settings, basically you can select whether you want to use only one antenna or do diversity on both (which I have to). It works fine everywhere except in the living space where the signal has to cross two walls with aluminum-foil fiber insulation in them.

That's what I had scoped out really well when installing it. Channel 2 was best and still is, nothing else going on on this channel.

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Joerg

It's planned. That area is the only one where I hadn't strung CAT-5 yet. Made myself an access hole last year so now I can get there. But not before spring, right now it's ugly down there. But: I want to be able to just schlepp the little netbook to the living quarters so my layouter can ping me with a new round of Gerbers when he does one of those urgent jobs.

Yep, just like this dreaded ATSC we'll get in June :-(

I wish WLAN had an option in the protocol where you could set it to 300k or so in order to ruggedize a connection. But it's always full bore.

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On a sunny day (Thu, 19 Feb 2009 12:32:35 -0800) it happened Joerg wrote in :

I ask this, because I once had also very strange intermittent problems, and could not find it, until I noticed that it appeared every time heavy traffic drove by, so vibration of the house. Scoped the mains, and there were these impulses every now and then. Went looking found the mains fuse that was not turned in strong enough (one of those white ceramics ones), and it had formed a burned black contact. Removed it, cleaned it, and the fuse holder, and problem gone. The vibration of the traffic made that contact work like a microphone.

Such an intermittent contact can reset the processors in printers and other equipment, while most of the time a voltmeter would show a correct voltage, you just need to scope long enough.

Reply to
Jan Panteltje

Well, the Brother MFC printer isn't intermittent. Other than a backlit LCD it's dead as a door knob. The Tektronix only emits intermittent hissing sounds.

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Regards, Joerg

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Joerg

Noise on channel 2 is quite low.

That's what I am afraid is taking place :-(

The strange thing is that while it works it does so at top speed. When it quits it's totally dead for many seconds until it catches again.

Yes, but probably would require also upgrading some of the computers but is an option. One problem is that "modern" computers are dumbed down. All the "diagnostics" you get is five bars. No dBm levels, packet statistics and such, much better in the good old days. OTOH it's even worse with "modern" TV sets. All the diagnstics you get there is a picture or a blue screen.

The 2nd AP may actually be a great idea. Thanks. Only thing is that the 'puters would have to be trained to switch when packet losses mount. Which is probably something a "modern" OS cannot deliver.

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Joerg

Did you check the full width of this channel? 802.11b channels overlap, a lot. Channel 2 would conflict with anything on Channel 1 (or with devices below that frequency by several MHz) and with anything on channels 3 through 6.

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Dave Platt                                    AE6EO
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Dave Platt

802.11b does support multiple data rates, If I recall correctly, 1 Mb and 2 Mb rates use a modulation which is backwards-compatible with the original 802.11 DSS system, while higher rates (there are several) use a different modulation.

Whether any given 802.11b chipset firmware provides the ability to specify an upper rate limit, and whether any given vendor's drivers expose that to the UI, is another issue.

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Dave Platt                                    AE6EO
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Dave Platt

That's normally done by the chipset, its firmware, and the driver.

You'd set up the two APs on the same Ethernet segment, with the same ESSID, on non-overlapping channels. When the PC scans and looks for an AP for the ESSID it has been told to use, it'll normally try to associate with the one having the stronger signal... which would be your living-space AP if you're in that area.

There's sometimes a "sensitivity" adjustment (in the AP admin settings) which can be used to give PCs a hint of how far apart the different APs are, and thus at what point the PC card/firmware/driver should decide to drop its current AP association and re-scan and try again with a different AP.

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Dave Platt                                    AE6EO
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Dave Platt

Double NATing ?

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                     Baron.
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Baron

Well, there is something (weak) operating on Ch1, and stuff on Ch6. Not much choice. Unless I'd pull the main breaker on some neighbor's houses ;-)

Same scenario as years ago and Ch2 turned out to provide the best SNR.

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Joerg

Essentially the router part is disabled. It only acts as a hub, with one port being wireless.

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Joerg

On a sunny day (Thu, 19 Feb 2009 13:13:57 -0800) it happened Joerg wrote in :

Really that looks like a reboot, my Linksys does the same when I very shortly flip the power. Mine runs Linux, and it simple needs some time to reboot. Bad things on your main may have killed you printer and scope too. I see a correlation.

I do not believe in the multipath theory.

Reply to
Jan Panteltje

However, that won't fix this problem. Strong signals don't mean much in multipath. Same with the TV where often an analog channel comes through strong and clear but their ATSC says "no signal".

It always connects alright and the signal is strong but then randomly cuts out anyhow, for about 30-60secs at a time. Very annoying. I tried to place the AP in different rooms but since they are all heavily insulated the problem is more or less always there. RF bouncing back and forth I guess. An AP smack dab in the living room would not likely pass the inhouse permit process, SWMBO would loudly object ;-)

I wish they'd look for data integrity instead. In multipath you can have a strong signal with lousy data integrity and a weak one that's excellent. For example, the local Ch29 digital almost has the field strength to light a CFL in the box but the picture falls apart all the time. A few Bay Area channels 100 miles away are weak but work most of the time. Of course, TVs don't have any data integrity indicators either so it's a wild guess whether you can watch a movie to the end or not.

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Joerg

This stuff is heavily filtered here. The wallwart of the Linksys puts out its regular 17V. Says 12V but it's always been that way, I guess it is unregulated. I monitored it during drop-outs and the regular beacon activity continues. That would not happen if it rebooted.

You live in a country with no hills. The highest one you have is >100km away and only 322.5 meters high. I know, since I lived on the north slope of it.

VERY different out here. On analog TV you can see ghosting on the next line, not just a few cm down a line. Doesn't faze it but completely throws DTV off the rocker.

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Joerg

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