[TriLUG] solved: can't connect to wap when move it to alternate location

[Joseph Mack NA3T]

On Tue, 13 Apr 2010, Joseph Mack NA3T wrote:

> On Mon, 12 Apr 2010, Neil L. Little wrote:
>
>> Crap, they went out of business! Ok there was a place 
>> over on Yonkers road in Raleigh but I see now they went 
>> out of business last year.
>
> I did find a place that would rent a spectrum analyzer. 
> Only $1901/mo. At that price, I should buy one.

Story so far ...

Linksys wap works fine at home, take it to the friend's 
store, I can't connect to, or find the wap with netstumbler 
or iwconfig. Take the wap home it works fine again. Same 
thing with two other Linksys waps and another laptop 
(admittedly the same type of wifi card). There are two other 
waps in the vicinity I can pick up, which I assume are 
operating fine. Since my laptop can pick up other waps and 
not my waps, I assume the problem is with my wap(s) and not 
my laptop(s). If I disconnect the antennas to the wap, I can 
connect fine till I'm about 10' away. If I put paperclips in 
the antenna jacks, I can connect till I'm about 20' away. If 
I put the antennas back on, I can't connect at all.

The store had a 5.8GHz phone system, which I eliminated as 
the cause, by turning it off. My 900MHz phones at home had 
no effect on the wap. Pete Soper told me that some 5.8GHz 
phones use 2.45GHz for one of the directions, which would be 
in-band.

The waps were non-operational during the week, but when I 
returned to the store in the weekend, when I could safely 
mess with the network, the wap worked fine. Presumably the 
interference isn't constant.

hypothesis: wap is going CDMA deaf. Is the interferring 
signal in-band or out-of-band? A spectrum analyser ($$$$) 
would be just the thing here. Was there a bank of microwave 
ovens on the other side of the wall?

John Mitchell kindly lent me his wi-spy v1, an in-band 
2.45GHz band scanner. I expected it would take a little 
while to figure this thing out, but it was very easy to use. 
I could identify waps easily. It didn't take long to find 
that 4 different microwave ovens were radiating from ch7-11. 
I was surprised that they were all on the same freq and 
relatively narrow - I thought they were 100's of MHz wide. 
Looking with google, I find they're quite tightly regulated 
freq wise. The microwave ovens weren't detectable more than 
20' away.

For John's device there is a Linux driver, by the kismet 
people, which I got going in short time, and which gives a 
nicer display than the windows wi-spy display. John's device 
used to be about $99, but isn't made anymore. It's been 
replaced by a $199 device with similar specs and there is a 
new $99 device which spec-wise is about half of John's v1 
device. I could have gone for the device I had at $99, but 
not the replacement at $199.

After a null trip to the store in the weekend when I found 
the wap working fine, I returned during the week when the 
wap wasn't working. Then the wi-spy to show no inband 
interferrence. All I saw was the waps I could already see 
with iwconfig.

Thus there was no in-band interference. It had to be out of 
band interferrence. I did the following

o put a 4db (and 8db) attenuator in the antenna line - the 
wap started working. Presumably the 3rd order intercept 
point had been dropped enough for the device to start 
working again.

o put a 4 pole 60MHz bandpass filter centered on ch 6 in the 
antenna line - the wap started working.

The interference was almost certainly out-of-band.

I didn't know the brand of the other waps in the vicinity 
which appeared to be working just fine. They weren't 
bothered by the putative out-of-band signal and I assumed 
they were getting the same amount of signal as were my waps. 
Presumably I could have walked around and found out what 
they were. I assumed they had a different RF frontend than 
the Linksys. Looking in the digi-key catalog, I find ceramic 
filters for wifi front ends at $0.80 each. Presumably the 
LinkSys box doesn't have a filter, and I wasn't up for 
surgery on it. Presumably I needed any other front end. I 
assumed all wap manufacturers had their own front ends, 
presumably many with filters. I bought a TP-Link wap from 
Intrex and it worked first time.

Thanks to John for his wi-spy.

Joe

-- 
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) wm7d (dot) net - azimuthal equidistant map
generator at http://www.wm7d.net/azproj.shtml
Homepage http://www.austintek.com/ It's GNU/Linux!