Reflection problem?

We setup a 2.5 mile link using 2 Tranzeo TR-6000 AP's with integrated 15 dbi
in a rural location.

AP 1 18 ft agl
AP 2 24 ft agl

There is one tree partially blocking line of site 75 feet from AP 1.
A couple of buildings at about the half way point that definately come in to
play. Otherwise it is wide open.

So we know this setup has fresnel issues but thought we would give it a try
anyway. When the radios are dialed up to full power 23db the throughput of
the radios is virtually useless. If we dial the power output on AP 1 down
to 18db the link works great. I'm assuming that we are getting a reflection
that is causing a high error rate and reducing the power output reduces the
strength of the reflection.

Is this typical of a reflection problem?
Could the tree be the source of the reflection or does a tree pretty much
only absorb?
Can a reflection off the ground cause this type of behaviour?

This is not a critical must be up connection, so I'll leave it for now since
it has been working good for a month.What are the odds, if nothing else
changes, of this link continuing to function reasonably well?

Any input is much appreciated.

Ian

"Ian" <(E-Mail Removed)> hath wroth:

>We setup a 2.5 mile link using 2 Tranzeo TR-6000 AP's with integrated 15 dbi
>in a rural location.

That would be a TR-6015F.
http://tranzeo.com/uploaded_images/1...f%20Series.pdf

>AP 1 18 ft agl
>AP 2 24 ft agl

What's an "agl"? Oh, Above Ground Level. That's not the way it's
usually specified. Try HAAT (Height Above Average Terrain) instead.

>There is one tree partially blocking line of site 75 feet from AP 1.

All trees are obviously identical. Is this tree directly in the line
of sight? Any particular type of tree? If the tree has lost all its
leafs for winter, you're gonna have a BIG problem in spring.

>A couple of buildings at about the half way point that definately come in to
>play. Otherwise it is wide open.

A couple means two. Are there two (or more) building in the way? Same
questions as with the tree. What type of building? How big an
obstruction?

>So we know this setup has fresnel issues but thought we would give it a try
>anyway.

It's always worth a try. The problem with Fresnel Zone incursion is
that the effects are truely weird and inconsistent. You could move a
few inches one way or the other with one end of your link and go from
fabulous performance to zilch. Same with moving any reflectors or
obstructions. You might get a good connection today, but tommorrow
might be different.

>When the radios are dialed up to full power 23db the throughput of
>the radios is virtually useless. If we dial the power output on AP 1 down
>to 18db the link works great. I'm assuming that we are getting a reflection
>that is causing a high error rate and reducing the power output reduces the
>strength of the reflection.

Wrong. The power in the incident (direct) path varies directly with
the reflected path. There are no transition effects. If you have a
reflection issue, it should not be affect by power level. The easiest
way to test for a reflection is to just move one end of the link. If
the signal level and particually the S/N ratio varies sharply and
radically, you have a multipath or reflection problem. Find an
antenna position where the effect is minimal and consistant.

>Is this typical of a reflection problem?

Hell no. However, what might be happening is that your connection
speed is varying from 11Mbits/sec down to 5.5 or 2 or 1 when you
reduce the power. That shuffles the deck and might explain the
magical improvement with reduced power. It might also be that the
TR-6015 is defective and generating crap in the internal RF power
amplifier at 23dBm but not at lower power levels.

Suggestion.... Try reducing the power at the other end and see if the
effect is identical. It might point to defective radio.

>Could the tree be the source of the reflection or does a tree pretty much
>only absorb?

My house is in the middle of a redwood forest. I can assure you that
trees only absorb and do not reflect. If they did reflect, I would be
in big trouble with our neighborhood WLAN.

>Can a reflection off the ground cause this type of behaviour?

Ground reflections are a bigger problem than what you apparently
indicate. At 2.5 miles and 2.4Ghz, the Fresnel Zone is a 37 ft
radius. Your 18ft and 24ft antenna heights are well inside the
Fresnel Zone at midpoint. Since you're measuring from the ground, I
suspect there are quite a few obstructions at midpoint within a 36ft
radius.
| http://www.terabeam.com/support/calc...esnel-zone.php
Never mind reflections. Think obstructions instead.

>This is not a critical must be up connection, so I'll leave it for now since
>it has been working good for a month. What are the odds, if nothing else
>changes, of this link continuing to function reasonably well?

That's easy. Do the numbers:
| http://en.wikibooks.org/wiki/FAQ_for...k_Calculations
Assuming no Fresnel Zone incursion (perfection).

TX power = +18dBm
TX coax loss = 1db (internal connectors)
TX ant gain = +15dB
Distance = 2.5 miles
RX ant gain = +15dB
RX coax loss = 1dB (internal connectors)
RX sens = -85dBm (at 11Mbits/sec CCK)
Fade margin = unknown
http://www.terabeam.com/support/calculations/som.php
I get a fade margin of 18.8dB. That's tolerable. 20dB would be
nice. No clue how much loss the tree, buildings, or ground are
contributing. Are you running at 11Mbits/sec or slower? If
5.5Mbits/sec, then your fade margin is 21.8dB which is good enough.
However, that again assumes perfection in the path.

I can't predict the reliability without calculating the effects of the
tree, buildings, and ground blockages. My wild guess (and I do mean
really wild) is these will contribute at least 8dB of attenuation.
That leaves a fade margin at 5.5Mbits/sec of 13.8dB which will
probably function, but not very reliably. You can convert fade margin
to reliability using the table in the FAQ. 13.8dB is about 95%
reliability. That means you'll have 438 hours of outage every year.

--
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

>
> What's an "agl"? Oh, Above Ground Level. That's not the way it's
> usually specified. Try HAAT (Height Above Average Terrain) instead.

Thank's

>>So we know this setup has fresnel issues but thought we would give it a
>>try
>>anyway.
>
> It's always worth a try. The problem with Fresnel Zone incursion is
> that the effects are truely weird and inconsistent. You could move a
> few inches one way or the other with one end of your link and go from
> fabulous performance to zilch. Same with moving any reflectors or
> obstructions. You might get a good connection today, but tommorrow
> might be different.

If you don't mind I would like to tap into what you have learned from
experience. When you say it's always worth a try, do you tend to find that
if you set it up and it works, even though on paper it could be problematic,
(I'm thinking in terms of fresnel zone incursions) that it tends to continue
to work or that you end up with big headaches and would be better off
staying away if the numbers don't work out.
>
>>Is this typical of a reflection problem?
>
> Hell no. However, what might be happening is that your connection
> speed is varying from 11Mbits/sec down to 5.5 or 2 or 1 when you
> reduce the power. That shuffles the deck and might explain the
> magical improvement with reduced power. It might also be that the
> TR-6015 is defective and generating crap in the internal RF power
> amplifier at 23dBm but not at lower power levels.

Interesting, I talked to Tranzeo support and they suggested trying to reduce
power because in their words "The radios are designed to work over a long
range they may just be hitting each other too hard". It didn't make much
sense to me ( I always thought more was better) but I tried it and it worked
so I didn't argue. In fact lowering the power seems to have stabilized
things at 11Mbits, at higher power it seemed to be hunting for a speed. At
these lower power settings it seems to be working flawlessly.

Thanks again for a swift and informative response.

Ian

"Ian" <(E-Mail Removed)> hath wroth:

>When you say it's always worth a try, do you tend to find that
>if you set it up and it works, even though on paper it could be problematic,
>(I'm thinking in terms of fresnel zone incursions) that it tends to continue
>to work or that you end up with big headaches and would be better off
>staying away if the numbers don't work out.

I wish I had a rule of thumb for such situations. What I usually find
that testing for fade margin will determine the ultimate reliability
of the link. I usually carry a pile of attenuators to insert in the
coax line between the radio and the antenna. If I can't insert 10dB
of additional attenuation and still have a functional link, then
there's gonna be trouble in the future. In systems where I can't
insert and attenuator, I just cover up part of the antenna. Half the
dish is -3dB. Covering 3/4 of the dish is -6dB. Make sure you use
some kind of RF absorbant material (i.e. black foam or wet towel) and
not something reflective.

Duh, I forgot to mention that you should temporarily fix the wireless
speed when doing this test. Covering up half the antenna will
probably cause the link to slow down. That hides the effect of signal
loss.

There are lots of other issues that commonly screw up links. The
biggest is interference. One day it works, the next day it doesn't.
It's very difficult to check out a link in the presence of
interference. My usually method is to rely on diagnosis by
substitution. If I have SNMP running, I'll have a history of what is
considered "normal". I can usually deduce the cause by the pattern
and timing.

>Interesting, I talked to Tranzeo support and they suggested trying to reduce
>power because in their words "The radios are designed to work over a long
>range they may just be hitting each other too hard".

Bull shit. Back to the calculations. This time, the exercise is to
figure out how far the radios need to be apart before they overload.
Most 802.11b hardware just doesn't have much dynamic range. Typically,
they will overload at about -20dBm input to the receiver. Going back
to:
TX power = +23dBm
TX coax loss = 1db (internal connectors)
TX ant gain = +15dB
Distance = Unknown
RX ant gain = +15dB
RX coax loss = 1dB (internal connectors)
RX level = -20dBm (overload point)
http://www.terabeam.com/support/calculations/som.php
Distance to overload is 0.022 miles or 116ft. That's quite a bit less
than 2.5 miles. I'm not sure of the -20dBm overload point. However,
it's close enough as it will take a HUGE change in overload point to
create overload at 2.5 miles.


>It didn't make much
>sense to me ( I always thought more was better) but I tried it and it worked
>so I didn't argue. In fact lowering the power seems to have stabilized
>things at 11Mbits, at higher power it seemed to be hunting for a speed. At
>these lower power settings it seems to be working flawlessly.

I think you just proved my guess. It's not suppose to do that.
There's something wrong at high power with one of your radios. My
guess is that there's some envelope distortion at high power.

Asking again: What wireless speed are you running?

--
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

A Spectrum Analyzer would be so much easier.

JM

"Jeff Liebermann" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
>>It didn't make much
>>sense to me ( I always thought more was better) but I tried it and it
>>worked
>>so I didn't argue. In fact lowering the power seems to have stabilized
>>things at 11Mbits, at higher power it seemed to be hunting for a speed. At
>>these lower power settings it seems to be working flawlessly.
>
> I think you just proved my guess. It's not suppose to do that.
> There's something wrong at high power with one of your radios. My
> guess is that there's some envelope distortion at high power.

I'll try a little more testing (adjusting power at the other end, perhaps
bench test making sure to avoid the overload point) and see if I can get an
RMA

> Asking again: What wireless speed are you running?

Radios are set at best automatic but would like them to run at 11Mb/s. In
reality they are used primarily for Internet sharing and remote desktop
operation of one computer with very little file transfer, so I could live
with lower speed.

Ian

"JM" <(E-Mail Removed)> hath wroth:

>A Spectrum Analyzer would be so much easier.
>JM

OK, I'll bite. I really detest one line answers.
What would you expect to see on the spectrum analyzer?
Easier that what? Easier than doing the calculations?

Hint: The signal level at the receive end will be about -60dBm.
That's the decorrelated CW equivalent for a single carrier. Spread
that out over 25Mhz of spread spectrum, with an IF bandwidth of
250KHz, and you will be looking for:
-10 log (25Mhz / 250KHz) = -20dB
an observed signal level that would be about -80dBm. Do you have a
spectrum analyzer handy that can see an SS signal that's -80dBm
(assuming a similar +15dBi antenna and 2.5 miles away)?

Assuming you do see something, what would you look for? Envelope
distortion can't easily be seen on a spectrum analyzer. Multipath and
Fresnel zone effects cannot be seen at all. You might see envelope
distortion with a sampling scope by looking at the raw RF envelope,
but that's rough using spread spectrum. At best, you might see some
additional bandwidth consumption on the spectrum analyzer.

Here are some awful photos of the spectra coming from an WRT54G with
the power cranked up. That's about as much useful info as I would
expect from a spectrum analyzer trying to troubleshoot this problem
assuming the transmitter power amp has a problem.
http://explorer.cyberstreet.com/wrt5...raloutput.html



--
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

On Sat, 18 Feb 2006, in the Usenet newsgroup alt.internet.wireless, in article
<(E-Mail Removed)>, Jeff Liebermann wrote:

>"Ian" <(E-Mail Removed)> hath wroth:
>
>> When you say it's always worth a try, do you tend to find that
>> if you set it up and it works, even though on paper it could be
>> problematic,

Minor presumption - you have the equipment needed to make the try, or
can borrow it for as close to free as possible. If you have to buy the
equipment before you can test, it may not be worth it.

>I wish I had a rule of thumb for such situations. What I usually find
>that testing for fade margin will determine the ultimate reliability
>of the link.

In the 1970s, we were running a test program in the Central Valley, and
needed a datalink from there back to Ames. The local telco would rent a
wide band wire link at some outrageous monthly fee, or we could try a
wireless link. A direct shot is obviously out (both sites under 200 feet
above sea level with a 2500 foot range of hills between), but we had
access to a spot on a tower on Mount Diablo. The Diablo to Ames shot was
a piece of cake, but the test site to Diablo missed line of sight by 130
feet at mid-path. What the heck, let's give it a try. With 18 dBi dishes
at both ends, it's working, but the fade margin was only about 10 dB. OK,
Andrew has these nifty 33 dBi dishes for only an arm and a leg (about what
the telco wanted every six months) - buy two. Now, after we install
these dishes, and the link is running flawlessly, we go back and document
everything. As part of the documentation, we fly a helicopter near Diablo,
and using the radar at the test site, we're able to see that we really would
have needed a 600 foot tower on either end to get line of sight, but it's
working so who cares. About a year later, we discovered that there is a
LARGE wire fence on the top of the ridge near mid-path that seems to be
acting as a knife-edge.

Things are great until about 1988, then we notice dropouts in the
link - and testing on Diablo shows that the received signal strength
is down 30 dB from normal. Antennas, cables, transmitter and receiver
are all OK... WTF? Turns out the farmer who owned the fence at
mid-path had torn it down. Adding a second repeater on Mount Oso with
10 dBi Yagi antennas (salvaged from another project) between the test
site and Diablo solved the problem, except that we had pay another agency
for the tower space on Oso, and con the Frequency Manager to give us the
second channel he had insisted we'd need back in 1970.

Old guy

"Moe Trin" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> On Sat, 18 Feb 2006, in the Usenet newsgroup alt.internet.wireless, in
> article
> <(E-Mail Removed)>, Jeff Liebermann wrote:
>
>>"Ian" <(E-Mail Removed)> hath wroth:
>>
>>> When you say it's always worth a try, do you tend to find that
>>> if you set it up and it works, even though on paper it could be
>>> problematic,
>
> Minor presumption - you have the equipment needed to make the try, or
> can borrow it for as close to free as possible. If you have to buy the
> equipment before you can test, it may not be worth it.

I usually have enough stuff around to do a test.


> In the 1970s, we were running a test program in the Central Valley, and
> needed a datalink from there back to Ames. The local telco would rent a
> wide band wire link at some outrageous monthly fee, or we could try a
> wireless link. A direct shot is obviously out (both sites under 200 feet
> above sea level with a 2500 foot range of hills between), but we had
> access to a spot on a tower on Mount Diablo. The Diablo to Ames shot was
> a piece of cake, but the test site to Diablo missed line of sight by 130
> feet at mid-path. What the heck, let's give it a try. With 18 dBi dishes
> at both ends, it's working, but the fade margin was only about 10 dB. OK,
> Andrew has these nifty 33 dBi dishes for only an arm and a leg (about what
> the telco wanted every six months) - buy two. Now, after we install
> these dishes, and the link is running flawlessly, we go back and document
> everything. As part of the documentation, we fly a helicopter near Diablo,
> and using the radar at the test site, we're able to see that we really
> would
> have needed a 600 foot tower on either end to get line of sight, but it's
> working so who cares. About a year later, we discovered that there is a
> LARGE wire fence on the top of the ridge near mid-path that seems to be
> acting as a knife-edge.
>
> Things are great until about 1988, then we notice dropouts in the
> link - and testing on Diablo shows that the received signal strength
> is down 30 dB from normal. Antennas, cables, transmitter and receiver
> are all OK... WTF? Turns out the farmer who owned the fence at
> mid-path had torn it down. Adding a second repeater on Mount Oso with
> 10 dBi Yagi antennas (salvaged from another project) between the test
> site and Diablo solved the problem, except that we had pay another agency
> for the tower space on Oso, and con the Frequency Manager to give us the
> second channel he had insisted we'd need back in 1970.
>
That is a good lesson. If you don't have full control of all obstructions in
the way, even what you think may be a permanent non moveable structure can
change. When it comes to wireless the unexpected can always come back to
bite you.

Ian

On Mon, 20 Feb 2006, in the Usenet newsgroup alt.internet.wireless, in article
<v6lKf.3109$Nr5.2987@clgrps13>, Ian wrote:
>
>"Moe Trin" <(E-Mail Removed)> wrote

>> If you have to buy the equipment before you can test, it may not be
>> worth it.
>
>I usually have enough stuff around to do a test.

Not everyone does. Still, I hate to think how many extremely fragile, but
functional antennas (usually, shallow horns built out of sheet aluminum) we
cobbled up to run a test.

>> About a year later, we discovered that there is a LARGE wire fence on
>> the top of the ridge near mid-path that seems to be acting as a
>> knife-edge.
>>
>> Things are great until about 1988, then we notice dropouts in the
>> link - and testing on Diablo shows that the received signal strength
>> is down 30 dB from normal. Antennas, cables, transmitter and receiver
>> are all OK... WTF? Turns out the farmer who owned the fence at
>> mid-path had torn it down.

>That is a good lesson. If you don't have full control of all obstructions
>in the way, even what you think may be a permanent non moveable structure
>can change.

Some of it is local knowledge. The fence in question was about 1000 feet
or 300 meters long, and 6-10 feet / 2-3 meters tall and seemingly made of
"chicken wire" (about 1 inch hexagons) - certainly it had very little
effect on our X band (9 GHz) radars. I still have no idea why it was
there - following an East/West section line about 3 miles from the nearest
road. There was also a (common here in the West) 3 strand barbed wire fence
normally used to divide cattle grazing areas that was still in place in 1995.

>When it comes to wireless the unexpected can always come back to bite you.

True. In this case, it worked for us for about 16 years, so we had no
reason to complain. We really didn't _know_ that the fence was the the
cause until it was pulled down, and even that is a 'best guess' based on
chance aerial observations (we flew over the area, often at fairly low
altitudes several times a week).

Old guy