Re: Reflection problem?
- From: Jeff Liebermann <jeffl@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Sat, 18 Feb 2006 18:08:16 -0800
"Ian" <ian_agerskov@xxxxxxxxxxx> 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/118_12_5_TR-60xxf%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/calculations/fresnel-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_alt.internet.wireless/Wi-Fi#Link_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 jeffl@xxxxxxxxxxxxxxxxxxxxxx
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
.
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