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PA2AGA > HDDIG 25.09.99 03:25l 185 Lines 7898 Bytes #-9766 (0) @ EU
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Subject: HamDigitalDigest 99/239G
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length of a bit is going to be about that divided 9600 or about 18.6
miles -- right. So are we assuming then for his weird path of 25 miles,
he's getting a direct path of 25 miles, and perhaps at the knife
edge of the mountain in between there are going to be a bunch of
paths, all coming in at slightly different lengths across the top of
the mountain range -- as each one deflects off a slightly different point.
So, maybe some signals are sneaking in on a path that's say 34
miles long or 1/2 bit late.
Is there any horse sense, generally about FSK,
that say, how many db below the main signal, would a path that's
say, 1/4 of a bit late, need to be before the channel is reliable? Is a
path a few db down that's 1/2 of a bit late really bad? Here across
the SF Bay, I assume that even the line of site paths, still have that
line of site path, and another reflection off of the bay -- and I've heard
off the Bay Bridge, of all things. .
----
And finally, say, we warp ahead in time to the future where everything
is Spread Spectrum, and FSK is long forgotten. Is Spread Spectrum,
because of it's immunity to multi-path, going to actually make these
billiard ball paths -- that is up and over mountains and bouncing off
buildings and things useable? Or is something else going to bite us,
even with Spread Spectrum.
(Btw, it occured to me that if Alligator thinking is bad, bad, bad,
then the HDTV people might have a little problem here in the
SF Bay area, with all it's hills.)
>.
------------------------------
Date: Wed, 22 Sep 1999 20:03:16 -0700
From: "Hank Oredson" <horedson@att.net>
Subject: Let's look at real numbers for TNC software sales
Cathryn Mataga <cathryn@junglevision.com> wrote in message
news:0mfG3.2731$1i3.42652@nuq-read.news.verio.net...
> Gary Coffman wrote in message ...
<your link must be perfect!>
> Okay, I'm still trying to figure out why his 25mile 'bad path' link
> would necessarily have a BER, as you say, above 1e6. Is there
> any reasonable way for him to really measure this? Maybe a
> problem here is that we 'think' we might have links that are good,
> but they have bit error rates, good enough so the the link 'seems'
> okay, but really the errors are bad enough to cause network problems.
> And that what's really needed are some good tests.
We've run them. Posted the results here in this newsgroup several
times, in the FLEXNET discussions.
> 1e6 bits, that's 125,000 bytes or about 490 256 byte packets. Now,
> let's say, he digipeated 500 256 byte packets of random data from his
> system, to the other end and back, we should find that he should miss
> more than a couple, if his BER is worse than 1e6 -- right? Since that
> would have been 2*500*256*8 bits going through the link. That is twice
> for the trip up and back. I'd be curious to know the results of this kind
> of measurement to know how good or bad the link really is. Could
> this be a way to decide if new links are going to hurt or help the
network?
> That is simply do measurements of bit errors, and if they're low enough
> it's okay, however the thing works.
I measure packet loss, not BER. There are some good technical reasons
to do it this way. The main reason is that errors tend to be bursty.
So BER gives some clues about a link's performance, but does not
tell the story you want to hear: how well does the link work in practice.
In practice, my "bad" 25 mile link runs at about 1% packet loss.
Sometimes it is better, sometimes worse, depending on how wet the
trees on the ridge to my north happen to be, how many bad electric
fence chargers are nearby, whether a C-130 is right in the path, etc.
My "real bad" link is over a 40 mile perfect line of sight path.
10W and 10 db yagis both ends. Frequent SERIOUS noise sources
along the path. The link runs either 0% packet loss, or 50% loss,
depending on the noise situation. Changes from hour to hour.
This is a backup link, so it does not usually get heavy use, unless
the primary link has failed.
My "good link" is another 25 mile path. Can see the antenna at the other
end from the shack window (using a telescope). No trees in the
way. No noise sources in that direction (big surprise, it crosses
5 miles of industrial area and 10 miles of city). Occasional packet
loss, nothing to worry about.
> Also, I'm also trying to figure out what exactly is the mechanism that
> brings in the errors -- assuming that if he did the measurement, we'd
> find BER's much worse. For the speed of light of 186000mps, the
> length of a bit is going to be about that divided 9600 or about 18.6
> miles -- right. So are we assuming then for his weird path of 25 miles,
> he's getting a direct path of 25 miles, and perhaps at the knife
> edge of the mountain in between there are going to be a bunch of
> paths, all coming in at slightly different lengths across the top of
> the mountain range -- as each one deflects off a slightly different point.
Both antennas see some of the same mountains, both the coast range
(to the west) and the cascades (to the east). There are many (dozens)
of local hills of 1000 - 2000 foot elevation in all directions. Portland
terrane is a mess for radio. It's even worse. Channel 10 (PBS) is
40 miles north of here. Continue that path 25 more miles to Mt. St.
Helens. A bit east of path is Mt. Adams. Line of sight to the transmitter.
If my TV antenna is pointed exactly right, there are only a few ghosts.
So we get all kinds of scatter on every link path. It's a fact of life here.
The Bay area is quite similar: mountains on both sides, some cross-valley
hills that break things up, etc. etc.
> So, maybe some signals are sneaking in on a path that's say 34
> miles long or 1/2 bit late.
>
> Is there any horse sense, generally about FSK,
> that say, how many db below the main signal, would a path that's
> say, 1/4 of a bit late, need to be before the channel is reliable? Is a
> path a few db down that's 1/2 of a bit late really bad? Here across
Yes, it can be a problem due to the timing and amplitude jitter it causes.
> the SF Bay, I assume that even the line of site paths, still have that
> line of site path, and another reflection off of the bay -- and I've heard
> off the Bay Bridge, of all things. .
.... and the Oakland hills, and Mt. Hamilton, and Loma Prieta, and the
whole of the Santa Cruz mountain ridges, etc.
>
> ----
>
> And finally, say, we warp ahead in time to the future where everything
> is Spread Spectrum, and FSK is long forgotten. Is Spread Spectrum,
> because of it's immunity to multi-path, going to actually make these
> billiard ball paths -- that is up and over mountains and bouncing off
> buildings and things useable? Or is something else going to bite us,
> even with Spread Spectrum.
>
> (Btw, it occured to me that if Alligator thinking is bad, bad, bad,
> then the HDTV people might have a little problem here in the
> SF Bay area, with all it's hills.)
Gary is just using a catchword meaning "I don't run high power".
When both systems have similar power output, and similar receiver
noise floor, there is no "Alligator effect". Gary was simply being
disingenuous: nobody with any knowledge would build "Alligator"
links, it is a waste of money.
--
... Hank
http://horedson.home.att.net
>.
------------------------------
Date: Wed, 22 Sep 1999 18:18:05 -0700
From: "Hank Oredson" <horedson@att.net>
Subject: Let's look at real numbers for TNC software sales
To be continued in digest: hd_99_239H
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