HowToFixComputers.com

-= Hawk =- · Guest

On Thu, 21 Dec 2006 19:48:48 GMT, "Poul Starz" <Starz@Astromail.com>
scribbled:

Timothy Daniels · Guest

$Bill · Guest

-= Hawk =- wrote:

Todd H. · Guest

"Timothy Daniels" <TDaniels@NoSpamDot.com> writes:

Timothy Daniels · Guest

"Todd H." wrote:

Ed Nielsen · Guest

Been running out at ~52-54dBmV for about 5 years now with no issues.
I'd much rather run at 52dBmV and be able to shout and be heard above
the noise that inherently makes its way into the line than keep my cable
modem running down around 40dBmV.

CIAO!

Ed N.

Todd H. wrote:

Ed Nielsen · Guest

Lower packet loss with the modem pumping out more suggests that there
may be some noise (impulse, ingress) hopping onto your drop. Where did
they pad your cable? Right at the modem? If you can get to the main
splitter, try padding there. Don't worry about the input -- you have
plenty of room there to do what you want. You can go down to ~-12dBmV
before there would start to be any cause of concern. Heck, I used to
run at ~-12/52 for about 4 years until some plant issues caused me to
alter that. Now, I'm at +7/49, and I hate it. One of these days soon,
I'll get around to swapping out my DC to fix it.

CIAO!

Ed N.

Timothy Daniels wrote:

Timothy Daniels · Guest

I think you got that backwards. First, the modem appears to be
reporting a positive offset - a level *above* some reference level,
not a negative offset from some reference. Thus, with the splitter
in the line, the downstream level drops in magnitude from 10 dBmV
to 9 dBmV (this morning, and during the day yesterday, the downs-
stream level with the splitter in the line is/was 7 dBmV). And the
upstream level rose to 49 dBmV from 47 dBmV with the splitter
(today it's at 50 dBmV with the splitter.)

That's agrees with the model that the Transmit upstream level varies
counter to the downstream Received level, and it also suggests that
the upstream level is a positive value, not a value below a reference
level. Given that, it looks as if the high downstream level was set
too high for there not to be a splitter since the packet loss *increases*
when I substitute a barrel connector for the splitter.

This would make sense if the cable company always assumes that
there would be one or two TVs hung off the same line - a likely
scenario for the average unit in a condo building, but just not the
scenario in my condo unit.

*TimDaniels*

"Ed Nielsen" wrote:

mediumhappy · Guest

Wanted to post a follow-up to my original post.

On the advice of a friend I phoned Comcast's Philadelphia headquarters.
I told them the history, requested that they perform an ingress test at
the pedestal, and told them that my next step, if things couldn't be
resolved, would be to involve the Public Utilities Commission and the
FCC, as well as my legislators.

Within an hour there were three Comcast (not contractors, but Comcast
employees) trucks at my house. The feeds between the house and the
pedestal, the pedestal and the pole, and all the splitters on the
outside of my home, were replaced. There was a bad distribution
amplifier on our block, and all the remaining amplifiers in our
development were "out of alignment" - this was causing noise which was
destabilizing the signal and knocking me (and likely, many others) off
the 'net. They also replaced splitters and feeds inside my home, tested
signal strength at every outlet, and installed a weatherproof enclosure
outside of my home to enclose the splitter. All in all they worked in
and around my home until almost Midnight. The regional manager for
infrastructure gave me his cell number, and told me to call him
directly if there were any other problems. Finally, the folks at
Comcast corporate called me and rolled back my monthly rate to $19.95
for internet access, as a token of good will and as recompense for the
problems I experienced.

I have installed monitoring through dslreports.com (very good
suggestion, thanks!) and will be emailing the results to Comcast after
a week, so that they can see if the downtime correlates to known
outages, or if there are other problems in our area that they have to
fix. They've promised me that they will stay on this problem until I am
satisfied that it's fixed.

All in all I have to say they've gone out of their way to make me
happy. I think there's something to be said for jumping over the local
folks and bringing their corporate management into the equation - they
were able to get the right people focused on the problem, and they made
sure that someone on their end retained accountability and ownership
for my problem - definitely the right step. It should not be necessary
to so to these lengths to get a persistent yet intermittent problem
resolved, but apparently until Comcast revisits their escalation
procedures and does more to ensure customer satisfaction, it IS
necessary.

Thanks to all who commented on my post. Nice to know we're all in a
similar boat.

Pax,
D.

mediumhappy wrote:

Timothy Daniels · Guest

It's just amazing what action you can get when you go directly to
the cages that have to be rattled. In my case, I went directly to
the office of the field ops supervisor and got 2 trucks and 3 technicians
out the very next day. I think they reason that if you go that far to get to
them, the next step you'll take is to go to the government offices which
just love to make notes for the next regulatory hearing. It's too bad
that a customer has to so far and expend so much time and energy to
get any real response over "unplug and re-plug in your modem, then
restart your computer".

*TimDaniels*

"mediumhappy" wrote:

Ed Nielsen · Guest

That "reference level" (as you call it), like most numbers that have a
(+) or a (-) before them is 0. DOCSIS says that cable modems have an
input level window of -15 - +15dBmV, which simply means that as long as
the input signal level is within that window, it will work. The output
level window, as dictated by DOCSIS, is +8 - +58dBmV. Again, as long as
the output level of the cable modem is within that window, it will work.

The input level, of course, is determined by the plant/drop system. For
example: There is a line extender (amplifier) at the pedestal that
feeds your house. [Single family home with a white picket fence and a
couple of elm trees in the front yard out in the suburbs. Can't forget
the 2 dogs and a cat, a swing on the front porch, a vegetable garden,
and a few fruit trees out back, etc.] The tap on the output of that
L.E. is a 26 value tap, and the length of your drop is 100', and the
DOCSIS frequency is 600MHz, and it is a QAM256 system. The L.E is
running out at 44dBmV (no slope, the output is flat), which equates to
18dBmV coming out of the tap. 100 feet of RG 6 loses ~5dB@600MHz, so
you a level of 13dBmV at the groundblock. In a QAM256 system, the data
carrier runs at 6dB below the nearest analog channel, so that 13dBmV
becomes 7dBmV. Place a cable modem right at the groundblock, and it
will see an input level of +7dBmV. Say you have a TV set in the family
room and your computer is in a room 25' from the groundblock. A 2-way
splitter loses ~3.9dB at 600MHz and the 25' drop to the cable modem
loses 1.25dB. That 7dBmV at the groundblock then becomes (+7-3.9-1.25=)
1.85dBmV at the input of the cable modem. Suppose you have 4 TV sets.
The input level at your cable modem would then be (7-7.9-1.25=)
-2.15dBmV. Change the attenuation and the input level changes by the
same amount. A 30dB wide input level, cable modems have.

On the output side in the same scenario, the return amp in the L.E.
wants to see ~10dBmV. The pathway from cable modem is the drop and the
tap. Let's say that the upstream frequency is 22MHz. RG 6 loses
~1.25dB/100' at 22MHz. The pathway to that amp has the signal losing
1.25dB (drop) and 26dB (tap). 10 (required input level to the amp) + 26
(tap) + 1.25 (drop) would have the cable modem pumping out 37.25dBmV.
One TV (2-way splitter) and the cable modem 25' from the groundblock
would have it pumping out (10+26+1.253.6+.31=) 41.16. Four TVs would
necessitate that the modem transmit at 44.56dBmV.

The CMTS (Cable Modem Termination System (in the headend)) wants a
signal level of 0dBmV at its input. No wide window like cable modems
have -- it wants 0dBmV. If the attenuation at a cable modem changes,
the CMTS tells the modem to increase or decrease its output level
accordingly. If a cable modem runs so as to hit the CMTS with too hot
of a signal, the CMTS tells it to turn itself down. Too little and it
tells the modem to turn itself up. When you remove the splitter from
your drop, you reduce the attenuation of your drop by 3.5dB, which means
that there is 3.5dBmV more signal hitting the CMTS than previously, so
the CMTS tells the modem to turn itself down. Put it back in and the
modem is told to crank up the power to compensate for the increase in
attenuation. That is why the transmit levels vary when you
remove/insert the splitter.

If noise gets into the line, whether it be in the drop or somewhere in
the plant, the CMTS can't hear the cable modem as well as it needs to,
so it tells the modem to "speak louder," same thing as you do when
conversing with someone on the sidewalk and a loud car drives by. Noise
can be ingress from a local broadcaster or impulse noise such as from a
blow dryer or vacuum cleaner.

CIAO!

Ed N.

Timothy Daniels wrote:

Eric · Guest

Ed Nielsen wrote:

Timothy Daniels · Guest

Yup, that all sounds right. :-) And it sounds like the cable modem
is receiving and transmitting the right signal levels. But why would
the packet loss increase when the barrel connector is substituted
for the splitter, thus causing the upstream signal level to rise from
+47dBmV to +49 dBmV as a result of the downstream signal level
dropping from +10 dBmv to +7 dBmV? I'd think that the louder
outgoing signal would better the signal-to-noise ratio. as seen by
the network.

*TimDaniels*

"Ed Nielsen" wrote:

Ed Nielsen · Guest

It isn't packet loss that causes the modem to pump out more -- nor is it
because the downstream has changed. [You could use a couple of diplex
filters (not the satellite ones but similar in purpose) that separate
the forward path from the return path, pad the downstream down as much
as you want (or even amplify it) and you would see virtually no change
in the transmit level of the cable modem.] It is merely that because of
the reduced attenuation on the drop, the cable modem is hitting the CMTS
too hot, so it is told to turn down.

That suggests to me that there might be something (noise) getting into
the drop between the cable modem and the splitter/barrel location. When
you insert the splitter, you lower the signal level hitting the CMTS, so
it tells the cable modem to increase its transmit level. The signal
level from the splice location back to the lockbox remains pretty much
the same whether the splitter is in there or not -- it is only in the
portion of the drop from that location to the outlet that sees the
increase/decrease in signal level as the modem's transmit level changes.
Could that piece run parallel to some A.C. lines for awhile? There
could be something that is induced onto and is riding on the shield
until it finds a spot where it can leak onto the center conductor. Have
you tried grounding the splitter? If there is something riding on the
shield, that could give it somewhere to go where it will do no harm.

CIAO!

Ed N.

Timothy Daniels wrote:

Warren · Guest

Ed Nielsen <egnlsn@comcast.net> wrote: