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| Author |
Message |
Timothy Daniels
Guest
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 Posted: Mon Dec 25, 2006 3:25 am Post subject: Re: SB5120 & Comcast Woes |
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I never posed that it was packet loss that caused the modem
to increase its upstream signal level.
You say that it is the CMTS at the headend that tells the modem
to turn its signal level up or down. And when the splitter is substituted
for the barrel connetor, the output level of the modem is increased.
That suggests that the splitter itself is attenuating the upstream signal
from the modem to the CMTS, as nothing else has changed. It may
also be the splitter which is mangling the packets. I'll search around
for another splitter to see if that helps.
*TimDaniels*
"Ed Nielsen" wrote:
| Quote: | 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:
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:
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:
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:
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:
Without the 2:1 splitter, the downstream signal level rises to 10
dBmv
(from 9 dBmV) and the upstream signal level drops to 47 dBmV (from
49 dBmV). The tracerts show a quadrupling (or more) of timeouts with
the splitter removed. It seems that the lower transmit level
hinders the
packets' survival rate, and that the splitter, in reducing the
receive level,
causes the modem to drop its transmit level and thus increases the
timeout incidence. Is that right?
BTW, *I* put the splitter between the modem and the RR network
for analog cable TV reception of local stations. Whenever the techs
were here, the splitter had been replaced by a barrel connector
(don't ask). As far as TW is concerned, my only service from them
is RoadRunner. But no instruments measured levels within my condo
unit. The only measurements that they took directly were on the level
from the amp in the vault at the curb and on the output from the
building's
amp that resulted from their padding down the signal coming from the
curb amp. The only measurements that they took specific to my modem
were the remote readings reported to them from by cell phone from the
central office. They said that it gave them more information than
what
I got from the modem's webpage and, presumably, what they could learn
by tapping into the line between the wall outlet and my modem. So
it's
strange that the timeouts are so much less frequent when I put my 2:1
splitter in the line - it is not what they thought the usual
configuration was.
*TimDaniels* |
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Timothy Daniels
Guest
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| Posted: Mon Dec 25, 2006 3:40 am Post subject: Re: SB5120 & Comcast Woes |
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"Warren" wrote:
| Quote: | Ed Nielsen wrote:
It isn't packet loss that causes the modem to pump out more --
That's an important thing to keep in mind.
|
Yes - if someone had said that. I certainly didn't.
| Quote: | Of course if the train doesn't arrive at all, the passengers can't arrive,
either. So a crappy RF signal could cause packet loss, but packet loss
doesn't have to be related to a crappy RF signal. And packet loss won't
have any affect on the RF signal levels.
|
If by "crappy" you mean "weak", yes, as strong signals can also
be noisy and the noise can cause bits to be misinterpreted and
thus "lost", causing a bad checksum for the packet and a request
for a retransmission and thus a lowered data throughput, i.e. lower
bytes-per-second of good data. But whether any circuitry can
adjust the power to affect the signa-to-noise ratio as measured by
a high retransmission rate, I don't know.
*TimDaniels* |
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Ed Nielsen
Guest
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| Posted: Mon Dec 25, 2006 9:20 pm Post subject: Re: SB5120 & Comcast Woes |
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|
Sorry -- I had misread something you had written. Warren had given a
good analogy of that, though.
Technically, a splitter is a power divider. In fact, as you probably
already know, in the commercial world that is what they are called.
When the voltage is cut in half (half of the voltage goes down each leg
of a 2-way splitter), there is a 3dB reduction in signal strength.
Actually, it's 3.01. Throw in a little more for imperfect materials
used in the manufacturing process and we have ~3.5. That number varies
slightly with different manufacturers and it also varies a little with
frequency. As the frequency increases, so does the loss through the
splitter. The insertion loss through a 2-way splitter is typically from
~3.3dB (5MHz) to ~4.2dB (1GHz). A 4-way loses ~6.7dB (5MHz) to ~8dB (1GHz).
It is possible that a splitter could cause disruption amongst the
packets. Make sure that the one you have in there has a high return
loss on all ports. Also, of course, solder-backed (no epoxy).
HAVE A VERY MERRY CHRISTMAS!
CIAO!
Ed N.
Timothy Daniels wrote:
| Quote: | I never posed that it was packet loss that caused the modem
to increase its upstream signal level.
You say that it is the CMTS at the headend that tells the modem
to turn its signal level up or down. And when the splitter is substituted
for the barrel connetor, the output level of the modem is increased.
That suggests that the splitter itself is attenuating the upstream signal
from the modem to the CMTS, as nothing else has changed. It may
also be the splitter which is mangling the packets. I'll search around
for another splitter to see if that helps.
*TimDaniels*
"Ed Nielsen" wrote:
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:
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:
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:
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:
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:
Without the 2:1 splitter, the downstream signal level rises to
10 dBmv
(from 9 dBmV) and the upstream signal level drops to 47 dBmV (from
49 dBmV). The tracerts show a quadrupling (or more) of timeouts
with
the splitter removed. It seems that the lower transmit level
hinders the
packets' survival rate, and that the splitter, in reducing the
receive level,
causes the modem to drop its transmit level and thus increases the
timeout incidence. Is that right?
BTW, *I* put the splitter between the modem and the RR network
for analog cable TV reception of local stations. Whenever the techs
were here, the splitter had been replaced by a barrel connector
(don't ask). As far as TW is concerned, my only service from them
is RoadRunner. But no instruments measured levels within my condo
unit. The only measurements that they took directly were on the
level
from the amp in the vault at the curb and on the output from the
building's
amp that resulted from their padding down the signal coming from the
curb amp. The only measurements that they took specific to my modem
were the remote readings reported to them from by cell phone from
the
central office. They said that it gave them more information
than what
I got from the modem's webpage and, presumably, what they could
learn
by tapping into the line between the wall outlet and my modem.
So it's
strange that the timeouts are so much less frequent when I put my
2:1
splitter in the line - it is not what they thought the usual
configuration was.
*TimDaniels* |
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Todd H.
Guest
|
| Posted: Tue Dec 26, 2006 11:05 pm Post subject: Re: SB5120 & Comcast Woes |
|
|
"Timothy Daniels" <TDaniels@NoSpamDot.com> writes:
| Quote: | Without the 2:1 splitter, the downstream signal level rises to 10 dBmv
(from 9 dBmV) and the upstream signal level drops to 47 dBmV (from
49 dBmV). The tracerts show a quadrupling (or more) of timeouts with
the splitter removed. It seems that the lower transmit level hinders the
packets' survival rate, and that the splitter, in reducing the receive level,
causes the modem to drop its transmit level and thus increases the
timeout incidence. Is that right?
|
Hi Tim,
Something isn't making sense. When you remove the splitter, it does
make sense to see the downstream signal increase and the upstream
decrease (the modem doesn't have to dial up as much gain to sync
upstream). however, I wouldn't make any correlation between that
normal operation of a lower upstream gain and why you see more packet
loss without the splitter.
That 10dBmV downstream level is mighty damned hot in comparison to
happy values I normally see. Perhaps it's more of the issue than
the upstream level.
| Quote: | BTW, *I* put the splitter between the modem and the RR network
for analog cable TV reception of local stations.
|
Ah ha... is this thing designed for teh relevant bandwidth of that
cable system though...
| Quote: | Whenever the techs were here, the splitter had been replaced by a
barrel connector (don't ask). As far as TW is concerned, my only
service from them is RoadRunner. But no instruments measured levels
within my condo unit.
|
Techs at my place always hook their meter to my premises cable after
unhookin git from the back of my modem. This gives them an apples to
apples validation of what the modem is telling them for a downstream
signal level at least, and gives another datapoint as to what the
cable modem inside their tester thinks it needs for upstream gain to
sync up.
| Quote: | The only measurements that they took directly were on the level from
the amp in the vault at the curb and on the output from the
building's amp that resulted from their padding down the signal
coming from the curb amp. The only measurements that they took
specific to my modem were the remote readings reported to them from
by cell phone from the central office. They said that it gave them
more information than what I got from the modem's webpage and,
presumably, what they could learn by tapping into the line between
the wall outlet and my modem.
|
| Quote: | So it's strange that the timeouts are so much less frequent when I
put my 2:1 splitter in the line - it is not what they thought the
usual configuration was.
|
I agree that seems strange, but it's also a good datapoint that tells
us that the downstream level being so hot might be the root cause
problem here.
Get them back out there. See if they'll pad down the downstream to
get it more around 0dBmV.
And quit stealing cable service from them you cheap bastard. Ask them
about BASIC BASIC really really basic cable service to see if they
have it. Comcast doesn't advertise this service here, but it
exists... and for $9 a month for about 20 channels of service, it
dropped my cable modem charge by $15 netting me a $6/month savings.
Best Regards,
--
Todd H.
http://www.toddh.net/ |
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Timothy Daniels
Guest
|
| Posted: Wed Dec 27, 2006 8:46 am Post subject: Re: SB5120 & Comcast Woes |
|
|
"Todd H." wrote:
| Quote: | "Timothy Daniels" writes:
Without the 2:1 splitter, the downstream signal level rises to 10 dBmv
(from 9 dBmV) and the upstream signal level drops to 47 dBmV (from
49 dBmV). The tracerts show a quadrupling (or more) of timeouts with
the splitter removed. It seems that the lower transmit level hinders the
packets' survival rate, and that the splitter, in reducing the receive level,
causes the modem to drop its transmit level and thus increases the
timeout incidence. Is that right?
Hi Tim,
Something isn't making sense. When you remove the splitter, it does
make sense to see the downstream signal increase and the upstream
decrease (the modem doesn't have to dial up as much gain to sync
upstream). however, I wouldn't make any correlation between that
normal operation of a lower upstream gain and why you see more
packet loss without the splitter.
That 10dBmV downstream level is mighty damned hot in comparison
to happy values I normally see. Perhaps it's more of the issue than
the upstream level.
BTW, *I* put the splitter between the modem and the RR network
for analog cable TV reception of local stations.
Ah ha... is this thing designed for teh relevant bandwidth of that
cable system though...
Whenever the techs were here, the splitter had been replaced by a
barrel connector (don't ask). As far as TW is concerned, my only
service from them is RoadRunner. But no instruments measured levels
within my condo unit.
Techs at my place always hook their meter to my premises cable after
unhookin git from the back of my modem. This gives them an apples to
apples validation of what the modem is telling them for a downstream
signal level at least, and gives another datapoint as to what the
cable modem inside their tester thinks it needs for upstream gain to
sync up.
The only measurements that they took directly were on the level from
the amp in the vault at the curb and on the output from the
building's amp that resulted from their padding down the signal
coming from the curb amp. The only measurements that they took
specific to my modem were the remote readings reported to them from
by cell phone from the central office. They said that it gave them
more information than what I got from the modem's webpage and,
presumably, what they could learn by tapping into the line between
the wall outlet and my modem.
So it's strange that the timeouts are so much less frequent when I
put my 2:1 splitter in the line - it is not what they thought the
usual configuration was.
I agree that seems strange, but it's also a good datapoint that tells
us that the downstream level being so hot might be the root cause
problem here.
Get them back out there. See if they'll pad down the downstream to
get it more around 0dBmV.
|
I'm afraid that I'd have to show that the speeds are dependent on
the downstream signal level, and right now that doesn't seem to be
the case. The techs seemed to be happy when they dropped it from
15 dBmV to 9 - 10 dBmV (measured at the modem with a barrel
connector between the modem and the wall). Right now, I can't see
a significant difference in ping timeouts between the barrel connector
and the splitter. Of more annoyance right now are the long delays
seen with html downloads and the news server.
| Quote: | And quit stealing cable service from them you cheap bastard. Ask
them about BASIC BASIC really really basic cable service to see if
they have it. Comcast doesn't advertise this service here, but it
exists... and for $9 a month for about 20 channels of service, it dropped
my cable modem charge by $15 netting me a $6/month savings.
|
I did ask Comcast about analog local channels a couple months ago
(which is what I steal), and they said they weren't taking new orders for
it because it was going away in December - so I continued to steal it.
Now that my service is from Time Warner, I don't know what the deal is.
But I like stealing it. :-) The offerings are so bad that it keeps me from
wasting more time watching cable TV. Hey, when you're an unemployed
student who can't afford to heat the condo, ya gotta do what ya gotta do.
*TimDaniels* |
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Dutch
Guest
|
| Posted: Wed Dec 27, 2006 9:09 am Post subject: Re: SB5120 & Comcast Woes |
|
|
While scrubbing graffiti off the
comp.dcom.modems.cable,alt.online-services.roadrunner walls, I heard
Timothy Daniels say:
| Quote: | "Todd H." wrote:
[...]
And quit stealing cable service from them you cheap bastard. Ask
them about BASIC BASIC really really basic cable service to see if
they have it. Comcast doesn't advertise this service here, but it
exists... and for $9 a month for about 20 channels of service, it dropped
my cable modem charge by $15 netting me a $6/month savings.
I did ask Comcast about analog local channels a couple months ago
(which is what I steal), and they said they weren't taking new orders for
it because it was going away in December - so I continued to steal it.
Now that my service is from Time Warner, I don't know what the deal is.
But I like stealing it. :-) The offerings are so bad that it keeps me from
wasting more time watching cable TV. Hey, when you're an unemployed
student who can't afford to heat the condo, ya gotta do what ya gotta do.
|
And how much heat would the $600 or so your paying Comcast or TWC per
year buy? ;-)
--
Dutch |
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Ed Nielsen
Guest
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| Posted: Wed Dec 27, 2006 9:38 am Post subject: Re: SB5120 & Comcast Woes |
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|
That is an excellent splitter, although even excellent ones can have an
occasional bad apple in the lot. Other than the suggestion to try
grounding that splitter, all I can probably say is good luck pinpointing
the cause if the issue. I read your other post about what you are
seeing, and it really sounds like a network issue rather than something
local.
CIAO!
Ed N.
Timothy Daniels wrote:
| Quote: | Now the splitter seems to make no difference in packet loss (as measured
by "ping -t"), except to drop the downstream signal level to the modem
to 6-7 dBmV and to cause the modem to raise its signal level to 50 dBmV.
The splitter is a TVC unit that's rated for use to 1 GHz, and it's
solder sealed.
On pings to yahoo, google, and mit, packet loss is reported as 2-3%
(although
I observe twice that) and to rr.com it's reported around 10%. There seems
to be no pattern, now.
*TimDaniels*
"Ed Nielsen" wrote:
Sorry -- I had misread something you had written. Warren had given a
good analogy of that, though.
Technically, a splitter is a power divider. In fact, as you probably
already know, in the commercial world that is what they are called.
When the voltage is cut in half (half of the voltage goes down each
leg of a 2-way splitter), there is a 3dB reduction in signal strength.
Actually, it's 3.01. Throw in a little more for imperfect materials
used in the manufacturing process and we have ~3.5. That number
varies slightly with different manufacturers and it also varies a
little with frequency. As the frequency increases, so does the loss
through the splitter. The insertion loss through a 2-way splitter is
typically from ~3.3dB (5MHz) to ~4.2dB (1GHz). A 4-way loses ~6.7dB
(5MHz) to ~8dB (1GHz).
It is possible that a splitter could cause disruption amongst the
packets. Make sure that the one you have in there has a high return
loss on all ports. Also, of course, solder-backed (no epoxy).
HAVE A VERY MERRY CHRISTMAS!
CIAO!
Ed N.
Timothy Daniels wrote:
I never posed that it was packet loss that caused the modem
to increase its upstream signal level.
You say that it is the CMTS at the headend that tells the modem
to turn its signal level up or down. And when the splitter is
substituted
for the barrel connetor, the output level of the modem is increased.
That suggests that the splitter itself is attenuating the upstream
signal
from the modem to the CMTS, as nothing else has changed. It may
also be the splitter which is mangling the packets. I'll search around
for another splitter to see if that helps.
*TimDaniels*
"Ed Nielsen" wrote:
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:
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:
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:
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:
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:
Without the 2:1 splitter, the downstream signal level rises to
10 dBmv
(from 9 dBmV) and the upstream signal level drops to 47 dBmV (from
49 dBmV). The tracerts show a quadrupling (or more) of
timeouts with
the splitter removed. It seems that the lower transmit level
hinders the
packets' survival rate, and that the splitter, in reducing the
receive level,
causes the modem to drop its transmit level and thus increases the
timeout incidence. Is that right?
BTW, *I* put the splitter between the modem and the RR network
for analog cable TV reception of local stations. Whenever the
techs
were here, the splitter had been replaced by a barrel connector
(don't ask). As far as TW is concerned, my only service from them
is RoadRunner. But no instruments measured levels within my condo
unit. The only measurements that they took directly were on
the level
from the amp in the vault at the curb and on the output from
the building's
amp that resulted from their padding down the signal coming
from the
curb amp. The only measurements that they took specific to my
modem
were the remote readings reported to them from by cell phone
from the
central office. They said that it gave them more information
than what
I got from the modem's webpage and, presumably, what they could
learn
by tapping into the line between the wall outlet and my modem.
So it's
strange that the timeouts are so much less frequent when I put
my 2:1
splitter in the line - it is not what they thought the usual
configuration was.
*TimDaniels* |
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Todd H.
Guest
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| Posted: Wed Dec 27, 2006 11:54 am Post subject: Re: SB5120 & Comcast Woes |
|
|
"Timothy Daniels" <TDaniels@NoSpamDot.com> writes:
| Quote: | I'm afraid that I'd have to show that the speeds are dependent on
the downstream signal level, and right now that doesn't seem to be
the case. The techs seemed to be happy when they dropped it from
15 dBmV to 9 - 10 dBmV (measured at the modem with a barrel
connector between the modem and the wall).
|
This barrel connector... is it a standard female to female coupler or
does it have any attenuation built in?
| Quote: | Right now, I can't see a significant difference in ping timeouts
between the barrel connector and the splitter. Of more annoyance
right now are the long delays seen with html downloads and the news
server.
|
Intermittent problems... oy. That sucks.
In my experience with this crap, packet loss is what it boils down to.
If you can demonstrate packet loss, and you have eliminated your
router from the situation, they should be sending a tech. Unless you
own your modem, in which case, I'd throw some money at trying a new
modem.
I've had these problems with two different providers. After months of
a parade of techs coming out and saying levels looked swell, I finally
got them to replace the cable modem and voila, all problems went
away.
In the second instance, the issue was about cable length and
splitters, and later, number of directional couplers, and finally the
grade of directional coupler.
--
Todd H.
http://www.toddh.net/ |
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Todd H.
Guest
|
| Posted: Wed Dec 27, 2006 11:58 am Post subject: Re: SB5120 & Comcast Woes |
|
|
"Timothy Daniels" <TDaniels@NoSpamDot.com> writes:
| Quote: | Now the splitter seems to make no difference in packet loss (as measured
by "ping -t"), except to drop the downstream signal level to the modem
to 6-7 dBmV and to cause the modem to raise its signal level to 50 dBmV.
The splitter is a TVC unit that's rated for use to 1 GHz, and it's solder sealed.
On pings to yahoo, google, and mit, packet loss is reported as 2-3% (although
I observe twice that) and to rr.com it's reported around 10%. There seems
to be no pattern, now.
|
Even 2-3% is unacceptable.
In one instance, I had an interesting datapoint to present to the
cable company. I was seeing a marked difference between pinging the
external and internal interfaces of my cable modem.
Internal, by standard is 192.168.100.1, external IP varies of course,
and you can find yours by surfing to http://whatismyip.com/ or the
status pages of your cable modem should tell ya.
In my case, I was seeing no packet loss to the internal interface, but
several % to the external interface. They replaced the cable modem
and the problem went away. YMMV.
Best Regards,
--
Todd H.
http://www.toddh.net/ |
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Timothy Daniels
Guest
|
| Posted: Wed Dec 27, 2006 1:05 pm Post subject: Re: SB5120 & Comcast Woes |
|
|
"Warren" wrote:
| Quote: | Most -- nearly every -- cable provider won't burn a public IP address on
the external interface of a cablemodem since it will never need to be
accessed from outside the company's network. It will be assigned a
private range IP, usually a class A address (10.x.x.x).
|
Please tell me more about this 10.x.y.z. IP address. That address,
the first upstream node in my tracerts, is the only address that has no
ping dropouts. Is that the modem as seen by the PC? Everything
else upstream has a 2% and higher dropout rate.
*TimDaniels* |
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Warren
Guest
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| Posted: Wed Dec 27, 2006 1:05 pm Post subject: Re: SB5120 & Comcast Woes |
|
|
Todd H. wrote:
| Quote: | Internal, by standard is 192.168.100.1, external IP varies of course,
and you can find yours by surfing to http://whatismyip.com/ or the
status pages of your cable modem should tell ya.
|
whatismyip reports the IP address of your computer (or router), not the IP
address of your cablemodem's external interface.
Most -- nearly every -- cable provider won't burn a public IP address on
the external interface of a cablemodem since it will never need to be
accessed from outside the company's network. It will be assigned a private
range IP, usually a class A address (10.x.x.x).
And since the cablemodem is only a bridge, you will not be able to find
it's address reported by any outside sources, especially any off the
network.
The Motorola Surfboard modems do not report the external IP address of the
cablemodem on any of it's status pages, either. And I don't recall seeing
any cablemodem that will report this address to the end-user. Essentially,
the most likely way you might be able to find out the IP address of the
external interface of your cablemodem is if a CSR tells you what it is.
And that would only be accurate at the time they tell you. (And they're
not supposed to tell you what it is, anyway, so it may not be available to
them as it once was.)
--
Warren H.
==========
Disclaimer: My views reflect those of myself, and not my
employer, my friends, nor (as she often tells me) my wife.
Any resemblance to the views of anybody living or dead is
coincidental. No animals were hurt in the writing of this
response -- unless you count my dog who desperately wants
to go outside now.
Shop for networking gear:
http://www.holzemville.com/mall/linksys
http://www.holzemville.com/mall/netgear |
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Timothy Daniels
Guest
|
| Posted: Wed Dec 27, 2006 1:05 pm Post subject: Re: SB5120 & Comcast Woes |
|
|
"Ed Nielsen" wrote:
| Quote: | ... it really sounds like a network issue rather than something
local.
|
Yeah, which means that it won't get solved until other
customers run "ping -t" and discover the same problem -
which is unlikely. This is not the first or second or third
time in which I've had to diagnose a system problem for
a utility company, and it's hell to convince them that I've
really spotted something.
I once noticed noise on my car's AM radio whenever I
drove past a traffic intersection. This went on for about
six months, but phone reps didn't know how to write up
a customer's report of their own failing equipment I finally
spent a couple days networking by phone through the
power company, and I finally got an engineer to look into
it. A week later he called back and said that it was a
failing transformer, and the final outcome could have
been an explosion.
*TimDaniels* |
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Timothy Daniels
Guest
|
| Posted: Wed Dec 27, 2006 1:05 pm Post subject: Re: SB5120 & Comcast Woes |
|
|
"Todd H." wrote:
| Quote: | "Timothy Daniels" writes:
I'm afraid that I'd have to show that the speeds are dependent on
the downstream signal level, and right now that doesn't seem to be
the case. The techs seemed to be happy when they dropped it from
15 dBmV to 9 - 10 dBmV (measured at the modem with a barrel
connector between the modem and the wall).
This barrel connector... is it a standard female to female coupler or
does it have any attenuation built in?
|
No, it's just a standard coupler (it's threaded on both ends on the
outside).
| Quote: | In my experience with this crap, packet loss is what it boils down to.
If you can demonstrate packet loss, and you have eliminated your
router from the situation, they should be sending a tech. Unless you
own your modem, in which case, I'd throw some money at trying a new
modem.
|
I'll drop by Time Warner tomorrow and try to borrow a modem -
they're only 5 blocks away. If I can eliminate the modem as a
cause of the packet loss, I'll be able to prove it's a network
problem. Otherwise, I'll have to sit on the phone for an hour again
to get Tier 3 to order another tech visit and the guy may or may not
show up. Line techs have been here, but they don't do anything
about network problems, and the tech with the substitute modem
10 days ago didn't try any pings or tracerts.
*TimDaniels* |
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Warren
Guest
|
| Posted: Wed Dec 27, 2006 9:31 pm Post subject: Re: SB5120 & Comcast Woes |
|
|
Timothy Daniels wrote:
| Quote: | "Warren" wrote:
Most -- nearly every -- cable provider won't burn a public IP address
on the external interface of a cablemodem since it will never need to
be accessed from outside the company's network. It will be assigned a
private range IP, usually a class A address (10.x.x.x).
Please tell me more about this 10.x.y.z. IP address. That address,
the first upstream node in my tracerts, is the only address that has no
ping dropouts. Is that the modem as seen by the PC? Everything
else upstream has a 2% and higher dropout rate.
|
You won't see either of the cablemodem's interfaces IP addresses in any
traceroute as the cablemodem is just a bridge. The 10.x.x.x IP address you
are likely seeing is just another of the many aliases of the CMTS. (Or it
could be some other router still within the cable company's network.)
--
Warren H.
==========
Disclaimer: My views reflect those of myself, and not my
employer, my friends, nor (as she often tells me) my wife.
Any resemblance to the views of anybody living or dead is
coincidental. No animals were hurt in the writing of this
response -- unless you count my dog who desperately wants
to go outside now.
Shop for networking gear:
http://www.holzemville.com/mall/linksys
http://www.holzemville.com/mall/netgear |
|
| Back to top |
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Timothy Daniels
Guest
|
| Posted: Thu Dec 28, 2006 8:28 am Post subject: Re: SB5120 & Comcast Woes |
|
|
"Warren" wrote:
| Quote: | Timothy Daniels wrote:
"Warren" wrote:
Most -- nearly every -- cable provider won't burn a public IP address
on the external interface of a cablemodem since it will never need to
be accessed from outside the company's network. It will be assigned a
private range IP, usually a class A address (10.x.x.x).
Please tell me more about this 10.x.y.z. IP address. That address,
the first upstream node in my tracerts, is the only address that has no
ping dropouts. Is that the modem as seen by the PC? Everything
else upstream has a 2% and higher dropout rate.
You won't see either of the cablemodem's interfaces IP addresses in any
traceroute as the cablemodem is just a bridge. The 10.x.x.x IP address you
are likely seeing is just another of the many aliases of the CMTS. (Or it
could be some other router still within the cable company's network.)
|
I've again asked the RoadRunner Tier I Nat'l Help Desk what that
IP address represents, and after checking with someone else, the
rep said that it was the modem. (Not that Tier I actually knows
something, but it would explain why pings to it were always returned.)
*TimDaniels* |
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