I have a Motorola SB5100, about a year old. This past week I have been
frequently losing connection to my ISP (Comcast Detroit).
Cable feed is separate drop from pole, just before modem hits a splitter
one (-3.5dB) leg to modem, other (-3.5dB) to a TV in same room (yes, I
subscribe to video service, and there are no traps anywhere along the
feed). This splitter and cabling is a commonly used arrangement, exactly
same as cable company installations.
Last week the SB5100 was suddenly experiencing frequent loss of signal,
"send" lamp blinking and "online" lamp out. Pulled up the modem
diagnostic pages, showed Downstream signal at -3 dBmV, s/n 35dB;
Upstream 55dBmV.
I removed the splitter and show, predictably, Downstream 0 to +1 or +2
dBmV, s/n 36-37dB, and with the splitter out, seem to be holding on to
the signal.
I am quite certain that in the past, signal levels with the splitter
have not been any different than what I'm getting now. Since I've been
running with this arrangement (splitter in system) for almost a year,
does this problem indicate something suddenly wrong with the modem, or
just marginal signal levels from Comcast. It would seem odd to me for a
modem to suddenly be sensitive to a small variation in signal level.
Anyone with similar experience?
I did call Comcast, attempted to explain the situation. A tech came by,
stuck a meter on the line at the pole and in the room and declared that
signal levels were at spec. He was predictably mystified when I showed
him the SB5100 Configuration Manager and the Signal page
http://192.168.100.1/signal.html.
He had no idea such a thing was available. So I'm not optimistic that I
can be successful at convincing Comcast that there may be a small tweak
on their end, headend, access point, line amp, to make this problem go
away. Or do I just lose the splitter and go back to over-the-air TV in
the computer room?

Chuck Reti
Detroit MI

Chuck Reti wrote:

Your upstream could be a bit lower and SNR is OK, but could be a tad higher.

See if it persists and you may want to borrow a modem to see if your
numbers change.

[This followup was posted to comp.dcom.modems.cable and a copy was sent
to the cited author.]

In article <chuckreti-78AB59.22360221042006@comcast.dca.giganews.com>,
chuckreti@comcast.net.invalid says...
I think the real issue is the upstream strength. 55dB is marginal. If
the Receive light is solid, but Send is blinking, the modem is trying to
establish an uplink.

It's hard to fix upstream problems. Upstream uses frequencies below
54MHz (TV channel 2). It is more sensitive to poor ground connections
which can let noise in. I would fully check all your connections, maybe
even loosening and retightening them to make certain it is a clean
contact.

I've heard of RF amps that can amplify BOTH directions, but they are
pricey.

--
If there is a no_junk in my address, please REMOVE it before replying!
All junk mail senders will be prosecuted to the fullest extent of the
law!!
http://home.att.net/~andyross

In article <chuckreti-78AB59.22360221042006@comcast.dca.giganews.com>,
Chuck Reti <chuckreti@comcast.net.invalid> wrote:
:I have a Motorola SB5100, about a year old. This past week I have been
:frequently losing connection to my ISP (Comcast Detroit).
:Cable feed is separate drop from pole, just before modem hits a splitter
ne (-3.5dB) leg to modem, other (-3.5dB) to a TV in same room (yes, I
:subscribe to video service, and there are no traps anywhere along the
:feed). This splitter and cabling is a commonly used arrangement, exactly
:same as cable company installations.
:Last week the SB5100 was suddenly experiencing frequent loss of signal,
:"send" lamp blinking and "online" lamp out. Pulled up the modem
:diagnostic pages, showed Downstream signal at -3 dBmV, s/n 35dB;
:Upstream 55dBmV.
:I removed the splitter and show, predictably, Downstream 0 to +1 or +2
:dBmV, s/n 36-37dB, and with the splitter out, seem to be holding on to
:the signal.

There is a problem with the upstream signal level. At 55 dBmV your
modem is screaming just about as loud as it can in order to make itself
heard, and if the upstream loss increases just a little bit you will
lose the connection. What is the upstream power level with the splitter
removed?

--
Bob Nichols AT comcast.net I am "RNichols42"

In article <e2dl83$5uc$1@omega-3a.right.here>,
Robert Nichols <SEE_SIGNATURE@localhost.localdomain.invalid> wrote:

Oddly/interestingly enough, upstream reads 55 dBmV either way. The
downstream numbers change favorably, 0 to +2 or +3; 37 db s/n with
splitter out as opposed to -3 dBmV / 35 db s/n with splitter in.
Am experiencing solid service, no drops with the cable direct into the
modem. May try a directional coupler instead of a splitter, about 1 dB
loss to the modem and 4 to 6 to the TV which would be fine.
Don't have any idea how to convince Comcast that it's probably something
on THEIR end. They'll just want to send out another guy with a wrench
and a crimp tool.
I'm still not suspicious of the modem itself, but may give Moto a call
as well.
--
Chuck Reti
Detroit MI

Chuck Reti wrote:

If you've got a next door neighbor with cable, maybe you could swap
modems and see if things change.

In article <chuckreti-5A8719.18130222042006@comcast.dca.giganews.com>,
Chuck Reti <chuckreti@comcast.net.invalid> wrote:
:In article <e2dl83$5uc$1@omega-3a.right.here>,
: Robert Nichols <SEE_SIGNATURE@localhost.localdomain.invalid> wrote:
:
:> There is a problem with the upstream signal level. At 55 dBmV your
:> modem is screaming just about as loud as it can in order to make itself
:> heard, and if the upstream loss increases just a little bit you will
:> lose the connection. What is the upstream power level with the splitter
:> removed?
:
:Oddly/interestingly enough, upstream reads 55 dBmV either way.

That confirms that the head end is telling your modem, "Yell as loud as
you can, I can barely hear you." The problem could be your modem not
putting out as strong a signal as it thinks it is, or it could be
trouble in the transmission path. The easiest thing to try is a
replacement modem. Why not just rent a modem from Comcast for a month
or two. If that fixes the problem you can buy a new modem and cancel
the rental. If the problem persists, you can now tell Comcast, "It's
your modem and your system, make it work."

BTW, when you phone in to tech support, they should be able to see
what your upstream signal level and SNR are at the head end.

--
Bob Nichols AT comcast.net I am "RNichols42"

Chuck Wrote: "Oddly/interestingly enough, upstream reads 55 dBmV either
way. The
downstream numbers change favorably, 0 to +2 or +3; 37 db s/n with
splitter out as opposed to -3 dBmV / 35 db s/n with splitter in. "

Looks like your modem is working properly. The cable techs are not.
Sorry you have to live in an area with lousy service. If you can go to
your local office (where the techs are dispatched), ask to speak to a
technical supervisor. Go around 8:30 or so, adn bring along a printout
of your signal.html page and explain the situation. HOPEFULLY, he will
send out a tech who knows what he is doing and get things resolved. Of
course, there's a high likelyhood that the tech sup. won't have a clue
either, but maybe he can find someone who does.

BTW: the upstream tx power level not changing sometimes happens when
you are attched to a Cisco CMTS (the headend equipment that converts
data to radio frequencies). Cisco designed their CMTS to be a little
more tolerant of level changes in favor of high carrier to noise
ratios, at least as far as I can tell. Other manufacturers are much
less tolerant of reciever levels. Often times I can see recive levels
from a misbehaving modem to be -5dB, where on an Arris pizzabox or ADC
'Cuda (yes, that's the name. The guy who first designed the thing has a
blue Plymouth Baracuda) will typically reset a modem if the receive
level falls below -1dB.

Thanks all, for the replies and information.

Chuck R.

On Sun, 23 Apr 2006 13:30:59 +0000 (UTC), Robert Nichols
<SEE_SIGNATURE@localhost.localdomain.invalid> wrote:

Comcast ran a separate line to my modem - avoiding all splitters but
the one on the access box. That fixed the upstream # and it has worked
great ever since. Only problem is that they cut the line short so it
can't be tucked away very well.

In article <hnjp42d1dvgovbhfj1f3jt17l801mg7l57@4ax.com>,
Jim T. <suenjim4.badaddress@comcast.net> wrote:

The line to my modem is also a separate drop from the pole, running
directly into the upstairs computer room. I'm taking one split off it to
feed a tv in that room.
I'm thinking the Motorola modem might be not as tolerant to my
marginally low signal levels as it should be. My neighbor has a
Thompson/RCA modem (Comcast self-install kit) and has -4.2 dBmV 33dB s/n
up and 54 dBmV down and claims he has no service drops.
--
Chuck Reti
Detroit MI

Chuck Reti wrote:

But does he get the same numbers if you plug it in at your house is the Q.