Moto SB5100 signal drop

Moto SB5100 signal drop
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Subject Author Date
Moto SB5100 signal drop Chuck Reti 04-21-2006
Posted by Chuck Reti on April 21, 2006, 10:36 pm
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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

Posted by $Bill on April 22, 2006, 2:27 am
Chuck Reti 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
> 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.

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

> 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?

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

> 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?

Posted by Andrew Rossmann on April 22, 2006, 10:59 am
[This followup was posted to comp.dcom.modems.cable and a copy was sent
to the cited author.]

chuckreti@comcast.net.invalid says...
> 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 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

Posted by Robert Nichols on April 22, 2006, 12:22 pm
: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.

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"

Posted by Chuck Reti on April 22, 2006, 6:13 pm

> 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. 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

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