Cable Modemery

[sechs]

I know that a number of folks here use cable Internet, so I'm hoping for some insight into my problems.

A couple of months ago, I switched to Comcast for a then-current promo. Initially, I had problems with general slowness and lots of dropping. The modem was showing lots of issues with upstream ranging, as well as with receiving IP address renewals.

A tech came out and said that my downstream power level was too high -- which seemed like an unlikely story. However, he slapped on an attenuator and re-provisioned the modem, and, the next day, someone climbed our poll and fiddled with the box up there; and things were generally better after that.

Recently, I've had connection problems again. Yesterday, the modem was unable to find an upstream channel for five hours. After soft rebooting it , as well as unplugging and replugging it, several times, I gave in and called Comcast support. They had me unplug the modem *and* disconnect the cable, then reconnect both; and that fixed the issue.

The modem that I use, a Motorola SBG900, is generally panned because it's combined modem/router/wireless, and the router and wireless bits are pretty crappy. I don't really use either of those, and the modem was "free" as part of the promo. As far as I can tell, the modem guts are the same as one of their older-model, and generally reliable, DOCSIS 2.0 Surfboard modems.

Question 1:
Can someone explain what disconnecting the cable from the modem does that disconnecting the power does not?

Question 2:
Any thoughts on what the real problem is here? It'll be a real pain to have Comcast roll a truck again to look at this.

 

[time]

I haven't had to get my hands dirty with cable modems since Motorola lifted their game a few years back. As I recall, you can check the signal level/noise yourself by browsing the modem's inbuilt http server on 192.168.100.1 (?)

If your signal is going through a passive splitter (1 in 3 out), you don't need an attenuator with a modern cable modem (AFAIK).

The only thing that disconnecting the cable could possibly do is allow the hub at the other end to detect the modem is missing, but I'd really expect the same effect by simply turning it off for a couple of minutes (or half an hour if so recommended).

In my experience, none of this is a substitute for the ISP resetting the port on the hub.

 

[time]

Regarding your second question, I'd check the S/N first - your answer may be right there. If all looks peachy, then you've got a flaky modem, and good luck convincing the ISP of that. In my several years on cable, I went through about 4 modems and 3 big fights with the ISP.

Of course, you should always try a different cable. Also, do you have any other outlets?

 

[mubs]

Is your electrical power clean? I've had really bizarre problems that went away immediately after putting in a quality surge/noise suppressor.

 

[sechs]

I haven't had to get my hands dirty with cable modems since Motorola lifted their game a few years back. As I recall, you can check the signal level/noise yourself by browsing the modem's inbuilt http server on 192.168.100.1 (?)

Here's a signal levels reading from before the technician visit:
Downstream
Signal to Noise Ratio 39.8 dB
Power Level 9.9 dBmV
Upstream
Channel ID 7
Frequency 30600000 Hz Ranged
Power Level 48.9 dBmV

And here's one that I just plucked:
Downstream
Signal to Noise Ratio 39.4 dB
Power Level 7.3 dBmV
Upstream
Channel ID 8
Frequency 25300000 Hz Ranged
Power Level 52.8 dBmV

If your signal is going through a passive splitter (1 in 3 out), you don't need an attenuator with a modern cable modem (AFAIK).

There was originally a 1-in, 2-out splitter where the drop enters the house; that was replaced with the attenuator, with the attenuation on the leg going to the cable modem. That line is furthur split by me at the jack between the tuner on on my computer and the cable modem.

The only thing that disconnecting the cable could possibly do is allow the hub at the other end to detect the modem is missing, but I'd really expect the same effect by simply turning it off for a couple of minutes (or half an hour if so recommended).

I suspected as much. I expect that the rep on the phone did that while I was futzing with the cables.

 

[sechs]

Is your electrical power clean? I've had really bizarre problems that went away immediately after putting in a quality surge/noise suppressor.

I don't know about clean power, but, like practically all of my equipment, it's plugged into a UPS. I only have basic monitoring of that particular UPS through my NAS, but another UPS plugged into the same outlet doesn't show any problems on a regular basis.

Sometimes, when I turn on the floor lamp, the voltage must sag, as I hear the UPS click on to boost the voltage.

 

[Stinker]

Sechs- THe signal coming into your house has problems or you got too many splits going on before your cable modem. Lots of techs on this forum will help you.

http://www.dslreports.com/forum/comcast

Most techs there will say 7dB is the upper (or lower) limit for power level. All of them will usually say that a dedicated line should go to the cable modem at the first split inside your house.

You want your downstream power level to be close to 0dB. Adding a 6dB attenuator in front of your cable modem should get you a good downstream number. Problem is, that will also cause your upstream power level to increase 6dB. Your upstream is already on the high side. But start with the attenuator and see what it does for you. They're like $5 if you shop Amazon.

This image is a little old but the numbers should still be in the ballpark. BTW, your SNR is great both up and down.

http://img101.imageshack.us/img101/8676/signallevels.jpg

 

[Stinker]

Antronix is a good brand.

Also, you can contact Comcast support directly here....

http://www.dslreports.com/forum/comcastdirect

 

[time]

There's nothing wrong with your modem stats, but you'd expect that because it's currently working ... You'd have to check when you started to have dropouts.

Because your S/N is so good, there isn't much to worry about. The extra 2.5dB attenuation (6 - 3.5) has pushed your upstream closer to the maximum, but as Stinker's graphic shows, even 54-55dBmV shows there is sufficient signal to be able to tolerate a 3dBmV variation (58dBmV means signal can no longer be maintained at the desired level).

If it happens again, I'd yank either the attenuator or splitter and see if that helps. The splitter is another variable all by itself, but I imagine you've already tried all that. I'd have to agree with your implied sentiment, that the original tech didn't achieve much at your house, and the solution was elsewhere (eg up the pole).

 

[Howell]

I've never felt as much the dunce as to read this chat and have no idea what it means. : )

 

[sechs]

Lots of techs on this forum will help you.

Actually. "lots of techs on this forum" simply told me to replace my modem without much additional inquiry into the situation. That's Comcastic!

 

[sechs]

There's nothing wrong with your modem stats, but you'd expect that because it's currently working ... You'd have to check when you started to have dropouts.

Because your S/N is so good, there isn't much to worry about. The extra 2.5dB attenuation (6 - 3.5) has pushed your upstream closer to the maximum, but as Stinker's graphic shows, even 54-55dBmV shows there is sufficient signal to be able to tolerate a 3dBmV variation (58dBmV means signal can no longer be maintained at the desired level).

The problem of course, is that it's intermittent. Bigger, perhaps, is that the modem is obviously crapping out trying to lock onto an upsteam channel. I can't get S/N information on that, because it's measured at Comcast's end.

If it happens again, I'd yank either the attenuator or splitter and see if that helps. The splitter is another variable all by itself, but I imagine you've already tried all that. I'd have to agree with your implied sentiment, that the original tech didn't achieve much at your house, and the solution was elsewhere (eg up the pole).

It happened again, and the fix was the same: call in, follow their instructions to disconnect and reconnect the cables, and tada!

I've arranged for another truck roll, anyway.

I suspect that the attenuator and fooling around at the pole was overkill, and I'm now running against some intermittent situation, which is, in turn, causing some barfing between my modem and the upstream concentrator. The technician should be able to take numbers at the wire going into the modem and swap the attenuator back to a simple splitter.

 

[Stinker]

Actually. "lots of techs on this forum" simply told me to replace my modem without much additional inquiry into the situation. That's Comcastic!

Wow. My experience was much different than yours. I guess it depends on how you pose your questions, if you come across with an attitude, and if you get lucky.

 

[Mercutio]

My opinion of Comcast is so low that I put up with service that was apparently about 10% of what I was supposed to be getting for around 18 months. I'm actually a little bit shocked that they're being as helpful as the are in your case, sechs. All I ever seem to get from them is "blame the last contractor they sent to do work."

 

[sechs]

Wow. My experience was much different than yours. I guess it depends on how you pose your questions, if you come across with an attitude, and if you get lucky.

Actually, it has to do with how they don't pose questions, come across with an attitude, and if they get lucky....

 

[sechs]

My opinion of Comcast is so low that I put up with service that was apparently about 10% of what I was supposed to be getting for around 18 months. I'm actually a little bit shocked that they're being as helpful as the are in your case, sechs.

I really dislike Comcast, but I really like a good deal. Another "you get what you pay for" situation.

I'm not really sure that they're being helpful so much as looking to collect that $50 fee for it somehow being my fault. They obviously haven't noticed the number of truck rolls in my neighborhood.

I'm planning on switching the Internet after the six-month promo is up. AT&T will certainly give me a nice deal, even if their service is slower (but stable!).

 

[Stinker]

I have at least two mainstream alternatives to Comcast for internet in my area. That probably has to do more with how the handle their customer service more than anything else I guess... For me Comcast gives me the best speeds and most options....at a lower cost than any of them. Hope you get your stability issue worked out!

 

[sechs]

Technician came out, saw that the cable was running through my UPS (for surge protection) and said that was the issue. Except that, when he took the numbers, it didn't make much of a difference whether the signal passed through it or not. Strike one.

Then he tried to blame my splitter. He swapped it with one that he had, but, alas, it didn't make any difference on the numbers. Strike two.

Finally, he settles on something almost reasonable: ingress on the cable running through the wall. Since I have some fairly comcastic RG59 here, it's actually a plausible explanation; the numbers apparently show a lot more noise at the jack than at the demarc. Except that I didn't have any problems running Internet with Astound on the same line. Tech had no explanation for that. Strike three.

So, as expected, he replaced the attenuator with a regular splitter and left it at that. It's been stable so far, but I'll wait and see.

 

[time]

Wow, that really sucks.

Still, there's an extra 2.5dBmV for you to play with ... wneddnce:

 

[Stinker]

Technician came out, saw that the cable was running through my UPS (for surge protection) and said that was the issue. Except that, when he took the numbers, it didn't make much of a difference whether the signal passed through it or not. Strike one.

Not too surprised he saw that as a potential problem. I had a Cyberpower surge strip that I had my coaxial signal running through on just one of my TVs and it was causing problems with the picture/sound on it. Took me a long time to figure it out because I never thought that would be the source of the problem! It was causing the SNR to take a big dive. Definitely not the situation in your case however since your SNR was high...

If you have Motorola cable boxes, you can get to the diagnostic screen for them by hitting the Select button on your remote within about 1 second of turning it off. Might help you to know what the signals are like at different parts of your coaxial network in your house.

One other thing I recommend is connect your cable modem directly to the line coming into your house (prior to any splits). Then, you can get an idea of what the power levels and SNR is before your home coaxial network comes into play by accessing the cable modem diagnostic screens.

 

[Pradeep]

Had an old Motorola surfboard. When TWC boosted speeds above 10mbit/sec on the dl, they had us bring in our old modems and they replaced them with new ones. Much smaller, even hotter running. But faster.

 

[sechs]

Had an old Motorola surfboard. When TWC boosted speeds above 10mbit/sec on the dl, they had us bring in our old modems and they replaced them with new ones. Much smaller, even hotter running. But faster.

I suspect that this was due to a DOCSIS version upgrade.

If I were to upgrade to two tiers up, I'd need a DOCSIS 3 modem. Where I am, this DOCSIS 2 modem (should) work fine.