SB5120 & Comcast Woes

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:

Reply to
Ed Nielsen
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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:

Reply to
Ed Nielsen

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:

Reply to
Timothy Daniels

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:

Reply to
mediumhappy

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:

Reply to
Timothy Daniels

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:

Reply to
Ed Nielsen

If you are in an area fed from poles (aerial), go outside and follow your drop to the line. You will see a small box called a tap. There should be a number showing. Subtract that number from your modem transmit level. Then subtract 3 dB for every 2-way splitter, 7dB for every 4 way and 11dB for every 8 way in the line between your modem and the tap. Subtract 1dB for every 100' of coax. The remainder should be equal to between 15 and 20dB (most systems that were rebuild by AT&T Broadband should be about 17dB).

Example:

26-4 tap 4 way splitter 150' drop 50' run to outlet 17dB amplifier drive level modem should transmit at 52dB

Usually, systems are designed to make modems run about 45-50dB. Lower value taps (further away from an amp) take into account increased cable loss, splitters, and other devices. The tap in the example above will be directly connected to an amplifier.

Reply to
Eric

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:

Reply to
Timothy Daniels

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:

Reply to
Ed Nielsen

That's an important thing to keep in mind. TCP/IP packets is just the data that's carried over an RF signal on the coax. You could have 100% or 0% packet loss, and it wouldn't have any affect upon how the RF signal levels are negotiated.

In other words, the railroad tower operator doesn't care if the trains have people in them or not, as long as they're arriving at and leaving the stations at the correct time. The railroad tower operator doesn't even know if the arriving train had any people on it when it left the last station, so even if it was full when it left station one, if it arrives at station number two on-time, it's a success even if the train is now empty.

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.

Reply to
Warren

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:

Reply to
Timothy Daniels

Yes - if someone had said that. I certainly didn't.

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*
Reply to
Timothy Daniels

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:

Reply to
Ed Nielsen

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.

Ah ha... is this thing designed for teh relevant bandwidth of that cable system though...

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.

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,

Reply to
Todd H.

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:

Reply to
Timothy Daniels

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.

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*
Reply to
Timothy Daniels

While scrubbing graffiti off the comp.dcom.modems.cable,alt.online-services.roadrunner walls, I heard Timothy Daniels say:

[...]

And how much heat would the $600 or so your paying Comcast or TWC per year buy? ;-)

Reply to
Dutch

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:

Reply to
Ed Nielsen

This barrel connector... is it a standard female to female coupler or does it have any attenuation built in?

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.

Reply to
Todd H.

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

formatting link
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,

Reply to
Todd H.

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