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MB8600 - Millions of uncorrected errors, low signal to noise ratio (SNR), slow gigabit

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MB8600 - Millions of uncorrected errors, low signal to noise ratio (SNR), slow gigabit

Hi!

 

I currently pay for Xfinity Gigabit cable internet.

 

My current hardware setup is:

Modem: Motorola MB8600

Router: Synology RT2600ac

 

My problem is I am not getting full gigabit speeds (~940Mbps) when I test via https://speedtest.xfinity.com/. In the past (e.g. a couple of days to a week ago) I did get full (~940Mbps) speed test results.

 

I have seen my downstream speed test results as low as 75Mbps, up to 130Mbps, all the way up to ~700Mbps, but the results are not consistent and can go for multiple hours with very slow results (e.g. < 300Mbps).

 

My upstream results seem to be as expected - up to 40Mbps.

 

The computer I test from is hardwired into my router. I am not testing over wifi.

 

Things I have already tried:

When I look at the connection page I see the following:

https://imgur.com/BH7MIIo

 

Downstream Bonded Channels

 

 Channel Lock Status Modulation Channel ID Freq. (MHz) Pwr (dBmV) SNR (dB ) Corrected Uncorrected
   1 Locked QAM256 36 645.0 -6.9 38.9 8 0
   2 Locked QAM256 8 471.0 -8.9 38.5 3 0
   3 Locked QAM256 9 477.0 -8.2 38.7 0 0
   4 Locked QAM256 10 483.0 -6.8 33.1 0 0
   5 Locked QAM256 11 489.0 -6.5 35.3 0 0
   6 Locked QAM256 12 495.0 -7.3 38.7 1 0
   7 Locked QAM256 13 507.0 -7.4 39.1 0 0
   8 Locked QAM256 14 513.0 -6.6 39.3 0 0
   9 Locked QAM256 15 519.0 -7.0 39.4 0 0
   10 Locked QAM256 16 525.0 -7.4 38.9 0 0
   11 Locked QAM256 17 531.0 -7.2 39.0 2 0
   12 Locked QAM256 18 537.0 -6.0 39.3 3 0
   13 Locked QAM256 19 543.0 -6.7 39.1 4 0
   14 Locked QAM256 20 549.0 -6.8 31.2 7355 17
   15 Locked QAM256 21 555.0 -7.5 37.0 15 0
   16 Locked QAM256 22 561.0 -7.0 29.6 3437 0
   17 Locked QAM256 23 567.0 -5.7 27.1 2885440 1196
   18 Locked QAM256 24 573.0 -6.8 28.7 35887 922
   19 Locked QAM256 25 579.0 -6.6 0.0 25541696 13741353
   20 Locked QAM256 26 585.0 -7.1 30.2 283 0
   21 Locked QAM256 27 591.0 -6.0 29.1 10241734 35233
   22 Locked QAM256 28 597.0 -5.6 37.3 2 0
   23 Locked QAM256 29 603.0 -6.3 38.8 12 0
   24 Locked QAM256 30 609.0 -6.2 39.1 3 0
   25 Locked QAM256 31 615.0 -6.3 36.5 3 0
   26 Locked QAM256 32 621.0 -5.3 36.0 15 0
   27 Locked QAM256 33 627.0 -5.8 39.2 8 0
   28 Locked QAM256 34 633.0 -6.7 39.0 11 0
   29 Locked QAM256 35 639.0 -6.6 39.0 17 0
   30 Locked QAM256 37 651.0 -5.9 38.9 8 0
   31 Locked QAM256 38 657.0 -7.3 36.6 13 0
   32 Locked OFDM PLC 48 850.0 -10.9 27.5 35631238 0

 

Upstream Bonded Channels

 

 

Channel Lock Status Channel Type Channel ID Symb. Rate (Ksym/sec) Freq. (MHz) Pwr (dBmV)
   1 Locked SC-QAM 1 5120 19.3 45.5
   2 Locked SC-QAM 2 5120 25.7 44.8
   3 Locked SC-QAM 3 5120 32.3 44.8
   4 Locked SC-QAM 4 5120 38.7 44.8


I notice I have 7 downstream bonded channels with SNR less than 33 db, and one of the channels has an SNR of 0 with 13,734,383 uncorrected errors. I have another downstream bonded channel with 25,208 uncorrected errors and low SNR.

 

In my event log I see a large number of messages like:

Warning (5) MDD message timeout...
Notice (6) CM-STATUS message sent...
 

I have tried calling Comcast tech support. We ran through the standard debugging steps and nothing seemed to have any effect.

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Diamond Problem Solver

Re: MB8600 - Millions of uncorrected errors, low signal to noise ratio (SNR), slow gigabit

Can you hook up your modem at the point of entry (to bypass your house wiring), and see if your levels change? If not, you’ll need a tech out to troubleshoot outside

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Regular Visitor

Re: MB8600 - Millions of uncorrected errors, low signal to noise ratio (SNR), slow gigabit

Hi @CCAndrew, thank you for the reply!

 

I don't know how to hook up my modem at the point of entry (to bypass my house wiring). Do you have any guidance on how to do that?

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Diamond Problem Solver

Re: MB8600 - Millions of uncorrected errors, low signal to noise ratio (SNR), slow gigabit

Try to find the ground block or the cable inside before it hits any splitters. Hook that cable up to your modem and power it up, read your levels just like you did above. It’s a way to prove it’s an outside issue vs inside issue. Also, if you do come across splitters, connectors etc are tight, anything loose could possibly give you the errors and SNR and signal loss. Basically, doing what you can before getting a tech out

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Expert

Re: MB8600 - Millions of uncorrected errors, low signal to noise ratio (SNR), slow gigabit

Some things that you can try before getting a tech visit;

In a self troubleshooting effort to try to obtain better connectivity / more wiggle room, check to see if there are there any excess/unneeded coax cable splitters in the line leading to the modem that can be eliminated/re-configured. Any splitters that remain should be high quality and cable rated for 5-1002 MHz, bi-directional, and no gold colored garbage types like GE, RadioShack, RCA, Philips, Leviton, Magnavox, and Rocketfish from big box stores like Home Depot, Lowes, Target, Wal-Mart etc. Splitters should be swapped with known to be good / new ones to test

If there aren't any unneeded splitters that can be eliminated and if your coax wiring setup can't be reconfigured so that there is a single two way splitter connected directly off of the drop from the street/pole with one port feeding the modem and the other port feeding the rest of the house/equipment with additional splits as needed, and you've checked all the wiring and fittings for integrity and tightness and refresh them by taking them apart then check for and clean off any corrosion / oxidation on the center wire and put them back together again, then perhaps it's best to book a tech visit to investigate and correct.

 

Good luck with it !



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Re: MB8600 - Millions of uncorrected errors, low signal to noise ratio (SNR), slow gigabit

Hi @EG and @CCAndrew -  thank you very much to both of you. You helped me solve my problem!

 

How I fixed my problem - short version

 

As both @CCAndrew and @EG said:

I connected the cable inside my house to my modem before it hit any splitters and my problems disappeared.

 

 

How I fixed my problem - long version

 

After I followed this advice from @CCAndrew :

"Try to find the ground block or the cable inside before it hits any splitters. Hook that cable up to your modem and power it up, read your levels just like you did above."

 

And this advice from @EG :

"...check to see if there are there any excess/unneeded coax cable splitters in the line leading to the modem that can be eliminated/re-configured..."

 

I got the following readings from my Mororola MB8600:

 

Downstream Bonded Channels

 

Channel Lock Status Modulation Channel ID Freq. (MHz) Pwr (dBmV) SNR (dB ) Corrected Uncorrected
   1 Locked QAM256 36 645.0 -2.0 39.6 0 0
   2 Locked QAM256 8 471.0 -4.1 39.5 0 0
   3 Locked QAM256 9 477.0 -3.5 39.6 0 0
   4 Locked QAM256 10 483.0 -2.1 40.2 0 0
   5 Locked QAM256 11 489.0 -1.7 40.1 0 0
   6 Locked QAM256 12 495.0 -2.5 39.6 0 0
   7 Locked QAM256 13 507.0 -2.8 39.7 0 0
   8 Locked QAM256 14 513.0 -1.9 40.1 0 0
   9 Locked QAM256 15 519.0 -2.1 39.9 0 0
   10 Locked QAM256 16 525.0 -2.9 39.7 0 0
   11 Locked QAM256 17 531.0 -2.6 39.7 0 0
   12 Locked QAM256 18 537.0 -1.6 39.8 0 0
   13 Locked QAM256 19 543.0 -1.8 39.9 0 0
   14 Locked QAM256 20 549.0 -2.4 39.7 0 0
   15 Locked QAM256 21 555.0 -3.1 39.5 0 0
   16 Locked QAM256 22 561.0 -2.2 39.7 0 0
   17 Locked QAM256 23 567.0 -1.4 39.7 0 0
   18 Locked QAM256 24 573.0 -1.9 39.4 0 0
   19 Locked QAM256 25 579.0 -2.3 39.5 0 0
   20 Locked QAM256 26 585.0 -2.4 39.6 0 0
   21 Locked QAM256 27 591.0 -1.4 39.9 0 0
   22 Locked QAM256 28 597.0 -1.1 40.0 0 0
   23 Locked QAM256 29 603.0 -1.6 39.9 0 0
   24 Locked QAM256 30 609.0 -1.9 39.8 0 0
   25 Locked QAM256 31 615.0 -1.6 39.8 0 0
   26 Locked QAM256 32 621.0 -0.8 40.0 0 0
   27 Locked QAM256 33 627.0 -1.5 39.8 0 0
   28 Locked QAM256 34 633.0 -1.8 39.7 0 0
   29 Locked QAM256 35 639.0 -2.2 39.5 0 0
   30 Locked QAM256 37 651.0 -1.4 39.7 0 0
   31 Locked QAM256 38 657.0 -2.6 39.4 0 0
   32 Locked OFDM PLC 48 657.0 0.0 35.8 89911756 0

 

Great signal to noise ratio (SNR) with no corrected or uncorrected errors except on the OFDM PLC channel, which I believe is to be expected. Amazing!

 

Upstream Bonded Channels

 

Channel Lock Status Channel Type Channel ID Symb. Rate (Ksym/sec) Freq. (MHz) Pwr (dBmV)
   1 Locked SC-QAM 1 5120 19.3 41.8
   2 Locked SC-QAM 2 5120 25.7 41.0
   3 Locked SC-QAM 3 5120 32.3 40.8
   4 Locked SC-QAM 4 5120 38.7 41.0

 

Speed Test Results (https://speedtest.xfinity.com/results - hardwired)

 

Download speed 900.4 Mbps
Upload Speed 39.5 Mbps
Protocol IPv6

 

Note: While I didn't get 940 Mbps (real world maximum over gigabit LAN), I wasn't directly wired into my modem. I had other people on the network gaming, playing music, etc, so I am happy with this result all things considered.

 

I wonder if there is any difference between doing the speed test over IPv6 vs IPv4?

 

Summary of how I fixed my problem

 

As both @CCAndrew and @EG said:

I connected the cable inside my house to my modem before it hit any splitters and my problems disappeared.

 

Since I only have cable internet (no cable TV), I don't need splitters. 

 

Thank you both so much!

 

It seems like it was either a bad splitter, poor connections between splitter and connected cables, a bad cable after the splitter or all of the above. Either way, I am happy now to be back to full speeds!

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Diamond Problem Solver

Re: MB8600 - Millions of uncorrected errors, low signal to noise ratio (SNR), slow gigabit

You’re welcome, you may want to still get a tech out sometime. Your downstream “shouldn’t” be in the negatives with no splitters. But definitely looks better

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Regular Visitor

Re: MB8600 - Millions of uncorrected errors, low signal to noise ratio (SNR), slow gigabit


@CCAndrew wrote:
You’re welcome, you may want to still get a tech out sometime. Your downstream “shouldn’t” be in the negatives with no splitters. But definitely looks better

Thank you for the advice @CCAndrew. I'll see if I can get a tech out to check things out at some point.

 

To help me learn more, what effect might negative downstream power levels have on my service?

 

Also, should I be worried about upstream power levels less than 42 dBmV? I read on https://www.speedguide.net/faq/what-cable-modem-signal-levels-are-considered-good-78 that ideally upstream power levels want to be above 42 dBmV.

 

Just trying to figure out what SNR and power levels I would see with ideal conditions.

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Diamond Problem Solver

Re: MB8600 - Millions of uncorrected errors, low signal to noise ratio (SNR), slow gigabit

Everything is a range but some numbers (usually in the middle of that range) are ideal. Upstream between 35 and 52, downstream 10 and -10 and SNR above 35 but closer to 39-41 is great. There’s nothing “wrong” with your levels, you’re in the range. There may not be anything wrong it could be a combination of low levels outside, distance from pedestal to the house and then distance to the modem. There is loss of signal over distance

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