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sunmast
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13 Messages

Tuesday, January 24th, 2023 6:35 AM

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MB8611 OFDM PLC pwr (dBmv) too high

I was having a lot of issues with my MB8611 modem too, but I realized the problem was due to too high pwr in downstream channels. It was solved (but not completely) by adding two splitters in the line, so the pwr of QAM256 channels are dropped into the -10 ~ 10 dBmv range (except OFDM PLC).

Now the connectivity is sort of stable, but the modem still reboots every 1-2 days. I suspect OFDM PLC is the issue because it still gets 14.4 dBmv. I'm hesitate to add more splitters, because I noticed the upstream pwr has increased a lot after two splitters being added. Now it's close to the 50 dBmv limit.

Please advice. Thanks.

  Downstream Bonded Channels  
  
   Channel Lock Status Modulation Channel ID Freq. (MHz) Pwr (dBmV) SNR (dB) Corrected Uncorrected
   1 Locked QAM256 20 531.0 7.0 42.6 0 0
   2 Locked QAM256 1 417.0 5.0 39.0 0 0
   3 Locked QAM256 2 423.0 5.1 41.4 0 0
   4 Locked QAM256 3 429.0 5.0 40.5 0 0
   5 Locked QAM256 4 435.0 5.1 39.6 0 0
   6 Locked QAM256 5 441.0 5.1 40.7 0 0
   7 Locked QAM256 6 447.0 5.3 40.8 0 0
   8 Locked QAM256 7 453.0 5.4 41.2 0 0
   9 Locked QAM256 8 459.0 5.5 40.4 0 0
   10 Locked QAM256 9 465.0 5.7 41.6 0 0
   11 Locked QAM256 10 471.0 5.8 41.0 0 0
   12 Locked QAM256 11 477.0 6.3 40.9 0 0
   13 Locked QAM256 12 483.0 6.2 42.2 0 0
   14 Locked QAM256 13 489.0 6.3 41.3 0 0
   15 Locked QAM256 14 495.0 6.6 40.9 0 0
   16 Locked QAM256 15 501.0 6.5 42.5 0 0
   17 Locked QAM256 16 507.0 6.5 42.2 0 0
   18 Locked QAM256 17 513.0 6.6 42.2 0 0
   19 Locked QAM256 18 519.0 6.8 42.8 0 0
   20 Locked QAM256 19 525.0 6.8 42.8 0 0
   21 Locked QAM256 21 537.0 6.7 42.3 0 0
   22 Locked QAM256 22 543.0 6.9 43.7 0 0
   23 Locked QAM256 23 549.0 6.9 43.8 0 0
   24 Locked QAM256 24 555.0 6.7 43.5 0 0
   25 Locked QAM256 25 561.0 6.9 43.7 0 0
   26 Locked QAM256 26 567.0 6.9 43.1 0 0
   27 Locked QAM256 27 573.0 7.4 43.0 0 0
   28 Locked QAM256 28 579.0 7.2 43.1 0 0
   29 Locked QAM256 29 585.0 7.7 41.7 0 0
   30 Locked QAM256 30 591.0 8.0 43.6 0 0
   31 Locked QAM256 31 597.0 8.0 42.4 0 0
   32 Locked QAM256 32 603.0 8.4 42.0 0 0
   33 Locked OFDM PLC 193 957.0 14.4 45.0 255401 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 16.4 45.0
   2 Locked SC-QAM 2 5120 22.8 44.5
   3 Locked SC-QAM 3 5120 29.2 44.5
   4 Locked SC-QAM 4 5120 35.6 44.0

Question

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Updated 

2 years ago

 • Edited

1.2K

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Accepted Solution

flatlander3

Problem Solver

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1.5K Messages

2 years ago

In transmission line theory, you got a potential for a reflection at every discontinuity (connector) in the line, including the one where the coax screws into the back of the modem/gateway.  That can be an overshoot, undershoot or critical damp.  It is frequency dependent.  Circuit board layout makes a difference at the frequencies of interest, although to a lesser degree (~1Ghz start doing something, usually ignored at 500Mhz).  That looks like a pretty fresh boot and may change quite a bit over time. 

What does it do on a clean run and does the uncorrectable error count on the ODFM PLC counter climb up -- the correctable errors are just part of the protocol and will happen all the time. That's actually "normal", and cost you nothing.  Hardware did it on the fly.  Uncorrectables are a problem.

Perhaps a single forward path attenuator with clean cable runs (good connectors) is a better plan than multiple splitters.  They're cheap.  Come in different attenuation values (-4, -6, -10dB, etc).  After a drunken neighbor ran over the cable box in the yard, I had to put one in after they "fixed the cable".  There's some voodoo involved (Bacardi...waving chickens over the modem.....) it's not fiber.

Upstream is trying to "push signal" -- if that's drifting up, it's having problems shoving signal and increasing the power.

(edited)

4

sunmast

Visitor

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13 Messages

@flatlander3​ thanks for the detailed information.

I'm not actually worrying about correctable errors. What's bothering me is the pwr level of the OFDM PLC channel which is outside of recommended -10 ~ 10 dBmv range. Do you think if it's a problem?

Reg. attenuator - if it's going to put more resistance in the downstream direction like splitters, I believe they'll also push up upstream power level...

2 years ago

0

flatlander3

Problem Solver

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1.5K Messages

@sunmast​ Yeah.  And reboots when you see the 15dBmv range and higher on downstream power.  I was seeing that with my stuff.  I couldn't find a 10dB attenuator at the time, so I just picked up a 6dB one for a test.  You might be able to find one you can get in a reasonable time frame in your market.  I'd post the amazon link, but the bot doesn't like that so just search for "forward path attenuator" on amazon.  If you see one with a plastic cap on it, that's just a shipping plug, it's actually threaded like regular coax.

My 6dB one could be a higher value, but it works for me to knock the signal down a bit.  It's really just a one directional filter (forward path), so it won't do anything for your upstream, but pushing through a couple of splitters plus the connectors may be causing that to drift up, and that really nice frequency dependent increase on your downstream power.  Ideally, the open ends on a splitter would be terminated anyway -- an open port is a stub tuner and unless it's intentional, that's a signal discontinuity too.  I don't like the wonky spread on signal power in the first place -- that's telling you there's something strange going on.  Like I say, there's some voodoo involved.  It might be upstream from your house anyway.

Since you have nothing besides the coax to the modem, you can screw it right to the back of the Modem inline.  For $8, it's better than rolling the dice on a service call and getting a bill for them removing your splitters and leaving the problem behind.  I have little faith in Xfinity.  If it works, then great.  If it doesn't, you're out $8 and doing a service call anyway, so your call on that. 

(edited)

2 years ago

sunmast

Visitor

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13 Messages

Ordered 2x "PPC FPA6-54 in-Line Forward Path Attenuator 6dB 75 Ohms for DOCSIS Cable TV Box & Modem for Fast Internet - New Version" from Amazon to -12db :)

Will test and report back. :)

2 years ago

0

flatlander3

Problem Solver

 • 

1.5K Messages

@sunmast​ Try em one at a time, on a clean cable run.  I thought I might need two, I did not.  I also broke into the cable box on my house.  What was there?  No ground. Out of the ground --> connector --> cable modem.

I had one modem fry from a close lightning strike.  Actually left black marks on the board in it.  I think they're supposed to make sure there's a ground when they install the cable, but they did not, so I hammered in a ground spike and heavy copper to a little cable ground block.  New cable to with the original Tyco connectors they use.  It made zero difference in signal, but I might survive a close lightning strike now.....or not.  Lightning is weird.

2 years ago

EG

Expert

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110.3K Messages

2 years ago

@sunmast 

Is there a drop amplifier on the coax line leading to the modem ? If so, try removing / bypassing it and see.

1

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sunmast

Visitor

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13 Messages

There isn't any amplifier :\

The connection is like this:

[the cable buried under my yard] - [1->2 splitter] - [about 15ft cable connecting into my house] - [1->2 splitter] - [about 3ft cable connecting to the modem]

Originally I thought stronger signal is better, so I removed all splitters in the way to the modem, and it only make things worse. Later I installed all of them back to get right power level.

None of these splitters are connected to TVs or any other cables/devices. The entire connection is connecting to my modem only.

2 years ago

0

Jlavaseur

Problem Solver

 • 

948 Messages

2 years ago

Here is my channel logs, works perfect no reboots, notice the ofdm plc channel id falls just outside of the 32 downstream channels, with a channel d of 33, i have no clue what that means, but its different then the other logs i see posted, @EG @flatlander3 ?

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flatlander3

Problem Solver

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1.5K Messages

2 years ago

@Jlavaseur​ I was looking at the frequency vs power in the original post.  That one looks like a classic, almost textbook transmission signal reflection to me. Can power double on a reflection?  Sure.  It's like sending a pulse down a jump rope attached to a wall.  It can also ring over time and really mess stuff up with transient states, or cancel out signal entirely.

I don't know what you got going for the errors on that one.  Couple errors as a transient event on a couple channels?  If it's working, I wouldn't worry about it.  Those OFDM errors fly by fast.  If they're super rare as a whole, maybe you can't see them.

1

Jlavaseur

Problem Solver

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948 Messages

@flatlander3​  this is getting taken wrong, I posted mine because it’s working perfectly, I thought it could be interesting and possibly beneficial to compare a good working log to one not so much, I found that one different and just thought I would try to expand on it to see if it actually had a impact..

2 years ago

0

Jlavaseur

Problem Solver

 • 

948 Messages

2 years ago

@flatlander3  i guess i am wondering if a qam256 channel and a ofdm plc channel can actually bond if there are in the 32 downstream channels, i have no clue about this stuff

1

0

flatlander3

Problem Solver

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1.5K Messages

@Jlavaseur​ Buried in the spec.  https://www.cablelabs.com/specifications CM-SP-CM-OSSIv3.1-I23-220819.  But that's not going to tell you how comcast does it.

2 years ago

0

Jlavaseur

Problem Solver

 • 

948 Messages

2 years ago

Ok so the specs on the mb8611 shows docis 3.0 32 downstream channels, docis 3.1 shows 2 ofdm plc channels, it doesn't really show them combined, that's where i am confused...

0

0

EG

Expert

 • 

110.3K Messages

2 years ago

@Jlavaseur 

 Please do not hijack someone else's help thread. Multiple posters' issues / discussions in a single thread becomes too complicated, confusing, and convoluted. And it's unfair to the original poster. Thank you.  

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sunmast

Visitor

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13 Messages

2 years ago

Following @flatlander3 's instructions, I installed 2x 6db attenuators and removed all splitters. The result looks great. Both downstream and upstream pwr level are very ideal now (except OFDM PLC but 7dbmv is still in +-10dbmv range). SNR is also excellent. It has been very stable since attenuators are installed. Only one T3 timeout in the event log and no reboots so far.

Connection
   Startup Sequence    
   Startup Step Status Comment

  
   Acquire Downstream Channel 531000000 Hz Locked
  
   Upstream Connection OK Operational
  
   Boot State OK Operational

  
   Configuration File OK
  
   Security Enabled BPI+
  


   Connection Status    
   System Up Time 2 days 00h:46m:53s  
  
   Network Access Allowed  
  


   Downstream Bonded Channels  
  
   Channel Lock Status Modulation Channel ID Freq. (MHz) Pwr (dBmV) SNR (dB) Corrected Uncorrected
   1 Locked QAM256 20 531.0 -1.0 43.2 12 0
   2 Locked QAM256 13 489.0 -1.6 42.7 0 0
   3 Locked QAM256 14 495.0 -1.4 42.9 0 0
   4 Locked QAM256 15 501.0 -1.5 42.9 0 0
   5 Locked QAM256 16 507.0 -1.5 42.8 0 0
   6 Locked QAM256 17 513.0 -1.4 42.9 0 0
   7 Locked QAM256 18 519.0 -1.4 42.9 0 0
   8 Locked QAM256 19 525.0 -1.2 43.0 0 0
   9 Locked QAM256 21 537.0 -1.1 43.1 14 0
   10 Locked QAM256 22 543.0 -1.0 43.1 11 0
   11 Locked QAM256 23 549.0 -0.8 43.2 0 0
   12 Locked QAM256 24 555.0 -0.9 43.1 0 0
   13 Locked QAM256 25 561.0 -0.9 43.0 0 0
   14 Locked QAM256 26 567.0 -0.6 43.1 0 0
   15 Locked QAM256 27 573.0 -0.5 43.3 0 0
   16 Locked QAM256 28 579.0 -0.4 43.3 0 0
   17 Locked QAM256 29 585.0 -0.1 43.4 0 0
   18 Locked QAM256 30 591.0 0.1 43.7 0 0
   19 Locked QAM256 31 597.0 0.3 43.6 0 0
   20 Locked QAM256 32 603.0 0.5 43.7 0 0
   21 Locked QAM256 33 609.0 0.6 43.8 0 0
   22 Locked QAM256 34 615.0 0.7 43.8 0 0
   23 Locked QAM256 35 621.0 1.0 44.0 0 0
   24 Locked QAM256 36 627.0 1.0 44.1 0 0
   25 Locked QAM256 37 633.0 0.9 44.0 0 0
   26 Locked QAM256 38 639.0 1.1 44.2 0 0
   27 Locked QAM256 39 645.0 1.1 44.0 0 0
   28 Locked QAM256 40 651.0 1.1 44.2 0 0
   29 Locked QAM256 41 657.0 1.3 44.2 0 0
   30 Locked QAM256 42 663.0 1.5 44.2 0 0
   31 Locked QAM256 43 669.0 1.6 44.3 0 0
   32 Locked QAM256 44 675.0 1.8 43.6 0 0
   33 Locked OFDM PLC 193 957.0 6.7 45.5 13069646 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 16.4 43.0
   2 Locked SC-QAM 2 5120 22.8 43.0
   3 Locked SC-QAM 3 5120 29.2 43.0
   4 Locked SC-QAM 4 5120 35.6 43.0

(edited)

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