Cable TV Internet Service Problem -- Solved

Recently, I have been bedeviled by unstable Internet service from COMCAST (Xfinity) in the Boston area. The following story is not
limited to COMCAST, and it could have been any Cable-TV Internet Service Provider.
Starting in late July 2021 and ending in early August 2021, for about one month total, my Internet service became very unstable. (Cable TV service was unaffected, so it was not that the physical cable path was interrupted.) Internet service had been rock solid since about 2016, when I purchased and installed the current cable modem, an ARRIS SB6183. My service tier is 200 Mbit/sec download, which is well within the capabilities of the SB6183, 686 Mbits/sec.
The main external symptoms were that browser access would randomly slow and hang, sometimes recovering, something not. Likewise email (POP account). Rebooting the modem (by cycling power) often but not always restored Internet service. As the month progressed, things got progressively worse, although it was always relapsing-remitting - it should spend all night trying to connect, failing, trying again, and so on.
Called COMCAST tech support, which forced me to deal with a robot lady that could only follow a script, the main point of which was to get me to power-cycle the modem. This power cycle often did work, probably because she was probably also sending a modem reset signal at the same time, but she never made that clear. (The robot lady here is in fact a machine, not just an unimaginative human, a droid.)
Got a human later, after calling back and answering that the problem was not fixed. The human came to the conclusion that my cable modem was too old and likely broken, and insisted that I call ARRIS and have them diagnose the modem, and only if ARRIS declared that the modem was not broken would COMCAST do anything like send a truck without charging an arm and a leg.
Called ARRIS. Cost US $50 (out of warrantee). Got a real human, in Chennai, India, who did seem expert in the modem internals, with perfect English and a very good telephone line, and was very patient.
COMCAST had claimed that ARRIS could do technical (ie, electrical) tests remotely, but this turned out not to be true. Anyway, I had collected lots of status data from the modem, which we discussed, and he concluded that the modem was not broken and did not need to be replaced
Now one mystery had always nagged me: While the Internet was up, the modem would report received signal powers (over 16 parallel RX channels) of +5 dBmV (decibels over one millivolt in a 75-ohm system), and a SNR of about 40 dB, and yet errors (both corrected and uncorrected) kept accumulating.
When the link was down, the received signal level would drop by 20 dB to -15 dBmV (which is in the DOCSIS 3.0 Spec Range) , and the SNR would still be 35 dB (also in spec range).
How does that work? With 35 to 40 dB SNR, there should be no errors. Neither COMCAST nor ARRIS were able to interpret that oddity.
Called COMCAST back. Went through the robot lady yet again, but on the third call, got another real human.
He really had no idea what went on inside modems et al, and insisted that if the associated firewall/router was not also rebooted when the modem was reset or rebooted, the ensuing chaos would cause the modem to be unable to measure incoming RF power (at around 500 MHz) correctly on a coax, the claim being that both Ethernet and Cable TV were both "electrical", so they could heavily affect one another Hmm. I'll have to think about that. For a very long time.
He was also of the opinion that Ethernet wires wear out and need periodic replacement. I declined to disassemble the house to get at the CAT5e cable that runs from basement (at the modem) to the 2nd floor (where the WiFi base station lives).
He worried greatly about cables and connectors in general, saying that they were very often the cause of such problems as I was seeing. After one hour discussing the issue, while his theories of causation were nonsense, this was his direct experience, and thus was my main takeaway.
Usually, connectors are the main cause of problems with "cables", and it had to be a coax cable, so I found an 11mm (7/16") open-end ignition wrench, and went around loosening and re-tightening all CATV RF connectors (Type F, for the record). The loosen-then-tighten drill is to physically disrupt any corrosion at contact points.
All connections were tight save one, the one where the coax from the modem connects to the RF feed coming from the nearby boiler room. The loose connector is in the wall plate under the table upon which the modem sits, along with my desktop computer, and that connector had managed over the years since 2016 to unscrew itself almost to the point of falling apart. That cable does move a bit when things are added or removed, and so on. Tightened the connector. Bingo!
And now the cause of the oddity became clear. The SNR is measured over time, to yield a stable value. But the connection was chattering between normal power (+5 dBmV) and open (-15 dBmV) fast enough that the SNR calculation didn't notice. This was also too fast for AGC et al to react, so symbols sent during the open period were lost. Some could be recreated by forward error correction, and some could not, and these errors accumulated steadily over time despite the stellar SNR.
The modulation type is QAM256, and the symbol rate on each QAM256 channel is 5120 Ksymbols per second. There are 16 downlink and 4 uplink channels. The AM component of the QAM symbols will have a fundamental at 5 MHz or so, far above the spectrum due to a rattling connector.
.<https://en.wikipedia.org/wiki/Quadrature_amplitude_modulation
A spectrum analyzer would not have found this problem either. I suppose if one envelope demodulated the received RF, the chattering would have been quite obvious - big AM noise peak in the 10 to 1000 Hz range, or the like. I would think that it would be useful for the modem to do this - loose connectors are pretty common I'd hazard.
Anyway, it all works now, the key symptom being that there are no new errors accumulating now.
And the takeaway is that if SNR exceeds 30 dB and yet errors accumulate, start re-tightening RF connectors. If that doesn't work, look for a cracked wire or connector, or a loose shield.
Joe
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"Joe Gwinn" wrote in message
............... The modulation type is QAM256, and the symbol rate on each QAM256 channel is 5120 Ksymbols per second. There are 16 downlink and 4 uplink channels. The AM component of the QAM symbols will have a fundamental at 5 MHz or so, far above the spectrum due to a rattling connector.
.<https://en.wikipedia.org/wiki/Quadrature_amplitude_modulation
A spectrum analyzer would not have found this problem either. I suppose if one envelope demodulated the received RF, the chattering would have been quite obvious - big AM noise peak in the 10 to 1000 Hz range, or the like. I would think that it would be useful for the modem to do this - loose connectors are pretty common I'd hazard.
... Joe
-----------------
+5dBm is nearly half a volt. Would an unsynced scope have shown dropouts in the envelope?
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On Sat, 14 Aug 2021 12:27:31 -0400, "Jim Wilkins"

A bandpass filter followed by a diode envelope detector feeding a scope would likely have worked. A spectral display may be best to prevent distraction, but the modulation amplitude was 100%, so it's going to be pretty obvious.
But what I'm thinking is that this kind of detection should be built into the modem, as ratty connectors are quite common.
Joe Gwinn
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"Joe Gwinn" wrote in message

A bandpass filter followed by a diode envelope detector feeding a scope would likely have worked. A spectral display may be best to prevent distraction, but the modulation amplitude was 100%, so it's going to be pretty obvious.
But what I'm thinking is that this kind of detection should be built into the modem, as ratty connectors are quite common.
Joe Gwinn
-----------------------
So the symptom was more complicated than intermittent complete loss of signal?
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"Jim Wilkins" wrote in message
............... The modulation type is QAM256, and the symbol rate on each QAM256 channel is 5120 Ksymbols per second. There are 16 downlink and 4 uplink channels. The AM component of the QAM symbols will have a fundamental at 5 MHz or so, far above the spectrum due to a rattling connector.
.<https://en.wikipedia.org/wiki/Quadrature_amplitude_modulation
A spectrum analyzer would not have found this problem either. I suppose if one envelope demodulated the received RF, the chattering would have been quite obvious - big AM noise peak in the 10 to 1000 Hz range, or the like. I would think that it would be useful for the modem to do this - loose connectors are pretty common I'd hazard.
... Joe
-----------------
+5dBm is nearly half a volt. Would an unsynced scope have shown dropouts in the envelope?
--------------------
I missed that you wrote 5 dBmV, not the dBm I'm used to. 5 dBmV is too small for a good scope display.
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On Saturday, August 14, 2021 at 5:39:12 PM UTC-4, Jim Wilkins wrote:

Some of the older Wavetek CATV Signal Level Meters have a built in spectrum analyzer function that you connect to an oscilloscope for the display. They are in the SAM series like the SAM III shown here:https://www.ebay.com/itm/144138486196 A cheaper way is with a SDR, a F to SMA adapter and your computer. There is good freeware to operate it from 100KHz to 1.7 GHz. https://www.ebay.com/itm/304033297816 0dBmV is 1 milivolt. Here is a good page comparing the older DOCSIS 3.0 to the current 3.1, including the RF frequencies used: https://www.rohde-schwarz.com/us/technologies/cable_tv/docsis/docsis-technology/docsis_technology_55513.html
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"Michael Terrell" wrote in message
On Saturday, August 14, 2021 at 5:39:12 PM UTC-4, Jim Wilkins wrote: ...

Some of the older Wavetek CATV Signal Level Meters have a built in spectrum analyzer function that you connect to an oscilloscope for the display. They are in the SAM series like the SAM III shown here:https://www.ebay.com/itm/144138486196 A cheaper way is with a SDR, a F to SMA adapter and your computer. There is good freeware to operate it from 100KHz to 1.7 GHz. https://www.ebay.com/itm/304033297816 0dBmV is 1 milivolt. Here is a good page comparing the older DOCSIS 3.0 to the current 3.1, including the RF frequencies used: https://www.rohde-schwarz.com/us/technologies/cable_tv/docsis/docsis-technology/docsis_technology_55513.html
------------------------
Thanks. Digital encoding is one of the most complex and difficult subjects I've studied, up there with chemical Thermodynamics. The 0 dBm (1mW, 50 Ohms) I assumed is 0.632V p-p on a scope. Usually I measured signals with a vector network analyzer.
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On Thursday, August 26, 2021 at 5:28:05 PM UTC-4, Jim Wilkins wrote:

Here is another useful website: http://rfcafe.com/
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"Michael Terrell" wrote in message ... Here is another useful website: http://rfcafe.com/
---------------------
Thanks. I've put solar and RF electronics aside for the time and my home project problems have all been calculating loads on steel beams, columns and their connections. That overhead gantry hoist did successfully lift and move a 2130 Lb oak log onto my sawmill. The fallen trees have all been cut into timber framing beams and very nice foot-wide knot-free boards.
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On Friday, August 27, 2021 at 8:22:43 AM UTC-4, Jim Wilkins wrote:

No problem. I have a bunch of other useful links, if you ever need them. I have registered a domain name, to rebuild the website that was on Earthlink before they died as a Broadband provider. Have fun with your woodworking/carpentry projects.
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"Michael Terrell" wrote in message

No problem. I have a bunch of other useful links, if you ever need them. I have registered a domain name, to rebuild the website that was on Earthlink before they died as a Broadband provider. Have fun with your woodworking/carpentry projects.
---------------------
First the wood has to dry for a year or three. I'm still dealing with maintenance I had to postpone while working overtime in another state and taking night classes. The wood was accidental, cleanup of two huge oak trees that blew down in my front yard. The straight, knot-free trunks were too good to become firewood.
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On Sunday, August 29, 2021 at 9:39:07 AM UTC-4, Jim Wilkins wrote:

Cabinet grade oak? :)
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"Michael Terrell" wrote in message
On Sunday, August 29, 2021 at 9:39:07 AM UTC-4, Jim Wilkins wrote:

Cabinet grade oak? :)
---------------------
One side of the trunk gave 8 defect-free boards 12' long and a foot or more wide, which I'll trim to 10" wide to fit my planer and 11' 6" long to fit the wall length. The overstuffed book shelves in there now are overlapped 6' 4" long boards that will become the side wall shelves and vertical ends. I cut that oak in the late 90's because it threatened a neighbor's new pool, and converted a band saw into a smaller sawmill to salvage its clear lumber.
My father's old cast iron Shopsmith has served pretty well to make custom cabinet doors and t&g flooring.
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On Sunday, August 29, 2021 at 4:03:21 PM UTC-4, Jim Wilkins wrote:

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That sounds like a plan. I have two of the T&G cutter sets from Harbor Frei ght for an old shaper/router. That way I din't have to keep taking them apa rt to change from T to G and back. Just use a gauge to make sure the height is correct. Since plywood is unaffordable at the moment, I'm planning on p icking up scrap pallets to break down and turn into T&G subflooring.
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"Michael Terrell" wrote in message
That sounds like a plan. I have two of the T&G cutter sets from Harbor Freight for an old shaper/router. That way I din't have to keep taking them apart to change from T to G and back. Just use a gauge to make sure the height is correct. Since plywood is unaffordable at the moment, I'm planning on picking up scrap pallets to break down and turn into T&G subflooring.
------------------
Have you tried to disassemble pallets before?
I use 40" x 48" pallets for the floors and end walls of my firewood sheds. The pallets that don't prove strong enough have been almost impossible to pry apart without splitting the slats, and the dirt in them dulls saw blades, so I cut them up with a chainsaw that I can file or grind sharp. I do a lot with recycled material but many damaged pallets are just firewood.
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On Sunday, August 29, 2021 at 6:16:22 PM UTC-4, Jim Wilkins wrote:

Yes, I repaired some by taking boards from one to replace them on other pallets, and some that I rebuilt without gaps between the boards. There is a tool to make the job easier, that you can buy or make. Here is an example: https://ussolid.com/pallet-buster-head-deck-wrecker-heavy-duty-skid-buster-tool-diy.html
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"Michael Terrell" wrote in message
On Sunday, August 29, 2021 at 6:16:22 PM UTC-4, Jim Wilkins wrote:

Yes, I repaired some by taking boards from one to replace them on other pallets, and some that I rebuilt without gaps between the boards. There is a tool to make the job easier, that you can buy or make. Here is an example: https://ussolid.com/pallet-buster-head-deck-wrecker-heavy-duty-skid-buster-tool-diy.html
------------------ I just wanted to warn you in case you didn't know how difficult they could be.
To reduce the cost of my firewood storage sheds I salvage the pressure-treated beams from replaced decks, from a neighboring contractor's scrap heap, so I have several large wrecking bars and nail pullers to deal with ringed and glue nails. The prying tool I used on the pallets is a 3-foot import version of the flat Wonderbar, with its edges ground sharper. It took some softwood pallets apart successfully but the nails in many hardwood ones were glued/rusted in too tightly and broke through the slats. My main pallet source is a garden center that stores their used pallets outdoors, indoor pallets might be cleaner and easier.
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On Monday, August 30, 2021 at 6:48:02 AM UTC-4, Jim Wilkins wrote:

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The pallets I get come from a local company, and were only used once, and a re stored indoors. The nails can be a problem on some pieces. I was getting some 10 foot pallets from a glass shop, until they hired a new guy who too k them for himself. The long parts were 2" by 8" boards.
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"Michael Terrell" wrote in message
The pallets I get come from a local company, and were only used once, and are stored indoors. The nails can be a problem on some pieces. I was getting some 10 foot pallets from a glass shop, until they hired a new guy who took them for himself. The long parts were 2" by 8" boards.
-------------------
The most useful ones I found were from a kitchen installer and had held counter tops. The 12' long timbers underneath are large enough that I used them as shed roof beams. The supply dried up when a new owner milked the company to death.
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On Monday, August 30, 2021 at 5:18:08 PM UTC-4, Jim Wilkins wrote:

A HVAC company was doing a lot of work, and received a lot of equipment on those 12 foort pallets, but Covid hit before I could start picking them up. There was a local pallet maker/repair business for sale back in 2000. A guy I worked with and I were thinking about buying it, but he backed out. It had a hydraulic powered tool to separate the boards to speed up repairs.
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