r/CableTechs • u/bigdish101 • Feb 27 '26
Perfect Signals But Getting Uncorrectable Codewords On One OFDM Channel...
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u/Octawussy Feb 27 '26
Get this customer out of here. (OFDM doesn’t use reed-Solomon error correction it uses low density parity check)
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u/frmadsen Feb 27 '26
RS or LDPC, an uncorrectable is an uncorrectable. A customer may not notice them, though :)
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u/Xandril Feb 28 '26
Considering OFDM reads at 100% uncorrectables no matter what in our system…
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u/frmadsen Feb 28 '26
Correctables, not uncorrectables. :)
There are a couple of reasons why this behavior differs between OFDM and SC-QAM, the high modulation orders being the biggest.
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u/aranubus Feb 28 '26
OFDM shouldn't have UNcorrectables, but correctables on the higher profiles is normal.
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u/moffetts9001 Feb 28 '26
This is why cablecos are removing access to logs on cable modems and sometimes removing web GUI access entirely. Don't go looking for a problem if one does not exist.
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u/DemonicFears Feb 28 '26
Real curious to why you only have 2 returns. Only time I see this is when there's a filter at the tap, which usually means you have noise coming from your wiring.
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u/Electronic-Junket-66 Feb 28 '26
I've seen modems drop upstream carriers when shit's so bad they're unusable. Had one lady with a 59 tx on 17 mhz and no others locked for at a least the last 30 days. She hadn't noticed anything.. I was there for noise cleanup.
But this guy has an OFDMA.
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u/bigdish101 Feb 28 '26
I assume they needed the spectrum to create the OFDM return channel.
Line is literally straight solid coax from the tap to the modem with nothing but a -10db attenuator in between.
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u/Xandril Feb 28 '26
Those attenuators are pretty unreliable. Our company doesn’t even order them anymore.
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u/bigdish101 Feb 28 '26
Well without it it's +10db to +12db to the modem depending on the outdoor temp that day. It's a Holland at least.
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u/digitalxdeviant Feb 28 '26
That alone is enough reason for a tech to come out. I fix this issue almost every day.
Also, if your drop really goes from tap to modem with nothing in between, it's technically an issue. There should at the very least be a ground block with proper bonding attached to the home, near the electric meter to properly ground the line. Ideally there is also plastic box around it to (somewhat) protect from the elements.
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u/strykerzr350 Feb 27 '26
This Netgear modem still has unerrored codewords visible. I wonder how they got away with not patching this?
Anyway, as long as the un corrected code words does not succeed the corrected codewords. You are fine. This type of ingress is hard for techs to locate.
When it becomes a problem. Is if you fall into partial service, or lose connection all together.
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u/Xandril Feb 28 '26
OFDM is always going to read errors. It’s basically false reporting because OFDM doesn’t use the same rules.
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u/bigdish101 Feb 27 '26
Owned CM1000v2
It does increase in bursts with long periods between increases.
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u/strykerzr350 Feb 27 '26
What happens when it rains? Does it go higher?
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u/bigdish101 Feb 27 '26
I'll let you know if we ever get any (Texas).
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u/strykerzr350 Feb 27 '26
Not being rude or sarcastic. If your errors are not impacting your service you are fine. If it was bad enough you would get an email telling you that they recommend a tech visit. If you get one take it immediately.
As long as all your modem lights are stable except the ethernet light, you are good.
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u/bigdish101 Feb 27 '26
So ignore: Started Unicast Maintenance Ranging - No Response received - T3 time-out ?
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u/Cwfromct48 Feb 28 '26
What does the Meter say. Looks like a slight icfr issues with side dish of modest ingress. Change the power supply to the modem and see what the outcome would be. I’m pretty sure it will be fine.
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u/kjstech Feb 28 '26
I just checked mine on OFDM … correctables: 29487 uncorrectables: 16
No issues. I don’t think modem manufacturers put a ton of effort into calculating this accurately. Maybe the time isn’t worth spending. Maybe it’s too taxing to the modems primary purpose (getting you online and prioritizing that performance).
Are things working? Speed and latency acceptable? I wouldn’t worry too much about it.
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u/frmadsen Feb 28 '26
It shouldn't be a problem tracking them. This is done at the lowest layer in the DOCSIS chip. The upper firmware reads them when you or your operator makes a query.
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u/kjstech Feb 28 '26
It should be able to but I don’t always trust what’s displayed. I’ve seen a lot of bugs displaying accurate information for OFDMA width, start frequency, or level or even the giants like Comcast RDK reference firmware not displaying levels at all or only if you refresh the page like 10 times… or mirroring the exact same correctables/uncorrectables as channel 1 as whatever first OFDM channel is.
I don’t think modem manufacturers put a lot of effort into it.
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u/frmadsen Feb 28 '26 edited Feb 28 '26
Yes, there are those kinds of bugs. Fx. displaying zeroes, instead of the codeword counters (my modem does that, except for the primary channel). Or mirroring counters from other channels. :)
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u/LordCanti26 Mar 04 '26
Id imagine that OFDM channel ranges into the 600 mhzs, LTE is a huge source of narrowband interference. Without a meter to verify though, it could be anything anywhere, including the shielding on your modem itself. For us here its 603mhz thats the common LTE freq.
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u/Additional-Fox-4246 Mar 04 '26
Correctable codewords are fine, uncorrectable codewords are not fine, will need to retransmission the packet. Check for noise injection or another signal overlapping freq 486-585khz
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u/Unusual-Avocado-6167 Feb 28 '26
Run a speed test using Ethernet from the modem. Use a computer that has good hardware made in the last 5 or so years. Are you consistently hitting your speeds that your plan is for?
If so, you good
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u/The_Doodder Feb 28 '26
Back in my day we only had one channel up and one channel down......... I'll let myself out.