I was asked to help with issues with a cable connection. Before I waste time over the phone with the ISP, I would like to better understand if the cable connection itself is ok. speed.cloudflare.com shows bad/poor/bad through ethernet, so I'm wondering if this is a cabling issue from modem/router to the ISP or if ISP is seriously overbooked. There's a OpenWrt AP behind it, which I could turn into a router and setup slow SQM, but that seems like a bandaid and not a proper solution.
These are hard to interpret without knowing what the ISP considers normal.
That said the rules of thumb are:
a) Modulation should be as close to the ISPs goal as possible. For a DOCSIS 3.0 Modem/cable plant, I think 256QAM for download and 64QAM for Upload are pretty much as good as it gets
If the modulation is significantly lower for individual channels that implies problems in the segment, as modulation is essentially shared in the whole segment. (This often affects the Upload direction and can manifest as reduced modulation channels that are only temporarily at a lower modulation, when the interfering source is active),.
b) Uncorrectables, this number should be low (zero is excellent) or it should not be growing at a noticeable rate, similar for Correcteds except a low baseline here is likely not a sign of trouble but the system working as intended. I note increased numbers of errors can be isolated to individual subscribers indicating local issues with RF noise, like a bad connector or marginally connected cable. (If these happen on the consumer side of the typical house amplifier these might not feed back into the full segment).
c) power and SNR to assess these you need information what your ISP aims for.
Here is a thread in a German DOCSIS ISP's forum discussing reference values for that ISP:
These might not be a perfect match but on principle other ISPs will have similar guidance (at least for the technicians).
Not being a DOCSIS expert, these readings look fine, so the issue might be transient (then please capture the stats again when the issue is acute). But could you post a screenshot of a full cloudflare speed test?
Only thing to add I guess other than @moeller0's excellent comment.
To start as an FYI, to make sure we're all on the same page. This is a 75 Ohm System hence dBmV. i.e. it's in dBmV not dBm. decibels millivolt not decibels milliwatt.
My experience is only really with a little VHF/UHF TV, but mostly looking at VDSL and 4G stats...
The basic math is something like 64qam needs 26db SNR, 256qam is like 33?
Seems to reflect what I've been able to google and read advice from different ISP's, with some extra margin on top.
Do your issues occur in bad weather / rain? I've had water getting into communications pits be a problem. IMO it's best to log it over a long period of time.
So I guess you have something like 3-5dB of link margin. From googling that looks close to what the minimum recommended by an ISP is. (Xfinity cable looks to say minimum is 35dB downstream, 31 upstream)
Similarly with the power xfinity says -7dBmV to 7dBmV is fine.
You should also ask ISP for what the upstream SNR is if possiblee?
Thanks for all the info. Really appreciated! I need some time to digest this. On a first glance I see down 256QAM and up 64QAM so that suggests good cabling and signal.