Upstream health rating: "Critical" for low but stable TX power – consider modulation, errors and stability

[HaxTheCook]

Greetings and thanks for Docsight!

I had a great time reviewing my Line and optimizing the inhous cables and outlets.
I now have a very stable Line but one suggestion:

The line status is "Critical", apparently driven by upstream TX power alone (e.g. US ch 3: 34.8 dBmV, "6.3 dB under target" of 41.1–47.0). At the same time the line is healthy:

• US SC-QAM all at 64QAM, OFDMA at 256QAM, at times 512QAM
• very few uncorrectable errors, no outages, 940 Mbit/s downstream
• no packet loss / negligible

Setup:
Vodafone Cable (ex-Unitymedia), own Fritz!Box 6591, [DOCSight v2026-11-02.1], "VFKD Thresholds", 1000/75.

Why TX power is low:
My in-house wiring is short and low-loss (I replaced a 17 dB through outlet with a 9 dB terminating one and renewed an old cable). TX power dropped from ~41 to ~33–37 dBmV afterwards. Since the CMTS ranges the modem to a fixed receive level, less path loss means less TX power, so a low value is not necessarily a bad line.

My Suggestion:
TX power is only a proxy for the real SNR at the CMTS, which the modem cannot see. It would help to also weigh:

• reached modulation (chosen by the CMTS based on its real SNR)
• uncorrectable errors and stability over time

An intermediate state like "outside nominal range, but stable" instead of "Critical" would avoid a permanent red status on a healthy line. A user-configurable lower bound would also work.

I don't know if this is really necessary but thats the one thing i would change.

What do you think?

[itsDNNS]

Low transmit power on its own is not a problem for your connection. The CMTS ranges the modem to a fixed receive level, so less loss in the home wiring simply means less transmit power. The current text for this state ("may not be reaching the provider reliably", "check cable connections") describes the high power case and is misleading here.

I also checked the Vodafone interface specification our profile refers to. For upstream it gives a nominal level range at installation and an absolute maximum, but no minimum, and no range at all for OFDMA. So "Critical" for low transmit power is a DOCSight convention, not something the specification states. Your values are still below the nominal range, so "outside the nominal range" is accurate, "critical" is harder to justify.

What I am currently looking at:

• correcting the wording for low transmit power
• treating "too low" differently from "too high", so that low power on its own would show as "Marginal" rather than "Critical"
• documenting which limits come from the specification and which are DOCSight conventions

Errors and modulation won't work as a counterweight. The error counters are downstream only, the modem cannot see upstream errors. SC-QAM upstream modulation is set per channel for all modems on it, so it says little about a single line.

I would still not show such a line as green, since less attenuation also lets noise from in-house wiring reach the shared segment more easily. If you want the value back in the nominal range, ask your ISP to check the levels, they can add an attenuator if needed.

[HaxTheCook]

Hey,

thanks for your reply. The thing i don't understand ist that putting in an attenuator.. wouldnt that just artificially weaken the signal so that the modem hast to send "louder" or ist my understanding there wrong? Why would that signal be clearer then?

Thanks for the additional info your right thats only downstream i didn't think of that at all. Oh why is the Modulation set vor all Modems on the line? isn't that really prone to failure? i thought its dependend on my line. that explains a lot.

Asking Vodafone to check the levels is less likely than winning the lottery. At least where i am living, thats why i startet tinkering myself in the first place. Last time i was in the hotling and the lady told me she will talk to the technical department while trying to sell me a mobile contract. The moment she realized i really don't want a mobile contract she just told me the number of the "technical department" and told me to call there. Turns out it was the same number i called in the first place. I'm really fed up with Vodafone :)

Habe gerade gesehen dass du ja auch aus Deutschland bist. Vielen dank für das Stück Software, macht Spaß damit zu basteln.

Viele Grüße aus BaWü

[itsDNNS]

Yes, that's all it does. The modem sends louder and the signal arrives at the CMTS at about the same level as before. Your own signal doesn't get any cleaner from that.

The point is noise. The upstream is shared, and whatever leaks into the cabling in every home on the node adds up at the CMTS. An attenuator also damps the noise that gets in between it and the modem, while the modem makes up the difference for its own signal. So it only helps against stuff picked up in your own wiring, it helps the node more than you, and a plain attenuator takes the same amount off your downstream. Your line runs fine, so I'd leave it alone. Certainly not worth another call to that hotline.

On the SC-QAM channels the modulation is part of the channel settings the CMTS announces to all modems, so everyone on that channel sends with the same one. I wouldn't call it fragile, it's more the lowest common denominator. It has to work for the worst modem on the channel, and if noise gets in somewhere the whole channel gets stepped down. OFDMA is different, there the CMTS can put modems on different profiles. So the 256/512QAM on that channel is closer to being about your line, though I can't tell from the FRITZ!Box value alone how much of it is really yours.

The changes are merged but not released yet. Low upstream power on its own is now Marginal at most, the text no longer claims your signal might not reach the provider, and low OFDMA power isn't rated anymore because neither the Vodafone spec nor DOCSIS gives a target level for it. The :latest image already has all of that, :stable gets it with the next release. Your line will still show Marginal, because two of your SC-QAM channels are below 37 dBmV.

Viele Grüße zurück!