How does a VDSL modem-router set or determine Output Power?

In FTTC setups, does the DSLAM in the street cabinet play any part in determining what level of power output (measured in dBm and which is recorded in the stats of some router-modems) is generated by the modem-router in the Upstream direction? I ask because, in my own VDSL2 setup, the upstream power output level being reported by my router-modem is unbelievably low, being a paltry 2.9dBm.

Is the power output level set entirely by what the user's modem decides, or does line length and line attenuation influence this? Does the DSLAM affect in any way the modem's output (upstream) level?

I'm on a nominal FTTC 40/10 account, and though I get the full 40M bps sync rate in the downstream direction, I get only a little in excess of 7M bps for upstream. This is with a brand new and quite expensive VDSL2 modem-router, BTW. Probably, that 7M bps is a trustworthy figure, given that the cabinet is some 750 metres away and that the copper section runs in heavily-used underground cables in the neighbourhood. My modem-router, however, is reporting that the output power level is just a mere 2.9dBm. The corresponding level of downstream power, ie power from the DSLAM, is 12.9dBm, one heck of a difference. Error rates on the upstream are low or even zero, depending on which ones you choose to consider, and there have been no auto-disconnects since first commissioning the modem-router. The corresponding SNR's, BTW, are 8.2dB (downstream) and 6.1dB (upstream).

Is influenced by a number of things.
Upstream Power Back Off is a mandatory requirement.
Distance to cabinet, distance from exchange, modem used, all influence power levels.

2.9dBm is not a low power level though. I regularly see lower, including my own line.
There are many lines

I'd guess you're on a Huawei cabinet as ECI cabinets report screwy power levels.

A quote from my black cat friend

In reply to a post by burakkucat:

The power is reported in units of dBm (decibels relative to one milliwatt), a ratio. Hence --
A reported power of 0 dBm indicates an absolute power of exactly one milliwatt.
A reported power more positive (larger) than 0 dBm indicates an absolute power greater than one milliwatt.
A reported power more negative (smaller) than 0 dBm indicates an absolute power less than one milliwatt.

We regularly see negative dBm values for upstream power. Yours isn't low.

Edited by j0hn83 (Thu 13-Dec-18 12:54:52)

Thanks for replying. Your comments are interesting.

First of all, may I ask what Upstream Power Backoff entails? I've never heard of that term before.

You say that, in your own case, your stats show an upstream power output level of even less than my 2.9dB. So, what level do you yourself have for that, then?

I'd agree that the particular modem used can influence the figure, because I've just changed (supposedly upgraded) from one particular Billion VDSL modem-router to a more-advanced Billion modem router, and have seen the upstream output power drop from 5.1dBm on the former modem-router to 2.9dBm on the new one.

If I remember correctly my electronics from 50 years ago, relative power (in dB's) is:

10 x log-to-base-10 of actual watts/1 watt (or actual milliwatts/1 milliwatt)

So, log-to-base-10 of actual watts = 0.29

Therefore (looking up the antilog on a table published on the Web), actual watts = 1.95 approximately.

Does that look right? These days I can barely recall how to use logs, but I think the above is correct.

In this situation we'd be working in milliwatts, rather than watts, so the actual wattage, over a metre, is 1.95 mW.

Edited by deleted (Thu 13-Dec-18 16:41:51)