How do I find out / calculate the SNR on VDSL2?

[g67575]

I have an Asus DSL-N16 router + modem, and I'd like to know what the SNR of my VDSL2 connection is.

The SNR in ADSL can be used to help calculate the 'max rate' (max connection speed) I believe, on ADSL connections. This is worked out by subtracting the Attenuation from the SNR of the line.

My downstream connection has been around 10mbps lower than the reported max rate, for several months now, so I'm curious if the reported max rate is accurate.

Or, is there a separate SNR for each VDSL2 band? As can be seen below:

 

[g67575]

The other thing that occurs to be, is that most lines will use an SNR Margin of 6dB, which I think is set by the equipment at the exchange or FTTC cabinet.

Normally, if I don't tamper with any SNR Margin settings, my line defaults to approx. 6dB, across all VDSL2 bands, which I assume it adjusts the line rate to ensure an SNR margin of ~6dB, whenever the modem syncs.

I've increased this to 8db, this reduces the FEC / CRC error rate below 100 per day (there's good reason to think this would result in higher sync speeds, because of how the DLM system works in the long term).

Anyway, I think the SNR might just be the difference between the reported SNR Margin, and Openreach's Target SNR Margin of 6DB (to complicate things, this can be set lower on some lines). My router reports an SNR Margin of 7.7-7.6 dB.

So, in my case, this would be an SNR of 1.6-1.7dB, depending on the Downstream band (D1, D2, and D3 on my line).

Or, the SNR might just be the lowest point on the graph shown above, so around 11dB.

 

[VersionMonkey]

It's been a while since I looked at this in detail, so what follows is probably not entirely accurate.

The SNR will vary based on atmospheric conditions - the phone line will be picking up radio signals (the twists in the pair mitigate this, but not completely) and RF propagation varies over the day. This was a much bigger factor with ADSL, due to the longer copper runs involved. But when you look at your modem stats, the SNR at that instant isn't necessarily the SNR at the time of the sync. Which could account for any difference between the DLM target and the reported SNR (unless you've just that instant triggered a resync).

DLM prioritises line stability - resyncs due to errors will cause the algorithm to push up the target SNR. An "average" line will probably end up at 6dB, as you've found.

I'm not sure how much weight errors that don't lead to a resync have in the algorithm; so I don't quite follow your reasoning for increasing your modem's target to 8dB. Surely even if you manage to get DLM to fall back to a 3dB target through doing this, then as soon you remove your 8dB limit to get the higher sync, your line stability will plummet and you'll end up back on 6dB?

Without researching it, my expectation would be that the reported near/far SNRs are an average across the used bands, but they could equally be the lowest value as you suggest.

My downstream attainable reported by the modem also typically wanders around 10-12Mb/s higher than the actual sync and my line is on a 9dB profile (crosstalk, I think). So that fits with you syncing at 8dB and seeing 10Mb/s lower.