Applying Downstream SNR offset to VDSL WAN --> DOES NOT increase Downstream Speed

Installing and Using OpenWrt
1

Hi All,

I am using my second BT Home Hub V5A, with OpenWRT, as a modem, in front of my main home Router.
The Router has a good modem - it is a Fritz!Box 7530 and I use the DECT in it, so I did not want to disrupt that functionality by flashing OpenWRT into the Fritz, though I knew it would have been possible.

Back to the OpenWRT modem - I am using this configuration as I realised that Fritz does not have an SQM, so I was experiencing bufferbloat all along my network on every device.

Everything works fine, but I have a question about the tweaks which I discovered I can apply to the DownStream SNR offset.

With the Downstream SNR offset set to 0dB, I get a good DS speed (I am on VDSL 17a profile, with a maximum of 80/20 from the ISP contract):
SNR measured by the modem for DS is 3.3dB [only 350 meters from the Exchange, apparently, according to FritzBox]
SNR measured for US is 6dB
Max Attainable Rate : 74Mbps
Data Rate received at the wall (FTTC) : 71.5MbpsBack to the OpenWRT modem
Data rate measured connecting to the LAN of the modem & using SpeedTest.net : up to 62-63 Mbps

I decided to make some tweaks to the SNR offset.

With SNR DS offset at -2dB --> DS speed remains unchanged, it seems so far / SNR measured is 1dB, I got, however some greater numbers for:
Max Attainable Rate : 81.8Mbps
Data Rate received at the wall (FTTC) : 76Mbps

Q1. Should I go back to SNR offset of 0dB?
Q2. Could I get penalised by the ISP for doing what I am doing (even though I am still achieving speeds within contract)?
Q3. Should I expect better DS speeds measured by SpeedTest at some point, if I stay on -2dB or if I go lower, even? (I know that when I first started receiving the service my SNR was adjusted gradually down from 6dB to 3dB - within the first 2 weeks, and SpeedTest took some time to measure higher speeds, so I wonder whether there is some IP caching in SpeedTest.net?)

Thanks

2

For question 2, in my experience in Australia, no you won't get penalised. However, if you lower the SNR target to a point where you introduce sufficient instability or error rates on the line, the ISP equipment may automatically put you on a "stability profile" e.g. 6dB and lock it there for a period of time before auto renegotiating starts again. This would result in a slower speed until it trains back down to an appropriate level.

In my opinion 1dB isn't a lot of margin. I've always had a good balance of speed and stability around 3dB. If you've got a clean line, maybe you'll get away with it.