Billion SNR tweak not working as expected

I just got a billion 8800AXL R2. Quite close to the cabinet so I get almost 80/20.

At this moment downstream SNR is 6.3, rate is 79828k, and max rate 79838k.

I understand that the quality of my line and about 80M rate is going to give me about 6.3db margin regardless, but, I expected tweaking the SNR down would increase the max attainable and it seems to have no effect.

The cabinet is ECI so not going to get a 3db margin. The line is quiet so it could stand a lower margin. The SNR tweak was one of the reasons I got a billion so I could still squeeze 80/20 out of it if the line degrades a bit.

Any insight on what is going on?

IIRC the SNR adjustment only works on ADSL lines.

The manual states (p.122) "There are no set values recommended as each ADSL line will be different" and makes no mention of VDSL lines which may or may not be significant.

In reply to a post by GonePostal:

IIRC the SNR adjustment only works on ADSL lines.

The manual and UI are not exactly explicit about it, but, that seems to be the case.

Rather sucks. I had a DSL-AC68U which once they had finished screwing up and aborting the G-INP implementation on ECI cabinets in 2016 has given me a rock solid 80/20 interleaved till it died last week (with a 2db SNR tweak and by rock solid I mean uptimes in months and average 1-2 CRC errors per day).

Now I have moved from what everyone says is the crappiest of them all Mediatek chipset to one of the best Broadcom chipsets and I end up with fastpath and slower.

I've had my 8800AXL R2 about 2-3 years, and like you, purchased as it was supposed to be a really good router.

Unfortunately, the 5GHz WiFi is flaky, constantly dropping out (even if really close to the device). We live in a bungalow with 1 wall between the router and most other rooms, and it still drops out every day!

Although other parts are stable (2.4GHZ, general routing - with uptime in months) its not been the best router I've used for the price I paid at the time (think it was about £150 when I got it).

I'm now looking to change to something with WiFi 6 as most of our end-user kit is compatible now.

When I was on VDSL I tried the same as you to alter the SNR, and it had no effect, so I'm assuming its implementation is incomplete or isn't designed to work on VDSL.

Openreach won't allow SNR reduced by modem tweak it won't work on FTTC. Only ADSL2+ but FTTC should reduced SNR target 3dB on it own depend on the cabinet as most ECI won't have that.

Edited by adslmax (Tue 05-Apr-22 23:25:09)

I use a Draytek V130 modem on my VDSL line and does "appear" to to respond to the SNR changes.

Lowering the SNR via the CLI causes the SNR displayed by the Web interface to drop accordingly and the line speed does go up.

It is an ECI DSLAM so there isn't anything else I can do.

The changes are actually sticky over reboots which surprised me. It gives me an extra 3 or 4 Mb/s and is totally stable

In reply to a post by adslmax:

Openreach won't allow SNR reduced by modem tweak it won't work on FTTC. Only ADSL2+ but FTTC should reduced SNR target 3dB on it own depend on the cabinet as most ECI won't have that.

You know that isn't true.
It's Broadcom who don't allow it on VDSL2 on their chipsets.

Lantiq chipset modems manage it fine.

Openreach simply can't stop you adjusting the downstream SNRM target at the modem.

In reply to a post by j0hn83:

Openreach simply can't stop you adjusting the downstream SNRM target at the modem.

It's negotiated between the modem and the cab during sync i.e. both 'agreeing' on a figure isn't it?

There's an option in my router to set targets at 5 different levels (tick boxes) for both directions. I'm very dubious whether it has any real effect because DLM seems to take control after a few days and after a couple of weeks it starts to lower the margin. That's when my sync drops and I go back to my stable 12dB down and 6dB up getting ~43Mbps/~8Mbps.

In reply to a post by longedge:

It's negotiated between the modem and the cab during sync i.e. both 'agreeing' on a figure isn't it?

DLM decides what target margin the downstream should use and the cabinet tells the modem. The modem does not *have* to take any notice.

Modem tweaks usually adjust the requested target not set an absolute value. If your tweak downwards makes DLM think the line is unstable it will increase the target and you may end up back where you were.

The default target is 6db. Quiet lines can be stable at less. DLM will go down to 3db, but, not on ECI cabinets. Maybe DLM requires G-Inp to go below 6db and ECI cabinets don't do G-Inp.

In reply to a post by haydnwalker:

Unfortunately, the 5GHz WiFi is flaky, constantly dropping out (even if really close to the device).

I have a couple of phones and couple of tablets on 5GHz and have had no problem so far. Firmware is 2.52.d18.