There is no such thing as a variation of G.DMT for BT.

The 448 and 888 Kbps limits are not part of the standard, but simply one of the many configuration options available in the DSLAM and any G.DMT compliant microcode will handle this.

if there is a faulty filter at the exchange end could selecting G.DMT modulation at my end somehow use a different non-faulty filter at the exchange? Possibly something to do with partial LLU?

No, the filters are either in DSLAM port or hard wired to it.

ADSL is known for being for more stable on some lines, simple explanation is that ADSL2 packs more bits into the same spectrum and in some scenarios this can make lines more unstable, even though ADSL2 does have more error correction ability.

ADSL2+ adds more frequencies on top of the changes from ADSL2, but as these are in the 1.1 to 2.2 MHz space this only is of help for shorter lines.

Beyond the changes above, some firmware for routers may simply be more debugged for ADSL, or ADSL2 or ADSL2+ without a lot of investigation impossible to say with any confidence, so users finding what works best for them in their particular set of conditions is the best way forward. The DLM systems that do exist try to automate a lot of this so that for the majority of the public there is no need for long discussions on the topic

So something must be causing problems with the frequencies when I select ADSL2 or ADSL2+ modulations from the router but not when I select G.DMT. As I mentioned previously using the Billion router's G.DMT setting (6 days ago) the stats are virtually the same as with ADSL2, although on the upstream there is an interleaving depth of 64 4 rather than 64 8, now no FEC's on the upstream, and an upstream sync of 1024Kbps rather than the fixed (?) TTB 1022Kbps.

Interesting.
I have similar 7800N on partial LLU. With comparable Attenuation. No errors reported.
If I only use G.DMT the speeds are 890 up 7780 down.
With ADSL2 and ADSL2+ only then down increases to 12100.
Do you have both G.DMT and ADSL2 ticked on your unit.
My ISP was reporting Errors on Up and increased SRN for both to 12.0 hence 7800N purchase.

I just have G,DMT ticked.

If I have both G.DMT and ADSL2 ticked then the router will sync using ADSL2. Also if I have G.DMT, ADSL2 and ADSL2+ ticked then the router will use the ADSL2+ modulation.

Previously instead of increasing the SNRM values with ADSL2+, due to high error rates, I chose to use ADSL2 with a forced downstream SNRM of 3dB (upstream remained at the default 9dB) and got a sync rate only slightly slower than ADSL2+ with a SNRM of 6dB. This I did in agreement with my ISP nearly one year ago.

However instability began just before Xmas so I reset the downstream ADSL2 SNRM to 6dB but unfortunately there was no improvement. Now I'm using the G.DMT setting with a downstream SNRM of 6dB and the line is performing really well

Edit: looks like you are syncing with interleaved ADSL1 when using the 7800N's G.DMT setting.

Edited by 4M2 (Mon 22-Jan-18 16:36:31)

Thanks
Now I understand why speed difference on G.DMT setting

Thought you might be interested to know that after a while errors started to increase with the Billion's DMT setting so I am now using the ADSL2+ setting with PhyR checked on the upstream and downstream. Seems to holding the connection really well with a downstream sync of ~11.5Mbps, 6dB SNRM and an attenuation of 37dB.

Similar to what I get on 36dB attenuation with ADSL2+. Never tried .biz route.
Do you ever get random wifi disconnections with your 7800N? I get these sometimes 3 or 4 in a day. Then maybe a week with none.