Here's a thing.
On my ADSL2+ connection using a BT HG612 (currently running the Wolfy firmware) I was studying the curious gaps in my bitloading, and note that all but one of them corresponds with an SNR per tone drop to 0 as well (see attached graph).
Roseway's marvellous DSLStats tells me that those tones are
110 474.375kHz
127 547.6875 kHz
188 810.75 kHz
191 823.6875 kHz
243 1047.9375 kHz
291 1254.9375 kHz
348 1500.75 kHz
All of these tones have a drastic drop in SNM and have a zero bitloading. Adjacent tones have a goodly number of bits allocated and no such SNM problems.
There's one other curious tone, 233 1004.8125 Khz but this does have a low bitloading of 2 and no drop in SNM.
I then discovered this thread in another place :
http://community.plus.net/forum/index.php?topic=105398.0Where
GrahamC notes in 2012 that he has gaps in his bitloading. And they are ... :
Gap1 474kHz-479kHz (Tone110)
Gap2 548kHz-552kHz (Tone127)
Gap3 811kHz-815kHz (Tone188)
Gap4 824kHz-828kHz (Tone191)
Gap5 1048kHz-1052kHz (Tone243)
Gap6 1255kHz-1259kHz (Tone291)
Gap7 1501kHz-1505kHz (Tone348)
Identical to mine! (excepting my tone 233 anomaly)
In that thread, he says he is in East Yorks, which is a good few hundred miles away from me, so it seems a little unlikely we share the same RF interference.
I notice
BaldEagle and a few others of this place commenting in that thread, has anything ever come of why there are those curious gaps? It does seem like some sort of software/hardware quirk to me.
Ian
Edit : Attributed DSLStats to wrong person