I went fttc with Plusnet at the beginning of January. IIRC my official distance to the cabinet is about 800m. I'm getting the full 40Mb down and 10 up and as my guaranteed minimum is 26 I'm not doing badly. S/N margin is usually 6.9 to 7.3 dB down and 6 to 6.2 up with very little continuous variation - only about a 0.2 dB or so "sawtooth" on the Routerstats graph. I've also noticed that when I'm downloading something at full chat (which speed tests usually yield around 36Mb/s), as e.g. a catch-up programme on the Sky box, the noise margin down drops by around 0.8 dB.
Now, everything has been rock solid, as described, until the other day (about 90 days) when an electrical fault tripped the ring main RCB and, of course, the router had to re-sync. This it did with the same up and down figures though I noticed that the maxima available had increased slightly. However the downstream S/N now had a sawtooth of about 1dB - i.e. rapidly varying up and down between about 5.8 and 6.8 dB.
Last evening I downloaded a BBC catch-up on the Sky box. I did not know it at the time but about when I did it (19:58 hours), according to the router log, the router must have re-sync'ed. This was probably as once the full chat download would start the noise margin would approach only 5dB and trigger a resync.
I didn't look at the Routerstats output last night but today the Routerstats graph is like it used to be - much flatter. The down S/N is around 7dB with only a tiny sawtooth, about 0.2 dB variation. I am back with my original connection.
Can someone please offer me a technical explanation?