In reply to a post by lockyatlrg:

I have enjoyed solid 80mb sync for the last 3 years 80001 down and 19999 up (only few meters from the cab)

But on the 8th of this month it's dropped to 79996 down and still 19999 up.

It looks like a rounding error in the modem, when it negotiates the sync speed. I have never had a speed of exactly 80000 - it has always been a few Kb below. Even when the attainable was well above 80Mbps.

It also doesn't readily change between synchronisations. I'm pretty convinced that the only time I've seen it happen was around the time that new firmware was being sent out to the modems.

I suspect you would also see a difference if BT made some configuration changes to the tones at the top and bottom of each band.