My attianable rate was 110mbit when I first checked, no idea if it was ever even higher, BT estimated me at 65mbit and my cabinet to socket distance is 200m. Documents I have read on crosstalk consider that with 20 lines active in a bundle on vdsl2 a average loss of sync speed is estimated at 40% so quite significant, 50% of 110mbit is 55mbit, so my 65mbit estimate I guess isnt too far of a 40% loss and I think thats what BT base their estimates on.
Reports from people like Qas who have shorter lines than me but lower attainable speeds the probable reasons are they have more crosstalk, vectoring apparently will get rid of this unpredictability as will get rid of most of interference aka crosstalk.
The graph that I think comes best to showing the effect of crosstalk on UK lines can be found in a Broadband Forum document on Vectoring
- See figure 6 on page 12. It covers profile 17a, but is for 26 AWG cable (0.4mm) while BT seems to use 0.5mm, which ought to give better speeds for the same distance (but no idea of the impact on crosstalk).
Ignoring vectoring, the blue crosses show the range of speeds seen at the same distance, and demonstrates how it varies. I imagine that BT's estimator responds with numbers that match either the lowest cross, or the (even lower) inverted triangle.
But seeing the graph leaves me dreaming of the day that vectoring comes in...
I also read in a few places Huawei are quite advanced on vectoring tech but ECI not as much, so I wonder if this will lead to a staggered rollout of the tech or will BT wait for both vendors to be ready, alcatel already rolling it out,
I've seen it suggested that BT's ECI equipment is limited to perform vectoring within the linecard (ie within one card on the shelf of the cabinet), and that new shelves are needed to allow vectoring to work across all lines on all 4 cards.
Perhaps new cabinets, such as the BDUK ones, will come with the upgrade.
No idea about Huawei though.
With this I think lines at 400m will get 80mbit sync (as they can do now with no crosstalk). I hope tho if vectoring is used speeds stay at 80 so people can enjoy healthy snr margins with DLM leaving them alone rather thn pushing for a slightly higher speed product of 100mbit and then have lines fighting with DLM again.
From that graph, it looks like 120Mbps at 400 metres ... but I know what you mean.
I suspect that BT's research guys are playing with this (and DLM) over in Suffolk, working out what is going to work best in reality, rather than theory.