to me waterfill seems bad as higher tones are more likely to generate ES.
I was thinking about this some more and that risk is minimal. In fact it's totally negligible on any line running at full capacity. Water-fill method only really affects the shortest of lines capable of getting full sync, by forcing more bits to be loaded at the higher end of the spectrum.
One of the other methods of bit load is the waterf
all method, which is fill up first then move to next tone. If you had a 40/10 provisioned line that was next to the cab, using the waterf
all method, this line would totally fill all 15 bits at the lower end of the spectrum and never use any of the higher frequencies. The water-f
ill method forces the line to use higher frequencies across the full spectrum range.
Trying to think of an easy way to explain the two types. Imagine all the individual bins are test tubes in a row, the test tubes are various lengths depending upon the SNRM. Waterf
all fills the first test tube with a hose pipe and the water then overflows to the next test tube until youve either used 80 pints of water, or filled each tube as much capacity as it can take.
Water-f
ill is a bottom up approach but based on the fact that water always finds its own level and you're simultaneously pumping 80 pints of water into each of the test tube from a hole in the bottom.. until the tube is full or the water runs out.
As mentioned above, UK DSL has been using the water-fill method since at least rate adaptive DSL (maxdsl and adsl2(+) with BE* & UKOnline).
Here's a screen cap of my maxDSL line syncing at 8Mbps quite clearing showing the water-fill method is in use, otherwise it would easily fill at 15 bits over the lower spectrum leaving nothing loaded in the higher frequencies.
Water-filling is much more efficient especially when you have multiple line lengths on the DSLAM and when used in conjunction with power management and/or masks allows more capacity for longer lines.
If you are going one step further and using DSM, you greatly reduce the risk of cross-talk on longer lines.
Whilst its good, it would be even better if they also used vectoring.
I'm not sure if the preference of loading U2 over U1 on shorter lines is a BT thing or global.. it could be more to do with Upstream PBO in U1 rather than a direct result of water-filling. I only mentioned it because its considered best practice to force capable lines to use the higher tones.