On Wed, 30 Apr 2014, Jan Ceuleers wrote: > On 04/29/2014 07:01 PM, Toke Høiland-Jørgensen wrote: >> However, as that graph shows, it is quite possible to completely avoid >> bufferbloat by deploying the right shaping. And in that case fibre >> *does* have a significant latency advantage. The best latency I've seen >> to the upstream gateway on DSL has been ~12 ms. > > I am not an expert, but I believe that this is due to the use of > interleaving. This is a method to improve the strength of forward error > correction by spreading out the effects of impulse noise on DSL lines > across multiple reed-solomon-protected codewords at the expense of > latency. You're exactly correct. ADSL2+ interleaving can be set to 0 (off), 4, 8 or 16 milliseconds in the downstream direction and 0(off), 1, 2 or 4 in the upstream direction (if memory serves me right, it was 8 years ago I did this last). So to avoid lost packets due to impulse noise, most set this to 16+4, and plus the regular encoding delay for ADSL2+, you often end up with around 25ms RTT to the DSLAM. -- Mikael Abrahamsson email: swmike@swm.pp.se