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