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: [email protected]
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