[Bloat] [aqm] the side effects of 330ms lag in the real world

[Pedro Tumusok]

On Wed, Apr 30, 2014 at 8:53 AM, Jan Ceuleers <jan.ceuleers at gmail.com>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.
>
> The topic is briefly discussed on the ADSL Wikipedia page.
>

Depending on the interleave depth you get extra latency. But there are also
different schemes you can use

Interleave, the one that basically came with ADSL.
Phyr, Broadcom evolution on interleave, that has less latency, I believe,
in most cases.
G.INP, which I have been told is a standardized version of Phyr.

All of these will affect the latency and by how much is also dependant on
the configuration you have done on your DSLAM. To add to the mess, not all
DSLAM's support phyr or g.inp and then up the complexity a bit with CPE
that supports one, two or all three of the technologies.

Ohh some ISPs might even not use interleave and just have the DSLAM setup
to do Fastmode/Fastpath or what the vendor decided to name the feature.

Pedro




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-- 
Best regards / Mvh
Jan Pedro Tumusok
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