[Bloat] ADSL2+ interleaving (Re: CFP: Workshop on Reducing Internet Latency)

[Mikael Abrahamsson <swmike@swm.pp.se>]

On Tue, 10 Dec 2013, Neil Davies wrote:

> We've seen this even when we've played with the settings through the 
> customer portal (I'm' in the UK - the provisioning portal allows many 
> such changes).
>
> Are you capturing any evidence of DSL modem state when this occurs?

Well, we saw the problems mostly on medium-short spans, such as 1000-1500 
meters. The packet loss wasn't as high as 20%, but it was consistant over 
time and showed up as errored seconds. I do not work there anymore, and 
this was 7 years ago.

Interleaving spans bits over longer period of time, so if you're running 
fast mode and you get a 1ms "hit" on the link, the FEC (forward error 
correction) gets too many bits corrupted and gets overwhelmed. If one 
instead has 16ms interleaving, a lot fewer bits from one packet gets 
corrupted, and FEC can correct the errors.

So your theory about line noise on specific frequencies is probably 
correct, you're not getting a big enough hit to trigger a re-train, but 
you're getting enough bit errors to cause post-FEC errors and thus packet 
loss. A re-train probably identifies the noise on those frequencies and 
uses them less.

In our DSLAM we could see the frequency band profile via a show command, 
have you done this?

-- 
Mikael Abrahamsson    email: swmike@swm.pp.se