[Bloat] ADSL2+ interleaving (Re: CFP: Workshop on Reducing Internet Latency)
neil.davies at pnsol.com
Tue Dec 10 03:07:37 EST 2013
Don't have direct access to the DSLAM, but do have access to that information in the modem - and yes and there appears to be a large change in certain frequency buckets.
The loss rates appear to vary with the offered load - i.e the "loss" only occurs if there was real packet data in that frequency band
On 10 Dec 2013, at 08:04, Mikael Abrahamsson <swmike at swm.pp.se> wrote:
> 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 at swm.pp.se
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