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: [email protected]
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