[AusNOG] Strange ADSL problem - troubleshooting / diagnosis advice wanted

[Paul Brooks]

On 10/09/2014 1:55 PM, Beeson, Ayden wrote:
>
> I'll have another look at actual router level access from the WAN but I'd still see
> it in my netflow collection from that interface.
>
>  
>
> I'm 99.99% percent it isn't coming from me or even to me directly, I thought the
> same as you and did a lot of digging to double check.
>
>  
>
> I have both SNMP and Netflow monitoring and logging configured on the modem and
> neither show any significant activity during these time periods on either the up or
> the downstream.
>
It may be a re-training bug between your modem and the DSLAM. ADSL2+ is supposed to
re-train to a new sync rate seamlessly without packet drops, whereas the older ADSL1
had to effectively drop the line, re-sync, generate a new bitfield table per tone to
cope with changes to background noise and cross-talk that caused a need for a re-sync.
Perhaps there is a bug causing the DSLAM and the modem to become confused about the
number of bits per tone, and which bits go in which DSL tone, on the transmission
path. That would cause a very high bit-error-rate, causing packets to be dropped when
checksums and fields come through corrupted.

Have you tried a different make/model DSL modem?

P.

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