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Author Topic: Resynchs and reconnects  (Read 1881 times)

Weaver

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  • Retd s/w dev; A&A; 4x7km ADSL2 lines; Firebrick
Resynchs and reconnects
« on: September 28, 2016, 04:26:01 AM »

Every week or two, very roughly, I see a line-down-line-up event reported on one of my three lines. I'm running them at a 3 dB doenstream target SNRM, and the typical SNRM reported at occasional snapshots after a line has been up for a few days is ~1.5 dB or less (very roughly).

I'm assuming that these events are modem-modem resynchs. Is the likely reason the failure of bitswap, things drifting beyond what it can cope with ?

How often do you see a resynch?

I'm assuming that these events reported are not some failure at the PPP level only. I can spot that some reported similar indications are administrative resets, probably due to kit being swapped out or upgraded or other reasons. I take it as one sign of such a case to be when CQM graphs (http://aa.net.uk/kb-broadband-cqm.html) get wiped out at the time of an event. Other than that it's just guesswork as to whether I'm seeing a straight loss-of-synch and modem-to-modem reconnection or some other kind of underlying event. Typical downtimes are 11 s or ~70s.
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Weaver

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  • Retd s/w dev; A&A; 4x7km ADSL2 lines; Firebrick
Re: Resynchs and reconnects
« Reply #1 on: September 28, 2016, 04:32:41 AM »

The frequency of these events is quite acceptable at the moment, and I expected problems when I first lowered the downstream target SNRM from the standard 6 dB to 3 (this is ADSL2). In any case, if a line goes down momentarily the other two lines are still functional, so I probably don't notice.

It's only going to be a possible problem with certain applications: How much of a practical disruptive effect it might have depends on how fast the routers can switch over and whether something like Netflix can cope with a short outage on one line - don't know what happens about retransmission with Netflix’s service. Clearly TCP won't care, retransmission will simply cover up such an event. Anyway, because I have several lines, the short interruptions may not be really disturbing me. If I start to use VoIP properly, this could be very bad news though.
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aesmith

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Re: Resynchs and reconnects
« Reply #2 on: September 29, 2016, 02:20:33 PM »

How often do you see a resynch?
Virtually never in our case, aside from self-inflicted disconnections.  I think the only time I've seen spontaneous resynch has been under fault conditions where either the error rate has gone though the roof or the noise margin has dropped sharply to below zero.

Regarding voice it would be interesting to test.  It would depend on how the load balancing algorithms at both ends deal with the loss of one circuit, for example if they maintain separate egress queues then when the line drops to you lose all the packets in that queue.   Also depends on how quickly the loss of the link is detected.  In terms of the end user, dropping say 1/2 a second of audio would be noticeable but not necessarily fatal, and from what you say it sounds unlikely that it would happen more than once in a call.   A quick looks suggests that you'd need a drop of several seconds before SIP should disconnect on loss of media, as usual with SIP it's a bit wooly.
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Weaver

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  • Retd s/w dev; A&A; 4x7km ADSL2 lines; Firebrick
Re: Resynchs and reconnects
« Reply #3 on: September 29, 2016, 02:45:17 PM »

> Virtually never in our case, aside from self-inflicted disconnections.

That's good to know. I suspect I would go back to ‘virtually never’ if I went back up to a 6 dB target SNRM.
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