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Author Topic: ADSL2 upstream performance suddenly improves massively, just after power cut  (Read 1551 times)

Weaver

  • Senior Kitizen
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  • Posts: 11459
  • Retd s/w dev; A&A; 4x7km ADSL2 lines; Firebrick

The other night we had a power cut over a large part of the island for an hour or so, two thousand households knocked out. When service was restored, I found that all of my modems’ upstream sync speeds had gone up, two of them dramatically so. The combined total measured upstream throughput was around 0.9 Mbps before, afterwards it was 1.34 Mbps, I make that 46%. One line improved its sync rate slightly, going from 510kbps up to 534kbps, but the other two went from ~400k each to 547k and 537kbps.

Any ideas why this sudden pleasing jump in sync rates should have happened?

The only thing I can think of is that a lot of neighbours’ kit had not come back on yet, perhaps all kinds of electrical items, not just DSL, so the neighbourhood had be electrically quieter. Can only pray that the improvement is sustained. The downstream speed hasn't changed much, whatever that means. Speculating, if we assume that the improvement is about reduced levels of low frequency noise, then that would be consistent with the idea that it’s mains equipment-related, as opposed to crosstalk and external RF from say radio stations

I'm using a Firebrick router to split the upstream load between the lines. I’m assuming that ATM overhead reduces upstream throughput so that it equals 87% of the sync rate quoted, and I've set the Firebrick to rate limit upstream to 99% of the 87% * sync_rate for each individual modem, slightly below 100% so that the Firebrick is control rather than the modems, if I've done my sums correctly. So according to my spreadsheet, we are achieving a TCP payload throughput (assuming that is what the speed tests are reporting, although they could have corrected for IP and TCP headers, to give a much more meaningful reported figure) that equals 96% of the combined 99%-rate-limited ATM payload rate.

To convert a speed test-reported modem throughput figure to something that I can meaningfully compare with ATM payload rates, I possibly have to adjust the speedtest-reported figure to account for overheads of TCP and IP headers, and I certainly have to adjust for overheads due to PPP + PPPoE + Ethernet + RFC2684 sec 5.2 + AAL5 trailers. Leaving aside TCP and IP for the moment, the PPP + PPPoEoA etc stack overhead of 44 bytes added to the ATM payload means an overhead of one extra ATM cell for a 1500 byte IP PDU, therefore that overhead alone accounts ~3%, so ~97% is the ideal maximum, which means that at 96% the Firebrick is doing a very good job.

However, before the power cut I never used to get such a high efficiency figure, so perhaps the Firebrick doesn't like a situation where lines differ greatly in their speeds, as one line used to be ~25% faster then the other two. Or alternatively, it could be a false result if the speed tester has improved somehow.
« Last Edit: October 27, 2016, 07:01:41 AM by Weaver »
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Weaver

  • Senior Kitizen
  • ******
  • Posts: 11459
  • Retd s/w dev; A&A; 4x7km ADSL2 lines; Firebrick

Update: unfortunately it was only temporary.

After a day or two, I re-examined the modems and re-ran the speed-tester result. The upstream measured throughput has now dropped to 1.11 Mbps, down from 1.34 Mbps after the power cut. This is still better than it was before the power cut but I doubt that even this lesser improvement will hold up for long.

Currently one modem has dropped back to roughly its old upstream sync rate, another is showing the same high 537 kbps u/s sync rate but at an unsustainable 0.5 dB u/s, whereas the target SNRM is 6 dB. So this one will resync back to its target at some point when it can't hang on any longer. I'm not noticing packet loss with that modem, so it's still coping for now.

Interestingly the efficiency percentage (of measured throughput compared with total ATM payload as discussed in the previous post) has dropped right back to 86% instead of its previous 96%, which was at the ideal maximum. I just don't know where this lost 10% has gone. Changing the Firebrick rate limit values to be very high, so that the modems are in control of limiting the throughput, rather than the Firebrick, seems to have no effect. I feel that somehow the Firebrick is to blame for this. I noted that 1.1Mbps is consistent with what you would get if all three modems were running at the rate of the slowest one.

I can rule a few things out. My rate limiter values were not way too low, as increasing the values to be extremely high does not give an upstream rate improvement. The Firebrick is not permanently running slow in every case, as the results straight after the power cut were ideal. And my calculations for overhead are not way off.

It could be that the speed testers are a bit rubbish and there is simply some performance loss or misreporting there, or problems with the path over the internet. I have tried several different speed testers though and taken the highest reported result.

Another possibility is that the speed testers somehow don't like mismatched line speeds, and the Firebrick is not to blame. I don't know whether the various testers are using TCP or not. Packet reordering or jitter caused by the three lines’ unequal speeds might somehow affect the numbers for all I know.
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Paul C

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Hi a bit a go we had a major high voltage fault which took out the power for most of the area, we get power cuts 3-4 times a year so I have rigged  up an inverter and battery to keep the lights on. we are have a long line about 10Km and prior to my present problems we had about 4megs normally but during the power cut router running on the inverter out of interest we re synced at 5meg then the power came back and the SNR dropped to about 1dB and fell over.
I put this down to the neighbors router re-syncing and the general noise on the line increasing, I think the lack neighbors is the reason we have the speed we have in the first place ;D
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