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I have an ongoing saga with my internet connection; where the latency periodically goes through the roof. In most cases, this seems to coincide with something uploading data.

I'm not having a lot of joy with the ISP (Sky), so I'm trying to do my best to gather information that might help. First off, I need to figure out if this is normal. I don't remember ever having this sort of problem in previous properties; but our phones didn't back up every photo/video recorded then (and we didn't have kids, so there were less of them!).

It's an ADSL connection; downstream is 11Mbs, upstream is 1Mbs. The usual culprit is Google+ on Android doing AutoBackup of photos + videos.

Here is a graph of ping times to the router, google.co.uk and sky.com for 20 minutes. The first 10 minutes there was nothing uploading (just some general surfing, and background chatter from a PC, two smartphones, two tablets). The second 10 minutes is during an auto-backup of a video to Google+ (which seems to take far longer than I would expect).

Timeout was set to 5 seconds; and all timeouts are recoded as 10000 in the data (so they go off the top of the chart). Both Green (Sky) and Red (Google) lines are pretty consistent, so the red line is mostly obscured by the green. The blue line has only one ping that was high at 2000ms.

The problem obviously isn't just that ping returns high numbers; the connection is utterly unusable during this time.

Ping times

Raw data is available here: https://gist.github.com/DanTup/800e72d05adf8a0a5cef

Is there something wrong; or is this normal for ADSL? I can imagine that Google might be able to receive my data faster than my line can transmit it and saturate the upstream; but I would expect people to generally be unhappy with ADSL if this was normal?

If it's not normal; any further tips on providing info that might help Sky investigate/fix it would be welcome.

Here are some router stats if they help. The initial line testing (DLM?) started at 2Mbs and went down, so someone at Sky manually set us at 11-12Mbs/1Mbs based on these stats.

Router stats

(Posted to Sky forums too)

Edit: Video

http://youtu.be/0V1TGfCnsKc

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  • Upload is used to send ACKs (among other things), TCP performance will go all to hell if it can't send ACKs reliably. Can you limit the uploads?
    – NickW
    Nov 18, 2014 at 17:19
  • I've seen this. you can test with jperf to confirm. You gotta limit the uploads or get a better line... Nov 18, 2014 at 17:24
  • Short answer (although not helpful at all) - nope. This is definitely not normal. What is even more discouraging, is the fact that your line parameters looks good enough. Does your ADSL lose the carrier and reconnect often ? Not sure from these pictures.
    – drookie
    Nov 18, 2014 at 17:25
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    It's entirely normal if you're saturating the line, it's not difficult to do.
    – NickW
    Nov 18, 2014 at 17:25
  • @EugeneM.Zheganin No, it doesn't often disconnect; though we sometimes restart the router trying to fix it. I wondered if the initial testing gave poor speeds due to some issue, and manually setting it to 12Mbs sort-of ignored this issue? Nov 18, 2014 at 17:26

2 Answers 2

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The problem is widespread enough that it even has a name. It is most frequently seen on the upstream, this is likely due to the upstream bandwidth being lower than the downstream bandwidth. The phenomena can happen on any link, but it requires more effort to get the problem on a fast link.

How much extra latency you get depends on various factors. I have observed this cause only 100ms of extra latency in some installations and as much as 60s of extra latency in others. It is not ADSL specific, my observations has mainly been with docsis modems.

If you feed all your traffic through a router that you control before it enters the modem, you can throttle traffic on the outgoing port on the router before it reaches the modem. If you throttle the traffic to 90% of the uplink capacity, you shouldn't see any latency caused by the modem.

There is no guarantee the router is going to do any better. But at least you can chose a router that can do it properly.

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Today Sky support let me talk to a techie, who said no, this is not normal. It is (as expected) normal for ping to rise; but not for the connection to be completely unusable. They can't see any faults or explanation. There's no noise on the line, error rates are low, etc.

They're sending a new router, and the investigation will continue if that does the same (which I suspect it will)!

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  • did a new router help? Oct 14, 2018 at 21:37
  • It's been a long time so I don't remember the details, but Sky did some tweaking of things and then at some point Fibre came along and we changed ISP. We've been on fibre (80/20) since and not had any issues like this. Oct 15, 2018 at 6:42

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