Puzzling network throughput behavior to my home

0

I wonder if anyone can suggest why I see this puzzling behavior:

When downloading something large to my home (via Comcast), I get very slow throughput (e.g. 12 Mbits/s). Then, if I start a speed test while the download is ongoing, it will slow down for a couple of seconds as I'd expect, but then suddenly increase to 5x the initial speed for the rest of the download. This is in a fully-wired network. Laptop->Switch->Firewall/GW->Modem(Cable)

What could cause this increase in throughput when running a speed test? Could my ISP be unthrottling my bandwidth when it detects increased demand? Could the speed test be triggering a TCP behavior (I would expect packet loss due to congestion to cause a decrease in window size which I would expect to decrease my throughput)?

BobDoolittle

Posted 2019-03-30T00:39:55.130

Reputation: 124

What's the modem showing for SNR in it's webgui? Might not be a bad idea to check the logs for errors. Does this behavior change when you reboot the modem? Does it seem like it takes an unusual amount of time to reboot an become ready? Please add this additional information to your question and not the comments section. – Tim_Stewart – 2019-03-30T02:43:21.010

1It coud be (and to me sounds like) Comcast gaming the speed test. Not an unheard if practice. – davidgo – 2019-03-30T18:59:43.957

@davidgo, they don't. Unless it's prime time during the evening, he should be bursting well above the speed he is paying for. Even at prime time he should be right around +5%/-5% of the cap. I've seen this behavior before with a defective STB causing excessive noise on the customers inside wiring. Unfortunately, I worked with these clowns out of high school. You might get 1/10 techs that understand what a decibel reading is when troubleshooting customers IW. – Tim_Stewart – 2019-03-30T20:46:56.443

1I acknowledge it could be the gateway/modem firmware/software issue - but see that when doing a speedtest the caps appear to magically vanish. – davidgo – 2019-03-30T21:32:18.833

1While I cant speak for Comcast, I do have insider knowledge that gaming speedtests is not an uncommon practice. – davidgo – 2019-03-30T21:33:34.237

@BobDoolittle - what happens if you disconnect your firewall and connect your PC straight to the cable modem? – davidgo – 2019-03-30T21:34:42.580

Answers

-1

A number of suggestions:

Maybe the server you are downloading from is slow, is limiting its speed or is located far away. Try downloading a file of similar size from another server.

A speed test will only give you a 'true' result if you use a speed test server that is fast, not limiting its speed and is located as close as possible to you.

Did you try to ping the server that you are downloading from? Note that the download server might be different from the website you start the download from.

NZD

Posted 2019-03-30T00:39:55.130

Reputation: 2 142

This isn't a question about why my download is slow. And it's not a question about my speed test, per se. It's a question about why running a speed test would SPEED UP a download in progress. One would expect the other way 'round - it should slow down. I still can't think of any explanation other than: My ISP must be gaming speed tests by unthrottling my connection when it detects one running. So I guess I should game my ISP by keeping a speed test running at all times in order to get good download speeds :-). Crazy times. – BobDoolittle – 2019-04-09T22:35:21.597

Yes, you are right; I misread your question. One additional thing you can try is to download the same large file from a Windows box and also from a Linux box. If the download times differs significantly, then the problem is linked to the TCP/IP implementation on your machine. – NZD – 2019-04-09T22:50:11.747

Asked: 2019-03-30T00:39:55.130

Viewed: 123 times

Active: 2019-03-30T02:18:44.953