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Lately when downloading large files from the internet, commonly streaming a video or downloading a large application the transfer speed starts out good - maybe 300-400 kilobytes/second. Then over time (next 15, 30 seconds) it slows down to at little as 75 kilobytes/second. Running a speed test often comes back with good results, but the speed test is done over such a short period of time that it seems to not be a real world test case.

I've heard that if this type of drop-off occurs that it's most likely a router problem. Can you guys confirm or deny this assertion?

In either case, what types of things should I look at to diagnose and resolve this?

Some info about my network:

  • cable broadband

  • fairly new Linkysys wifi router

  • devices experiencing this problem include iPad, iMac, and Apple TV.

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What is "cable DSL"? If it's over a phone line, it's probably DSL. If it's over a coaxial cable like cable TV, it's probably cable broadband service. – Spiff Sep 21 '10 at 5:13
"...but the speed test is done over such a short period of time that it seems to not be a real world test case." Not to mention that those speed tests report the absolute peak speed reached, however briefly it occurred. Watch the tests in action and decide whether you believe the final numbers. They're ISP-serving tests and totally bogus. – JRobert Sep 21 '10 at 10:45
@Spiff - yeah I guess you're right, it's cable broadband, not DSL – Marplesoft Sep 21 '10 at 17:16
@JRobert - good point - do you know of a test that will show true transfer speed over time and not just peak speed? – Marplesoft Sep 21 '10 at 17:16
I wish I did. At this point I resort to watching the test take place and sort of mentally averaging it. If I really needed the information, I guess I'd try to figure out the size of the file they send and then time it. – JRobert Sep 21 '10 at 23:57
up vote 1 down vote accepted

The speed of a connection is more closely related to latency than to how many megabits per second you're paying for. Megabits per second is really just the size of the pipe, not the speed. In order to truly test, you will need to bypass your router and plug directly into your modem. If you ping your DNS for an hour does the return time remain constant? if you increase the packet size, does it still remain constant? I'm willing to bet a lot of the issues stem from downloading over a wireless connection.

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Good point - I will do some testing plugging directly into the modem. – Marplesoft Sep 21 '10 at 17:14

What kind of speed are you actually paying for? Cable companies (like Comcast) have PowerBoost that will increase the speed at the beginning of a download that. My normal download speed is 2MB/s but usually the first couple of seconds Comcast sends data at 3.5MB/s and then it slows down.

The problem with the PowerBoost and speed testers is the boost is usually in effect with a large enough (but short enough) download from speed testing sites (like SpeedTest.Net).

If your speeds are noticably lower than what you pay for, you may want to contact customer support for your internet provider.

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Thanks for the thoughts but I'm more interested specifically in the drop off of transfer speed over time as opposed to questioning the overall bandwidth. – Marplesoft Sep 21 '10 at 4:02

If your ratio of bandwith upload/download is very asymmetrical it can happen that your upstream is easily congested. When the ACK-packets of your download-connection get delayed, the download speed will automatically be throttled.

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Depends a bit on your provider. My provider does this on purpose and even has a name for it "PowerBoost". Generally it's called 'traffic shaping' and a lot of ISPs do it to provide a more enjoyable experience for the majority of their customers.

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