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I've noticed that my broadband link always gives me a sawtooth shaped utilization graph when I'm downloading large files. It looks like this from Task Manager on Windows 7:

Sawtooth network utilization

When I'm doing large file transfers over the local network, this doesn't seem to be an issue and I get a nice and stable high speed transfer.

My network topology is like this:

PC <-- wired connection --> Linksys router (WiFi) <-- wired connection --> ADSL modem

How do I diagnose whether this is a problem with my router/modem or if it's a problem in my ISP's network?

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It could be how your ISP is throttling you. Are the peaks higher than what your ISP promises? – zdan Feb 6 '12 at 5:52
@zdan I think you're right. It does look like ISP throttling. Yes, those peaks are higher than my current plan. – Ragesh Feb 6 '12 at 10:27
up vote 3 down vote accepted

If you do not have this while transferring locally, then it sounds like you are experiencing throttling by your ISP to keep your speeds at what you paid for. What is likely happening is that it only happens when you have a really fast and sustained connection; the transfer gets faster but then your ISP kicks in to prevent you from getting faster than the service you subscribed to allows.

I’d bet that if you averaged the peaks and troughs out, you would find that the overall average is right around what your Internet connection speed is supposed to be. In other words, instead of keep the speed constant, the ISP is letting it spike, but then accommodates by dropping it. If this is how it is at all times of day and all days of the week, then it means that your ISP has poor network management (though ultimately you are still getting the speed you paid for). If it only happens during “peak hours”, then what might be happening is that your ISP is providing you with a “boost” where they actually let you go higher than than your service provides so long as the network has spare bandwidth.

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Thanks for the great answer. This does, indeed, look like my ISP is throttling the connection. – Ragesh Feb 6 '12 at 10:28
Does it happen at night as well? I experience the saw-tooth pattern sometimes when my normally 10Mbps connection gives 15Mbps (my ISP calls it “Speed Boost”), but normally (eg “peak hours”) when it tops out, it looks more flat. – Synetech Feb 6 '12 at 19:43

There are a bunch of possible reasons, but is probably worth reading because it covers one of the common causes in the set of connection technologies you list there.

To diagnose, nothing beats a process of elimination. Change each component out or eliminate it so you identify where the problem lies. Doing that with a highly repeatable test set is nice, but that just means "send the same data with the same software".

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