I am having the same issue on my business connection 5Mbps as in another posting on this site. As soon as any computer starts a download the latency on the first hop past our DFG provided by our ISP (Bell) goes off the chart.
This first hop is likely in our same building and is 1ms constantly, start a download, eg windows update, and it jumps to 200-1000ms.
I have spent hours on the phone with support all saying you have reach the max available bandwidth, it is normal for your latency to spike. But my reading tells me they are breaking something with TCP. I have run tests on a home Shaw connection and even on a Rogers LTE running downloads and reaching the max Mbps for my account but the latency does not go through the roof.
Am I right in my understanding that Bell is doing something to break TCP's built-in technologies to manage its rate based on the available bandwidth between the 2 end points?
Best Answer
Bell is telling you the truth. When you try to push 5Mbps (or more) into a 5Mbps connection, everything files into a neat little order (read: queue.) Your ping goes out without delay because there's no backlog. The reply, however, is now at the end of the queue. TCP is doing exactly what it's supposed to here -- the sender is filling the allowed receive window.
There are things you can do on your side of the line (QoS, WRED, etc.) to help reduce the effects, but this is the sort of thing you're going to see when there's a large difference between sender and receiver bandwidth. I've lived with it for years (T1, 6Mbps DS3, even 10Mbps cablemodem) You could ask the ISP to reduce the queue size on their side, but they're not likely to do it, as it'll result in packet drops.