said by xShades :[BC]
One thing I noticed is that when my modem transmits data on the upstream my latency goes through the roof. It seems that the jitter is connected to transmitting data.
Running a constant ping to the first hop outside of my location I'm getting pings from 9ms to 50ms. Thats horrible line quality and worthless for any type of real time gaming or VOIP.
If I push data on the upstream and watch my ping at the same time it spikes all the way to 150-200ms while the data transfers and then settles down to the horrible jittery base.
Yes. Uploading can have a drastic effect on downloading due to the need of TCP to acknowledge the receipt of packets back to the sender. The rising size of buffers on modern routers (an issue called "buffer bloat") has been shown to be aggravating TCPs reaction to upstream congestion.
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netalyzr.icsi.berkeley.edu/Netalyzer by ICSI was written to check for buffer bloat and a few other issues and is a good tool to run on your network. It will tell you if your router is being an issue.
My apple airport extreme has almost 1M of upstream buffer which is a pretty bad situation for lag/jitter when uploading AND trying to download using TCP.
If netalyzer shows a large uplink buffer on your router, you might want to do some research for a router with less uplink buffer and try that, or bypass your router and run the test again.
You'd also want to then test your jitter again while bypassed or on a non-buffer-bloated router. This will help you find out if the service is the issue, or if TCP ACK packets being buffered because of your uploading is the issue.
It's my opinion that TCP requires a fairly large amount of upstream bandwidth free to have fast and jitter-free downloading.