Why does upload traffic affect my download performance?

+2 votes
I have seen that my upload traffic affects my download traffic performance and have confirmed this by limiting my upload bandwidth on my router. What I would like to know is why.

If anybody could explain this to me I would greatly appreciate it :)
asked Apr 22, 2015 in ADSL by oXyc0doNe (360 points)

3 Answers

+3 votes
Hi Oxy,

Due to ADSL being asymmetric you will notice quite clearly that the issue is upstream traffic. Many people who are knowledgeable in regard to networks will attempt to classify the traffic and attempt to use QoS to solve this issue.

The problem with this is that traffic coming from the router itself does not go through the router and thus misses being identified and prioritised. Traffic is this statement is somewhat different to the usual definition of traffic. This traffic is TCP acknowledgement packets necessary for the downstream traffic to not keep resubmitting. A server when not receiving an acknowledgement within a certain time interval (TTL) resubmits the same data again.

As the router is the device that sends the acknowledgements or replies, this traffic was being drowned out by my normal upstream traffic and therefore waiting in a queue where it would be delayed and eventually become irrelevant in the equation.

By simply lowering the upstream limit - as you did - you essentially limited normal upload traffic to a rate below the max upstream limit leaving enough headroom for the 'unseen' acknowledgements. This immediately decreases the number of  resubmissions and repeated attempts to acknowledge the resubmissions, freeing up a huge amount of baggage on the network and immediately significantly improving performance including latency and therefore improving throughput and the quality of your connection.

I hope this explanation proves valuable :)
answered Apr 22, 2015 by Syb3r1us (1,220 points)
Well done. Get them points high for something nice coming your way.
0 votes
I think this happens because on ADSL the upload & download rate are not equal. With TCP IP every download packet needs an upload packet to send a response back to server you're requesting content from. So if you're upload rate is low, your download speed will be affected.
answered Apr 22, 2015 by theeandre (32,960 points)
0 votes
Weldone Syb3r1us. Well said!
answered Apr 23, 2015 by AfriDude (43,950 points)