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New Contributor

upstream / downstream

I have a question about the relationship between upstream and downstream traffic, more theoretical than anything, I think. I may be asking entirely the wrong question. How much free upstream bandwidth is necessary to maintain a specific downstream bit rate? For example, one of our internet connections is 5Mbps up and 5Mbps down. Let' s say I want to be able to download something at 4Mbps. How much free upstream bandwidth do I need in order to maintain 5 Mbps downstream? Is there a way to calculate this? Assume no traffic shaping has been applied -- the computer connected to the internet has full access to the entire bandwidth. Thanks for indulging my curiosity!
Valued Contributor III

This is kind of a tricky question. It depends on the type of traffic (HTTP, FTP, SSH, etc), the type of protocol (TCP, UDP...) and the packet sizes... I don' t believe there is a cut and dry method for calculating this.

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Esteemed Contributor III

Absolutely. Surely the ratio up:down will not be 1:1. You could easily ' measure' that ratio for your mix of traffic: - sustain a continuously heavy download - saturate the upstream and limit the bandwidth substantially until you see that the downstream is being limited Second guess would be what the providers offer. My VDSL line is rated 25/5 Mbps, so 5:1 should be sufficient to not choke the downstream. With ADSL, 16/1, 6/0.5 is common which translates to 16:1 and 12:1 resp.


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Esteemed Contributor III

It depends on alot of conditions; type of protocol within TCP, MSS, TCP_DELAY ( nagle), the type of server process, your SRTT, window-size, etc.... TCP is only as fast as the returned ACKs. So having a bigger or more importantly; a faster return path, can help with the overall TCP transfer speeds. the problem is ; " your last mile is just on part of he picture" Also network links are changing every secs with load%, the thing same with the routers, servers and switches,etc..... So can' t say or expect the same performance for ALL traffic every time. just my 3 cts due to inflation Now if you want to see how important link speeds are, build a lab, and set up a asymmertical link in size, and squeeze your return path and monitor your thruput. This would give you an ideal of how bandwidth effects your delivery.




New Contributor

Sounds like it' s a bit more complicated than I was anticipating! Thanks for the lesson, everyone.