You've probably picked up two or three points about upload affecting download, but in general it doesn't.
The two cases where I know it can are:
- for people on ISPs who allow Annex M for upload connections. Annex M allows up to 2.5Mbps upload compared to Annex A, (the default in the UK), which only allows around 1.3Mbps to 1.4Mbps. On medium to long lines Annex M can cause a fall in downstream connection speed.
- people doing P2P downloads with too many open streams. This can choke the upload with too high a volume of ACKnowledgements. As broadly speaking the downloading server of a stream will wait for the ACKs before sending the next load, clogging the upstream with these for a lot of streams will cause a waits/slowdowns in the download. Think of how traffic on a crowded motorway can slow to a halt even when there is no actual blockage, and imposing a 50mph can in fact get everyone through more quickly as it prevents that. (Queuing theory
). That's what happens to the upstream with too many streams. But! If you double the upstream sync rate, that's the equivalent of doubling the number of lanes on the motorway. So where this effect is concerned, increasing the upstream sync helps rather than hinders.
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