I have adsl (1024/256) and two computers connected to the internet via switch. Everything works well, until one computer starts uploading with full bandwidth. This affects the other computer, and the result can be seen in very decreased resolving and sending time.
This also happens if I limit the upload speed on both computers (let's say to 10 KB/s) - if they're uploading both at same time upload speeds from both computers add up (= 20 KB/s). But the thing is that download speeds aren't affected at all, not even if both computers download with full bandwidth at the same time (computer1 1024 and computer2 1024 also). Ergo, download is okay, but upload is kind of messed up.
I have to say that each computer has its own IP on the internet (and numbers follow one after another locally for LAN). Local network is working fine, but internet traffic can be tricky.
Network switch: LevelOne FSW-0508TX.