High-volume downloads can make simultaneous interactive connections practically unusable. Just like in the bad old days of modems, users press keys and then wait for seconds for the server to finally echo the character on screen. TCP/ IP needs enough free capacity in both directions. No matter whether you are uploading or downloading, packets still travel in both directions. The device at the receiving end of the download is expected to acknowledge the receipt of every packet it receives, and obviously the receipt travels in the upload direction towards the source.