Why does my download slow down when I'm uploading?

because there is only so much bandwidth available to use. if you are only downloading, that download can use as much as possible of the available bandwidth, but if you upload the same time as download, that upload has to use the same limited amount of bandwidth, so you lose download speed to give some to the upload.

@wenn: I suspect that’s incorrect. a DSL connection (in this case ADSL) dedicates frequencies to up- and downloading, although uploading can cause download speed problems with swarming P2P programs like Bit Torrent.

@saservp: Are you using Bit Torrent?

The bottleneck can be the bandwidth, but often it is the local processor getting saturated.

What is happening is this. When you transfer data your computer sends out what are called ACK Packets. It lets the computers you are getting data from know you received what was sent. When you saturate your upload sometimes those ACK Packets don’t get sent out. This slows things down since you are basically not allowing the other computer know that you are ready for the the next part of your download.

Most torrent clients allow you to limit your upload. I shoot for about 75% of my max upload. With your connection I would only upload about 25KB per second.

http://i.imgur.com/5YCZ9.png

@johnpowell: that was exactly the reason I was asking about Bit Torrent. However, it’s not yet clear that @saservp is using BT – if they’re not, it’s a very odd phenomenon indeed.

Processor and hard drive IO. Both the upload and download tasks access files on your hard drive.