An ADSL question we frequently get asked is "Why does web browsing, you-tube video watching or software downloading grind to a halt when I am uploading?" This is a good question as  ADSL is, after all. a full duplex technology, which means that it should be able to upload and download at the same time, albeit at vastly different speeds. So why is it that when one is uploading to flickr or you-tube that you cannot get to your new site?

ADSL slow download speeds when uploading, explained

The culprit here is your limited ADSL upload speeds, and the way TCP/IP, the newtork protocol that the Internet uses for communications, works. Let me explain. When you download a web page, picture or video over the Internet, the sending computer breaks the large piece of data down into smaller pieces called packets, and send these packets to your computer, using your download speed.

Your computer receives these packets and assembles them into the original, larger, piece of data i.e web page, image etc.  For each one of the packets your computer receives it sends an acknowledgement packet, or piece of data, back to the sending machine to say that it got the packet ok. This utilises your upload speed. If the sending machine does not receive the acknowledgement packets in time, it will reduce the rate at which it sends data until it can more accurately match the speed of sending data to the speed with which it gets acknowledgements from your computer. So if your computer cannot send the acknowledgement packets back quick enough, the sending machine sends at a slower rate, making it look like your download speed has dropped.

Now with that explanation behind us I can explain why your uploads cause your downloads to drop to a crawl. The basic problem is that your low upload speed, currently a maximum of 512kbps for the 4Mbps ADSL, is to blame. If you are uploading a picture of video, you are utilising so much of your upload bandwidth, that your computer cannot send the acknowledgement packets back fast enough to the sending computer. Since the sending computer does not get the acknowledgement packets at an acceptable rate from your machine, it drops the speed with which it sends them, hence your poor download speeds when uploading.

Slow ADSL download speeds when uploading - how to fix?

So what can be done to fix this problem? For most people uploading happens so seldom that it is not an issue, and they are happy to put up with the inconvenience rather than fiddle with their ADSL router, but if you are someone that plays games, or uses bit torrent and other file sharing applications what can be done to fix this problem?

The answer is not as complex as it might seem. All you need to do is throttle your applications upload speed so that it does use the whole available upload pipe. You should always leave sufficient upload bandwidth available to send acknowledgement packets at the fastest possible rate. You can usually adjust the upload speed settings on your ADSL router, using a combination of outgoing port number and max upload speed settings. Some of the application also allow you to specify a limit in the applications configuration options so you can change it there too.

So no longer do you need to wait for your upload to finish to download or surf the web!