Many ADSL users are surprised that when they trying and upload something via their ADSL connection, that their download speed seems to drop to nothing. User's are under that impression that if the upload speed is 4Mbps and the download is 512Kbps then you should be able to upload and download at those speed simultaneously. The conceptual model is that there are two "pipes" an up "pipe" and a down "pipe" that one should be able to "max out" at the same time.

Why does my ADSL download speed drop when I upload?

However, because of the way ADSL is designed, flooding a "pipe" in one direction will result in the other "pipe's" speed dropping to an unusable speed. Most users experience this when they are uploading a file and cannot surf the web at a reasonable speed any more. In the case of Small and Medium Enterprises (SME's) this can become a problem when the business decides to allow their remote users or sales staff access the internal network via VPN.

Throttle for your upload speed to allow usable simultaneous transmission

One way to prevent your download speed from being maxed out by your uploads, is to throttle the upload speeds so that they cannot flood the upload "pipe". Usually this can be done on your ADSL router or on your business firewall.

Another way to overcome this limitation, if you are a business or a wealthy individual :), is to make use of ADSL bonding. This allows you to aggregate up to 6 ADSL lines into one big pipe giving you 6 x the amount of upload and download speed!

ADSL still cheaper than digital leased lines

ADSL is still cheaper than using digital leased lines and, in truth, the inability of ADSL to handle simultaneous upload and downloads is done deliberately to force users to purchase the more expensive digital lines.