Is my ADSL full-duplex or not?

Recently one of my apps went out of control and began uploading continuously as fast as it could, more or less saturating the up channel. I noticed to my surprise that this coincided with an extreme drop-off of my down channel, to the extent that I could barely browse and was even having to wait for DNS lookups to happen. This made my ADSL unusable for practical purposes, so I had to kill the rogue app and stop the uploading. Then downloading returned to normal.

Being over 4 km from the exchange, I get about 2.8 Mbps download at best; this sometimes drops off a lot, possibly due to contention. I get about 0.37 Mbps upload at best. But when uploading continuously, the download almost disappears - maybe 20 Kbps at best.

I had always believed that ADSL provided separate and independent channels for uploading and downloading, although with very different bandwidths. But my ISP says that "ADSL is asynchronous meaning they both share the same capacity, so higher upload usage reduces the download availability". That doesn't look right to me. For a start, I don't see what asynchrony has to do with it. And I read on Wikipedia (the source of all perfectly authoritative correct information!) that:

"Currently, most ADSL communication is full-duplex".

and

"A /full-duplex/, or sometimes /double-duplex/ system, allows communication in both directions, and, unlike half-duplex, allows this to happen simultaneously. Land-line telephone networks are full-duplex, since they allow both callers to speak and be heard at the same time. A good analogy for a /full-duplex/ system would be a two-lane road with one lane for each direction".

The only weak link there is the "most" in the first quote. Is British (BT) ADSL an exception, in that the up and down channels share bandwidth? Or is it just that when the up channel gets saturated, the necessary acknowledgements etc. are slowed down?

Lastly, is there any reasonable workaround?

ADSL is full duplex, however - like any TCP/IP connection it requires spare bandwidth in order to transmit ACK packets, to acknowledge the data has been received. If you saturate your upload, then its unable to transmit this information - and as such your download will decrease.

Best thing to do, if possible - is if the application has a way to limit its upload - then limit it so you always have approximately 15kb/s (120kbits) of bandwidth available left over on the upload to allow for this data transmission to occur.

Unfortunately it is a flaw with most ADSL systems that once you start reaching in excess of 8mbps, then the upload becomes saturated anyway since its typically only 448kbps (good job its not still on 256kbps like it used to be )

Hope this helps.

Many thanks, BorgMaster, for your full, very helpful, and almost instantaneous reply! I have tried out your recommendation, and it seems to solve my problem. It's good to understand a bit more about ADSL - I had always assumed that uploads had no effect, probably because I was just sending short emails etc. I'll bear your comments about speeds in excess of 8 Mbps in mind - if I ever get more than 3 Mbps, that is! 8-)

I know your question has already been answered...but I just wanted to point out that your ISP is wrong about ADSL being Asynchronous - it is Assymetrical, which makes a lot more sense!

Thanks, JonRennie - I suspected as much, but didn't want to get side-tracked.