Uploading is killing my download speed

[#1]

I am using a web based backup service, and while I am uploading (at around 100-140 KB/s), my download speed drops from a normal speed of around 18 Mbps to 0.28Mbps.
I don't fully understand how this works, but is this correct? Surely I am using only a tiny part of my whole bandwidth for uploading, so why is downloading more than 50 times slower?

[#2]

Assuming that your figures are correct and you aren't on fibre you are saturating your upload as 140kBps (kilobytes per sec) is 1120 kbps (kilobits per sec)

[#3]

If your upload is unrestricted it will use all the available (upstream) bandwidth, 100-140kB/s is 800-1120kb/s (Bytes to bits) so is about maximum for a normal ADSL connection.  Saturating the upstream bandwidth will mean that acknowledgement packets for download have to be 'squeezed in' with the rest of the traffic so this will slow down your download.

[#4]

Thanks guys, that all makes sense now. Wish I could afford fibre 

[#5]

Some software allows you to limit upload or download speeds, it may be worth investigating whether you can restrict upload to, say 90kB/s (720kb/s) which should then leave plenty of space for the (very small bandwidth) download ack packets.

[#6]

If you are feeling adventurous and using a TG582N take a look at this:
http://community.plus.net/forum/index.php/topic,116556.0.html
I think I was a bit over excited when I posted that  however it did work extremely well.
You need to ensure that the Ack packets have a higher priority than your bulk upload traffic.

[#7]

I've got a Netgear DG834, and as far as I can tell there isn't anything I can configure to prioritise Ack packets.
Limiting the bandwidth available to the backup service hasn't made any difference.

[#8]

How much did you limit by?
From memory I think you would need around 3 - 400Kbps upstream to maintain an 18Mbps download.

[#9]

I put it down to 50 (control panel says that is kilobits per second) from 400.

[#10]

If you install http://www.softperfect.com/products/networx/ you'd be able to see if the limit was actually being applied.

[#11]

Thanks jelv; installed the monitor and found the restriction wasn't working.
Rebooted PC and upload is now running at 50-60 KB/s, and download at 13Mbps.
That will have to do until I can afford the extra tenner a month.