Why is ADSL so grossly asymmetric?

[#1]

I don't have a problem with my connection, I'm just curious. My download speed is regularly between 6 and 7 Mbps on a nominal 8 Mb line, so no complaints, but why is the upload speed, normally around 300Kbps, so much slower. I know this is normal, I just wondered why, if the line can support the higher speed, there is such a difference?

[#2]

There is only a limited frequency range that can be used for ADSL
This is all laid down in the national and indeed international standards to ensure everyone is using the same frequency bands around the world for each application.
The decision was made at the start for the standard broadband offering in the UK to make use of most of the frequency range for the download....and only use a small amount for the upload.
A bigger frequency range means more data can be transmitted and hence it will become faster.
The result is much faster downloads than uploads.
It would be quite possible to allocate more of the frequency range to the upload - but it must be taken away from that part of the range used for the download - so the download speeds would reduce.
I think there are some business products that are true symmetrical ie plain DSL but whether Plusnet have them available or not I've no idea and also whether a special router might be required I've also no idea.
Edit:
ADSL2+ works by extending the frequency range even higher which allows the download speed capability to go up much further.  However as the frequencies used get higher and higher the attenuation of the signal on the line gets MUCH worse as the distance from the exchange increases.  This is why longer lines derive no benefit from ADSL2+.  The higher frequencies used for ADSL2+ have simply attenuated away to a zero signal by the time it gets to their property.

[#3]

Hi x47c
Thanks for that very clear and concise explanation.

[#4]

Quote from: x47c

I think there are some business products that are true symmetrical ie plain DSL but whether Plusnet have them available or not I've no idea and also whether a special router might be required I've also no idea.

http://www.plus.net/business/sdsl/
And yes, a specific router would be required.