Why Can't ADSL Be Reversed?

John Macdonald asks: "An ADSL connection uses the asymmetric (that's the A in ADSL) bandwidth to provide much larger download than upload capacity. That's great for many situations, where people browse and collect, importing data far more than they export to the world at large. But there are some sites that could use the asymmetry more effectively the other way - with a large upload capacity and small download. This would work well for ftp and web servers, for example. So, why don't telcos provide this inverse capability? Is the hardware more expensive to run the other way? Is there just too little demand? Has nobody thought of it before? I'd guess that there is small enough demand that they prefer to only offer a symmetric, higher-speed, but also higher-priced, connection for such sites."

Comments

Apparently it's a signalling issue.

[Elequin]

From analog.com:

Some people have discussed 'reverse ADSL' - simply swapping two modems so that the high capacity direction is from the home to the CO. Unfortunately, in most cases this is not going to work. ADSL relies on all the 'loud' signals being located together (eg downstream sends are all at the CO), and all the weak received ones being located in a different frequency area, and physically separated. If you reverse this, then at the CO the loud 'send' of everyone else's downstream will be right where your reversed system is trying to listen to a very weak will be right where you are trying to receive the attenuated noisy weak high capacity 'upstream' - drowning it out. Conversely, your transmit signal will swamp everyone else. Given spectral compatibility constraints and 'good citizenship', this will limit reverse ADSL to perhaps 1000ft. Of course, up and down are arbitrary - what matters is everyone has to operate in the same direction. It is a little like driving; in the US people drive on the right; in the UK they drive on the left - either is fine, so long as you are consistent!

So it looks like you'd have to convince the telco to configure the entire DSLAM for reverse aDSL - for all the customers connected to it. Most likely not going to happen.

- Eric