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

Lack of demand, mainly.

[Mike1024]

Hey,

I would expect it's because there would be very low demand. They want to advertise 'Surf the web at 10* your normal modem speed', not 'Download and Upload, both at 5 times the speed of a normal modem'.

Furthermore, ADSL isn't designed for running web servers. You'd want a fixed IP, which they would then have to provide. You'd be connected constantly, not just when you were using it. You'd expect them to provide very good uptime, which would be disproportionately expensive for them to supply.

If you want a server, get a leased line or co-location. They're designed for running servers 24 hours a day, 365 days a year. If all you want is hosting, there are pleanty of hosting / virtual server companies around.

I'm afraid no-one's going to go to all the hastle of reversing ADSL speed caps, configuring fixed IPs, etc etc etc for a single user. It wouldn't be cost-effective. And there aren't all that many users demanding this service. Ask around some ADSL providers, by all means, but don't get your hopes up; I've never heard of anything like this being offered.

Michael

Re:Lack of demand, mainly.

[swright]

I agree with most of your points - but the fixed IP thing is more common than it seems (in the UK at least) - my ADSL comes with 5 static IPs (ethernet router not USB), and its well nice :)

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

Answers from a telco geek

[sydbarrett74]

I once asked this question to a buddy of mine who works for a telco and here was his answer. It has to do with NEXT and FEXT (near-end and far-end crosstalk, respectively). At your house, where you have maybe one pair of wires coming in, your ADSL modem can detect a signal because of a relatively high S/N ratio. Therefore, you can get a generous downstream bandwidth. Contrast that to the other end at the central office, where you have thousands of pairs coming in. Those thousands of pairs all induce crosstalk. In order to overcome the crosstalk, your ADSL device would have to output a stronger signal than is allowed under FCC regulations. So there is, according to my friend, a salient technical reason. However, other posters are correct. Asymmetry dovetails nicely with typical broadband usage. Joe and Jane AOLuser don't generate very much content at all. They are mostly passive consumers of content. It is only those of us who are more technically adept who have any interest in putting up web-hosting sites and such.