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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."
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
"Goodness me, how unlike the FBI to abuse the trust of the American public." -- The Onion
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
These connections are meant for consumers, those who consume, not those who spew.
It's the difference between a water supply (lots of download) and a sewer pipe (lots of upload). You try reversing either one, it upsets people.
I suspect the reason ADSL works at all is they can have a very expensive piece of equipment to hear a tiny signal sent from your ADSL modem, and can send a very strong signal that your modem can understand. This lowers the cost of the modem, which now only has to listen to a nice strong signal, and send a relatively weak signal.
The other half of the apple is that most consumers want high download, slow upload, and while the ISP may have a symmetrical connection, they sell the upload capacity to colocators and hosting customers. It's almost like selling the same bandwidth twice, but in our current low bandwidth society, the upload and download capacity really need to be considered seperately, and perhaps even charged seperately. Pulling info off the net is cheap, but pushing info onto it is expensive. This will continue to be true as TV and other broadband services supply more information than they accept.
-Adam
Can't be done because the entire DSLAM has to be reversed... and that just ain't gonna happen.
On a side note, I have a SpeedStream 5250 *S*DSL modem. I have been trying to get these to work back-to-back but without too much success. Technically it should work (SDSL can work both ways and the DSL chipset (a brooktree part which does HDSL, weird eh?) can work in slave or master mode. I'm planning on buffering the 5250's processor's datastream and injecting my own configuration commands to see if I can't get the DSL to sync. Getting the ATM channel to sync may be another matter altogether but they were cheap and I have some time to play. :-)
One thing I would love to ask the /. community for is the 5250dnld.exe program that used to come with a special disk with these modems. SpeedStream has removed all traces of the program and there are too many black gnutella clients which try to send you a "Fun Loving Criminal" win32 virus whenever you request an .exe. Any help or technical data (any rogue efficent networks employees out there?) would be much appreciated! Hell I havent even been able to find a console port, something I would have betted on there being. The flash upgrade program and (I'm assuming) the 5250dnld.exe program work by sending specially crafted 802.3 SNAP packets to the bridge.
It's a nice intellectual tech question, to which the answer seems to be (a) no, it's not that simple, because (b) no demand == no engineering effort to make it work.
:-)
There's also a practical question - how do you get a decent amount of upstream bandwidth from your home? The best answer appears to be the higher tiers of ADSL; here in Austin TX, the top tier is 1.5M/6.0M and hence you're getting (almost) T1 speed up. A colleague of mine runs a mini hosting service from his house as a sideline, using this technology. Comes with a static subnet for around $300 pcm, which is still a good bit cheaper than a T1. Like most minor pushers, he does it to feed his own bandwidth habit; 5.8M of that downstream is his to play with
T1's are also coming down a lot, and $700 is now the bottom end of pricing - this may be viable if you have some $$$ coming in to support it.
For myself, I use a $45 pcm cable modem, which only allows 0.3M up, but it's plenty for the range of services I host off my home box - my own usage is still msotly down (esp big downloads) and the steady 2.0M down at this price is good value.
The lack of a static IP on my service is an annoyance, but dynamic DNS is a good substitute, andI only ever change IP's on a reboot, which is typically once every 2-3 months; I have the source code for the specialised DHCP client daemon so I guess I could hack it to request the previous IP over again.
A few thoughts:
/want/. (ie, high traffic)
...
.. hand over your slice of the pie, buddy
... letting the masses distribute content brings way more headaches than letting them get access to other content
hosting is 24/7 - you probably can't download so much as to piss off your bandwidth provider (cause eventually you sleep, eat, etc), but if you offer uploads, thats what you'd
hosting brings content problems - telcos do not want to offer 'joe' the ability to host. many telcos actually forbid hosting, even if you use a dyn-ip solution or whatever. they just dont want to be bothered by the potential infringements, problems, illegal material, etc
hosting is a 'value add' thing - if you're giving access to content to people, you should (according to telcos or whatever) be making money from it, so
anyhow, forget the technical implications or limitations
"Old man yells at systemd"
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.
'He who has to break a thing to find out what it is, has left the path of wisdom.' -- Gandalf to Saruman
The "telco geek" was right when he noted that there'd be too much NEXT/FEXT (crosstalk). It only works because all of the signals are loud and soft together. The CO transmits at a higher frequency than the modem (frequency split) so the CO-end signals are weak at the modem and, more importantly, the modem-end signals are all weak at the CO. Run it backwards and the CO-end modem would clobber the signals coming in from remotes. The cable on the street was designed for voiceband, and has too much crosstalk at the 50 kHz-1 MHz band used for ADSL.
BUT there are alterantive technologies. SDSL is popular for business; that's Covad's big item. It was designed for symmetry, using echo cancellation on the same frequencies rather than frequency splitting. Of course it's more expensive -- SDSL cannot share the wire with analog phones, so you need a dedicated loop. Plus it is sold to businesses, which tend to use it more intensively than consumers (average bytes/month), so the ISP price is set accordingly.
A technology I really like is Paradyne's MVL. It uses "ping-pong" signaling on the wire, avoiding voiceband frequencies so it can share the loop like ADSL but not SDSL. MVL (also used, with modification, in AGCS' SuperLine) transmits a frame in one direction at a time. If there's nothing to send, it sends a minimum-length burst. So the bandwidth is adaptively directional, allowing you to have a client or server without takling to your provider.... It avoids crosstalk by staying below 120 kHz. Its peak speed is 1 MHz, depending on loop length, but that's still adequate for most users. Oh yeah, unlike ADSL, MVL will work on crufty loops with bridge taps, over 25,000 feet (at degraded speed), just so long as it's unloaded (no series inductors).
Not many telcos use this stuff, since it's not "standard", but a few do; I know a CLEC startup that does. (I've no interest in Paradyne; I just like the technology.) Rumor has it that Rhythms had some, but alas that will probably be confirmed by their pending asset auction....