Why Can't ADSL Be Reversed? 23
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."
Lack of demand, mainly. (Score:2, Informative)
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. (Score:2, Informative)
Re:Lack of demand, mainly. (Score:1, Interesting)
The question seems to be asking "Why can't I get something for nothing?"
Re:Lack of demand, mainly. (Score:2)
No problem. My ADSL comes with a static IP address range. That makes an ideal testing playground for a web site. But once you've got it all working, why would you want to leave it on the end of your DSL line rather than sticking it on your ISP's web server, or if it's more complex, have them host machines for you?
Apparently it's a signalling issue. (Score:5, Informative)
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
Re:Apparently it's a signalling issue. (Score:1)
It is simple economics (Score:1)
The cheap rates they offer for aDSL assume you are a home user. They have a different pricing plan if you want to use that same connection to run a buisness. A typical home user is going to be downloading in short bursts from time to time. A typical web server is probably going to see near constant uploads.
The telcos probably have some sort of a stocastic model to tell them how much capacity they need at the central office so that most of their users can use most of their bandwith most of the time. That determines how big of a pipe they need from the CO to the rest of the internet.
So you could probably reverse that 768/128 connection, but the telco would just charge you the same as if you were using a 768/768 because of the type of bandwidth you'd be using. I don't believe for a second that equipment costs enter into it at all.
They sell these things to consumers (Score:2)
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
Re:They sell these things to consumers (Score:2)
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.
There isn't really much difference in the "loudness" (amplitude) of the CO end or your end. If the CO end is "loud" then it will crosstalk like crazy.. it's just the way your upstream and downstream "pipes" are encoded. More bandwidth is given to the downstream pipe so you can get more bits per second. And to go the other way, less bandwdith is given within that pipe.
I guess a good analogy would be to think of your phone line as a sewer pipe. Within that sewer pipe you have your graywater pipe. Now the bigger the graywater pipe, the less the sewer pipe can move in the other direction, and vice-versa.
ADSL can't be easily reversed because the DSL chipsets aren't designed to reallocate bandwdith arbitrarily. The DSLAM is designed to use so much to send to you and your DSL modem is designed similarly.
Re:They sell these things to consumers (Score:1)
"backwards" ADSL (Score:3, Interesting)
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.
Re:"backwards" ADSL (Score:2)
Can't be done because the entire DSLAM has to be reversed... and that just ain't gonna happen.
Dammit I hit submit instead of preview.
What I was trying to say is that the head end is a DSLAM and the remote ends are just access devices. The remote ends are not capable of encoding the fast downstream and even if it were capable of doing so, it would likely interfere with everyone else.
ADSL in general is meant for consumers, not producers. Get yourself SDSL, HDSL (1.5 Meg bidirectional on two pair), HDSL2 (1.5 Meg biderectional on one pair) or something larger. But be prepared to pay through the nose.
Re:"backwards" ADSL (Score:1)
Is it just our local telco, or is HDSL2 (one pair) garbage? We've always had HDSL circuits and they work fine, but then they started putting in HDSL2 circuits between us (ISP) and T1 customers a while back. One of them had the circuit for about a month or two, was offline more than online, until the telco converted it to HDSL (and since then haven't had problems.) Another customer has an HDSL2 circuit, and to be honest, is down as we speak (has been all weekend - had to call telco the other day and have them test it just to get the thing to work!)
Unless something's up with our telco [bellsouth.com] then stay away from HDSL2 if possible. :)
Re:"backwards" ADSL (Score:2)
Is it just our local telco, or is HDSL2 (one pair) garbage?
Good question; I've only every worked with HDSL. I'm not sure what HDSL2's benefit is over SDSL -- We've used Pairgain's Megabit Modem 300s for quite a while now. They're good for 2.048Mbps full duplex and while I don't think they can use repeaters like HDSL, if you have that much distance to go I don't think you'd bugger with HDSL2 anyway.
Pragmatic Answer (Score:2)
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.
its not their business (Score:2)
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
Answers from a telco geek (Score:2, Informative)
The name of the game in ISP is sharing (Score:1)
In order to support this business plan, they have to solicit an audience who wants that kind of service (again, many
Why is there no reverse ADSL? Because it would only be of interest to those operating servers and consuming sufficient bandwidth that the consumer service plan is unsatisfactory. In short, someone who is using lots of bandwidth for a long period of time.
Why would an ISP go provide a service that would lose it money , and break its business plan? That would be dot-bomb logic (Or Crazy Eddie: "We lose money on each transaction, but we make it up on the volume!")
"Standard" ADSL would have crosstalk (Score:2)
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....