Theoretical bandwidth of DSL

[jimhsu]

The article on dailytech (http://www.dailytech.com/Vendors+Edge+Towards+1Gbps+DSL+Connections/article20001.htm ) had me thinking - what's the maximum theoretical bandwidth of various broadband solutions? I know for example that fiber gets less attenuation than copper twisted pair wires, and thus operates over MUCH longer distances with the same bandwidth.

Possibly relevant wikipedia article: http://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem

Anyone want to take a stab at it?

NB: Poster does not have a physics degree, but does have some ugrad physics/comp experience. Please try to explain what you're doing.

 

[airdata]

Cool post. I didn't know any advancements were being made w\ dsl. Whenever somebody tells me they have dsl, I generally estimate that they are running around 1-2.5Mbps.

825Mbps using copper wires for a distance of 400 meters. Huawei has also announced that it has been able to transmit data at up to 700Mbps over the same distance.

 

[jimhsu]

Yes, that 400m is exactly what is wrong with current DSL technology right now. Sure, you can get amazing speeds ... but within a block of your telco. Fundamentally (as far as I know), I think there's a limit that simply prevents DSL from being that fast once you're talking about thousands of meters. No amount of "software processing" or clever algorithms can compensate if you simply have more random noise on the line. I just don't know what that limit is.

Now if the noise is not random (then it wouldn't be noise, it would be unwanted signal), you could convolute the signal with an inverse of the unwanted signal and increase bandwidth that way. Can't do that if the noise is truly random.

 

[Modelworks]

The problem with DSL is the lines were not installed with DSL in mind. Remember this was originally a voice only network. Many of the installs were considered a success if you could hear the person you were talking to without much background noise. Even when modems were used they considered 14Kb as sufficient quality. The fact we could do DSL over those same lines without pulling them all out is a miracle. DSL can go very very far at high speeds if you install the lines properly to support it. AT&T uverse is fiber to the node but DSL to the home. AT&T realized that the problem with DSL was that the wiring between the central office and the home was the issue, not the technology itself.

With the proper wiring the distance you could use DSL would only be limited by the loss of signal in the wire. The problem is making it work with the existing sub-standard wiring networks.

 

[Modelworks]

Cool post. I didn't know any advancements were being made w\ dsl. Whenever somebody tells me they have dsl, I generally estimate that they are running around 1-2.5Mbps.

Been using DSL for years now , 10Mbps (20Mbps available) and never had it slow down from time of day users like I had with cable.
http://www.centurylink.com/Pages/Personal/SpecialOffers/internet6.jsp

 

[TecHNooB]

Very broad question.

The only realistic limit is who's willing to pay for the extra bandwidth. There's so many transmission techniques that it's hard to answer this question. Unless were strictly talking 1 type of medium, 1 lane, raw bits, no repeaters, etc. The term bandwidth also isn't very clear when you're in bit land. In other words, signals nowadays are often referred to as serial when the bits themselves are actually sent out in parallel. What makes it different from parallel is the use of buffers at the receiver end rather than relying on a really precise clock.

 

[Anteaus]

I've lived in a few places and have used both DSL and cable. On paper, DSL is generally considered the better of the two, but in reality cable almost always does better. Why? Because many DSL carriers are using old telephone infrastructure to deliver it and it simply isn't as good as newer cable installations.

In the end though, performance is highly subjective of what tech is available and how its used. Where I live, I pay for a 22 Mbps Cox pipe and I regularly see 25+ Mbps downloads (our system is unlocked due to lower than average bandwidth usage) whereas the local DSL provider is a old local telephone company who doesn't have the resources to upgrade their systems to a higher level. Move 50 miles in any direction (well you'd be in the water if you went south from here), and you'd would find a much different picture with local DSL beating out a subpar cable provider like Charter.

Also, I would like to remark both systems use shared bandwidth. Cable does it frontside, and DSL does it on the back end. Either way, the amount of users in your area DOES make a different whichever you choose.