What is DSL?
by: Blair Copeland (2011)
In the past there has been a lot of confusion over DSL so I thought I would make a couple of comments.
The name Digital Subscribe Line (DSL) is really the confusing part of this technology.
First, the DSL line is nothing special, it is just a POTS line.
Second, it is true that if the line has physical problems either in how it has been installed between the CO and the residential termination point, that the line may have problems. However, the only way to know is to send a signal over the line and measure the quality at the other end. If the signal does get to the other end and meets minimum strength and quality requirements, then the line is considered "good enough."
Third, the DSL signal is transmitted at a frequency not used by the POTS voice telephone equipment. By using this different frequency, you can get two channels on the same physical circuit.
Fourth, there is a piece of equipment placed on each end of the circuit that splits the channels off into different ports for difference services: one for the voice and one for the data.
There is a great resource on How Stuff Works