What is DSL

What is DSL?

OK, we know that the phrase stands for Digital Subscriber Line.

And we know that in the telco world, the subscriber is the customer, usually a residential customer.

And we know that when the telco want to sell "fast internet" connections, they answer with DSL.

Obviously, DSL is not using an analog modem over a POTS line.

The idea behind the name is that this is a digital line, not a POTS line. What is not clear is how "digital" it really is.

First, there is no such thing as a digital wire. All signals are sent down wires as electrical voltages. The "digital" part is that the signal is encoded in some binary encoding scheme, rather than being encoded using something like FSK (frequency shift keying) of an analog signal.

What are the types of DSL?

I know of at least three:

ADSL, asynchronous DSL, which piggybacks on a normal POTS line, providing relatively higher speeds for downloading and lower speeds for uploading
SDSL, synchronous DSL, which delivers the same speed up and down. The delivered speed is usually significantly lower than the "higher" speed provided for downloading on an ADSL line.
IDSL, DSL over ISDN. .

There may be more flavors, depending on your local telco.

How far can a DSL link go?

The answer depends. On lots of things, including how long the wire is from your house (actually from the "demarq" where the telco wires come into your house, and the telco's "central office".

Here is a cool engineering graphic of speed versus wire length for ADSL links.