what's the differences between ADSL and Extended ADSL?

ADSL, or asynchronous DSL, allows faster download speeds than upload speeds, e.g 768/256, where 768 is the maximum allowable download speed, in kilobits, and 256 is the max upload speed.

SDSL, or Synchronous DSL, has the same dl/ul ratio, e.g. 768/768

That's the only real difference. SDSL services are considerably more expensive than ADSL, intended for corporate applications where a greater upstream speed is necessary.

RaDSL isn't as popular now as it used to be, and when it was popular at all, it was only for business service, and it was expensive.  My guess is because the CO equipment required to support RaDSL is more expensive, though I could be wrong.  Mx speeds for RaDSL (well, in 2001) are 7mb down, 1mb up - but you have to be very close to the CO to get that speed.

Another flavor is IDSL, which is essentially an ISDN connection over a DSL line.  Speed is 144/144, or 128/128 if the ISP decides to cut the D channel out.

All kinds of DSL *except* IDSL require a dry copper pair from the CO to the endpoint.  No repeaters, no fiber, no load coils, no heat coils, no nothing.  IDSL is able to run over fiber and repeaters, etc, because of its origins in ISDN - a communication designed to run on analog phone lines.  For this reason, IDSL can be installed in sites that are too far out for other kinds of DSL.  However, like RaDSL, it tends to be expensive for the speed, and is not usually installed in residences.  (I know 144K doesn't sound like much, but it's a whole lot better than dialup.)

ADSL and SDSL are as jsa decribed above.  SDSL max speed is 1.5Mb/1.5Mb (gotta be real close).  ADSL I'm not sure about, I got out of the installation biz before it got big.

One of the things I know is that the 18,000 foot limit is nonsense.  What matters is resistance.  Yes, the longer the wire, the higher the resistance, but the quality of wire matters a lot more than the length.  I did plenty of installations for Telocity and Rhythms (remember them?) where customers at 22,000 feet got 784K or 1Mb SDSL speed, while customers at 13,000 feet could only get 208K.

When you go to your local phone company/DSL provider site and get your distance from the CO, all that is is a number out of a phone company database.  How did they get that number?  MapQuest.  For Ameritech/SBC in Chicagoland, I'm not kidding.  They will only do a real test of loop length if specifically requested.  That loop length test is a test of resistance, and length is calculated based on wire gauge and resistance.  This does not take into account other things which could cause high resistance, like bad connections or bad wires.

Getting CO distance from MapQuest also doesn't take into account that neighborhoods aren't necessarily fed along the best driving path from the CO to the site.  Your line could go half a mile past your house from the CO to a distribution box, then half a mile back, adding 5280 feet to your loop length that MapQuest won't ever tell you about.

The DSL providers now limit their customers to being within some distance from the CO, usually 15,000 feet.  This helps ensure that when they do the paperwork, provision the circuit, configure the routing and send the self-install kit to the house that it'll actually work without any additional effort.  The distance requirement is less a technical requirement than a profitiability one.

Sorry for going on.

and of course there is:
HDSL (symmetrical) - "HDSL (High bit-rate Digital Subscriber Line), one of the earliest forms of DSL, is used for wideband digital transmission within a corporate site and between the telephone company and a customer. The main characteristic of HDSL is that it is symmetrical: an equal amount of bandwidth is available in both directions. HDSL can carry as much on a single wire of twisted-pair cable as can be carried on a T1 line (up to 1.544 Mbps) in North America or an E1 line (up to 2.048 Mbps) in Europe over a somewhat longer range and is considered an alternative to a T1 or E1 connection."

and VDSL (asymmetrical) - "VDSL (Very high speed Digital Subscriber Line)  is a new technology expected to provide speeds as high as 52 Mbps downstream and between 1.5 and 2.3 Mbps upstream, but over shorter distances than ADSL - 1.3 km at 13 Mbps and 0.3 km at 52 Mps. Note that by deploying higher performance lines from exchanges to street cabinets, these speeds could be delivered to more homes."

ckratsch gives a really good synopsis of how DSL works and how the lines are qualified.  I used to know of a website that had all the information on the DSLAM and CO(s) serving each NPA-NXX but I cannot find it:( If I do, I'll post it.

I started installing DSL in 1999, and woke up from that dream job in 2001.  I do recall hearing about HDSL, but never came into contact with it.  And it sounds like VDSL was developed after my time with that industry.

I'm really quite disappointed over how the DSL industry faltered, failed, and was consumed by the telcos.  Especially considering that the telcos had at least a little to do with third-party providers failing.  It was the dotcom boom, and everyone was throwing money at everything, being loss leaders just to get market share.  That's why you used to be able to get dry-line 1mb SDSL at 20K feet for $40 a month.  The CLECs (third parties like Rhythms, Northpoint, Focal and Covad) were losing money hand over fist, because they depended on the ILECs (telcos) for the infrastructure.

DSL was originally developed in the 1960's, but the ILECs owned the copper, and they weren't going to lease lines to their competitors.  It wasn't until the 1990's that some gov't agency (FCC?) changed the rules such that ILECs were obliged to lease circuits to competitors, and at cut rates even.

So the ILECs followed the rules, and leased the lines.  But there were never any rules saying that the ILECs had to give good service to the CLECs.  Invariably, circuits that were supposed to be installed weren't, circuits that were installed were rewired after the fact, and the ILECs were generally uncooperative with the CLECs.  That's why there were so many situations where the ISP would point the finger at the telco, and the telco would point back at the ISP.  Meanwhile, the consumer is thinking, "Maybe I should just get cable."

Between CLECs throwing good money after bad on their end, and the ILECs making things more difficult than they needed to be, everyone went bust.  Except Covad, and they're just barely in the black now, and you never hear about them anymore, either.  The cable TV companies finally got their act together and upgraded infrastructure so that the kid on the block running Napster 24/7 wasn't using up everyone's bandwidth.

DSL, which is technologically sound, and I think would run on coat hangers now takes a backseat to cable.  Cable is technologically sound, too, but requires *new* infrastructure.  DSL runs on the same old copper wires that have been in the ground for decades.

I'm personally looking forward to the day that fixed wireless matures.  I had that at home for a while, and it was great service.  The ISP was a little sloppy, and the tower I was pointed at was oversold.  Word on the street is that 10mb up and down is available for a scant US$60 a month.  If you want a gigantic antenna on your roof.  Which I think is cool, and which my wife thinks is a horrible eyesore.

During that same period, I worked for a business grade regional provider. We did a few HDSL connections. I remember that the line qualification was the hardest part--there is copper and then there is good copper. Most of what we did was SDSL, it was a GREAT solution since leasing copper was so cheap, that is until the ILECs realized that they were losing out on a "business opportunity" and made it very hard to lease dry copper at a reasonable rate. Sounds like you dealt with the same issues we did. We dropped the high-speed DSL product for most and shifted to SONET DS1s. Kept the SDSL only in areas where we controlled the copper (in-building runs, etc.). VDSL bears watching, though I am sure it will face most of the same issues. Businesses are just better off with the telco-monitored circuits (DS, T, OC etc.).