igh-Bit-Rate Digital Subscriber Line (HDSL) Symmetrical (or Single-Line) Digital Subscriber Line (SDSL) Symmetric High-Speed Digital Subscriber Line (G.SHDSL) Asymmetrical Digital Subscriber Line (ADSL), including ADSL2, ADSL2+, and ADSL2-RE Rate-Adaptive Digital Subscriber Line (RADSL) Very-High-Bit-Rate Digital Subscriber Line (VDSL), including LR-VDSL2-12MHz and SR-VDSL2-30MHz Projections Versus Reality Once in a while, there is a great lesson to be learned from reflecting on market research. The first edition of this book says the following: "According to Cahners In-Stat Group (Business Communications Review, July 2001), the worldwide DSL subscriber base was expected to include some 7.5 million users in 2001, 12 million in 2002, 17 million in 2003, and 23 million in 2005." Generally, we find that most forecasts are not accurate, and in fact, far from reality-and usually the reality is much lower than the predicted out- come. In the case of broadband access, I'm happy to report that reality has far exceeded early expectations: Whereas in 2001, market researchers forecast 23 million DSL subscrib- ers by the year 2005, the reality is that, according to Point Topic ("World Broadband Sta- tistics, Q3 2005," www.point-topic.com), we had more than 125 million DSL subscribers worldwide as of September 2005-and by the first quarter of 2006, there were more than 150 million DSL subscribers worldwide ("Thriving DSL Surges Past 150 Million Worldwide Subscribers," DSL Forum, www.dslforum.org). How DSL Works The history of DSL goes back to 1988, when Bellcore (which is now Telcordia; www.telcordia.com) created DSL as a technique to filter out the incessant back- ground noise on copper wires and to allow clearer connections through the use of electronic intelligence in the form of DSL modems at either end of the twisted-pair line. In essence, the engineers came up with a way to carry a digital signal over the unused frequency spectrum available on the twisted-pair running between the local exchange and the customer. By usin