xDSL in a Nutshell

Digital Subscriber Line (DSL) technology enablesa digital modem to use a standard pair of copper telephone wires to carry far more data than is possbile using older, conventional methods. It employs sophisticated modulation schemes to achieve data rates of up to 50 Mbps under some circumstances. DSL variants are what the telecommunications business calls "last mile" technologies because they carry the signal just that las tmiile into yourhome rathe r than across or between countries. The best-known DSL variant is Asymmetric DSL (ADSL), which carries high-quality video data. "Asymmetric" here means that much more information goes to the subscriber than can be returned to the central office (CO). The downstream (to the subscriber) rate depends on distance: 8.4 Mbps at 9000 feet, which drops to 1.5 Mbps (just enough to support one MPEG video stream) at 18,000 feet. The upstream rate (to the CO) is only 16 to 640 Kbps. This makes ADSL better suited for distribution services (including Web browsing) than for any-to-any connections. On an ADSL line you can continue a phone conversation along with streaming digital video, and basic phone service is possbile even if the ADSL modem fails. High-data-rate Digital Subscriber Line (HDSL) uses more advanced modulation techniquest to de liver 2 Mbps at up to 12,000 feet without repeaters. SIngle-line Digital Subscriber Line (SDSL) delivers the same rates as HDSL over a sinlge line. BOth HDSL and SDSL a re symmetric: Data flows both ways at the same rate, and SDSL can often be used on existing standard phone connections. Very-high-data-rate Digital Subscriber Line (VDSL) iscurrently asymmetric, but with a higher data rate than ADSL: Downstream it can deliver from 13 MBPS at 4500 feet to 50 Mbps at 1000 (capable of carrying an HDTV signal). Upstream rates are around 2 Mbps. VDSL is intended mainly for implementing ATM LANs, and both VDSL and ADSL support error correction.

Implementing a DSL system involves two levels of switching. COs receive lines from the main public backbone and fan them out to local switching centers called distributed loop carriers (DLCs). THe DLCs contain devices called Digital Subscriber Line access multiplexers (DSLAMs). DSLAMs are the crucial new component needed for DSL. They combine LAN and ATM switches and routers into a single unit that multiplexes and routes different data types (including ATM and frame relay) to the xDSL service subscribers. In each subsc riber's home there's an xDSL modem, contained within a set-top box, network computer, PC, or other device.

Any of these DSL technologies might be a good candidate for a broadband phone system to replace (or bypass) ISDN, since running optic fiber into each neighborhood and then picking up the existing copper phone lines to subscirbers' homes requires far less investment than running fibre all the way into the home.