DSL: A Variety Of Flavors
Baskin-Robbins (31 flavors) and Crayola (64 colors) have nothing on Digital Subscriber Line when it comes to variety. Even though DSL has only a handful of basic "flavors," the different levels of service within each of those basic components makes for a rich - if ultimately confusing - array.
Start with DSL's five primary groups: Asymmetric DSL (ADSL), High-data-rate DSL (HDSL), ISDN DSL, Symmetric DSL (SDSL) and Very-high-data-rate DSL (VDSL). Of these five, the simplest - and least robust - is IDSL. Created mainly to reach customers that are located more than 18,000 feet from the nearest phone company central office, IDSL delivers low-speed service (usually 56 kilobits per second or 64 kbps, two ways), putting it in about the same performance class as Integrated Services Digital Network.
Things get more interesting and confusing from here. Both HDSL and VDSL - not to be confused with voice-over-DSL, or VoDSL - typically are used in special applications. For instance, local phone companies have used HDSL technology for decades to extend T1 (1.544-megabit-per-second) lines to corporate customers.
SDSL has emerged as the most attractive business-class DSL variety because it delivers high bandwidth in both directions. A typical SDSL service operates at half-T1 speed (768 kbps) two ways, although lower and higher speeds also are available.
Then there's ADSL, which has become the primary type of DSL service now offered; it can deliver data to subscribers at rates anywhere from 100 kbps or so to 7 megabits per second. The "A" - for Asymmetric - in ADSL signifies that the return path - from the subscriber to the network - is lower, typically ranging from 64 kbps for the slowest ADSL offerings to 1 Mbps for top-grade ADSL. For potential subscribers, the factor that makes ADSL confusing is that a service from a single provider can deliver a range of speeds, depending on factors such as proximity to the central office and service price. The factor that makes ADSL frustrating is that subscribers in many cases won't know exactly what kind of service they can get until the DSL provider installs it. That's because the physical condition of most local networks varies considerably, and any one of those variations could affect not only ADSL performance but also availability.