CertiGuide to Network+  9  Chapter 0111: Wide Area Networking       9  V  High-Bandwidth Individual Remote Access

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xDSL

xDSL is the umbrella term for all varieties of DSL.

HDSL (high bit-rate DSL) is the earliest version of DSL. One of its distinguishing features is that it offers symmetric data transfer speeds – the max upload speed and max download speed are identical. Due to this equal splitting of upload and download throughput, the maximum data rate is lower for HDSL than for ADSL. Data rates for HDSL range up to the capacity of a North American T1 line (1.544 Mbps) or European E1 line (2.32 Mbps).

ADSL (asymmetric DSL) is the most popular version of DSL for consumers today. It differs from the original HDSL in that it doesn’t offer equivalent transmission speeds on the upload and download sides. Rather, it’s set up to allow for greater download speeds than upload, based on the assumption that most consumers want fast access to downloadable content on the Internet (like streaming video), but don’t really require fast upload speeds.

ADSL is technically capable of dramatic speeds, up to 7.1Mbps download, and 1.1Mbps upload. (The now-extinct Sprint ION service intended to provide voice/data service at these rates over DSL, but a variety of technical and business issues caused it to be a non-starter.) Practically speaking, most residential ADSL customers communicate at speeds between 144Kbps-680Kbps, or perhaps 1.544Mbps, depending on the Telco and the level of plan they’ve purchased. Typical prices for ADSL service range from 2 to 3 times the price of dial-up Internet access – ie, from $40 to $75.

SDSL (single-line DSL) is the common “business DSL” technology because it provides the symmetric data transfer speeds of HDSL, which allow businesses to efficiently position file and/or web servers for access by the Internet community. It is also generally marketed with additional features attractive to corporate environments such as Service Level Agreements (SLA’s) that guarantee a specific percentage of DSL service uptime, or a specific maximum response time to problem reports. If you are a SOHO user who really depends on your broadband connection, and don’t mind paying extra, SDSL may also be for you. Additionally, if you’re not eligible for ADSL, you may be able to (again, pay more for less speed, and) subscribe to SDSL.

At a max of 1.5Mbps in each direction, SDSL is generally not available in speeds as fast as technically possible with ADSL (if you could get them), but at worst, at its slowest data rate; it’s faster than ISDN.

IDSL (ISDN DSL) is a slower DSL technology that runs at 144Kbps, approximately ISDN speed. Its main advantage is that while regular DSL is restricted to no more than a couple miles from the CO, IDSL tends to be available at locations up to 5-6 miles away from the CO.

VDSL (very high data rate DSL) is an emerging DSL technology currently under development that will offer a higher data rates over shorter distances than are covered by other DSL technologies. For example, it may offer speeds of approximately 50Mbps over lines up to 300 meters in length.

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