Types of DSL: ADSL and SDSL

The DSL Internet Service actually has “subdivisions” and it is good to know more about them to help you decide which to install in your home or in your small business. There are two types of DSL: Asymmetric DSL and Symmetric DSL.

Asymmetric DSL (ADSL)

This type of DSL connection gives the consumer more bandwidth when it comes to downloading as compared to uploading. Because of the reduced upload rate, Internet service providers are able to offer greater bandwidth in terms of downloading. This technology is best used by residential customers since they normally use more bandwidth for downloading. The normal rate for downloading is at 5 Mbps and 1 Mbps when uploading. Examples of Asymmetric DSL are: RADSL, VDSL and ADSL/G.Lite.

RADSL (Rate-Adaptive DSL) - is an ADSL technology from Westell in which software is able to determine the rate at which signals can be transmitted on a given customer phone line and adjust the delivery rate accordingly. Westell's FlexCap2 system uses RADSL to deliver from 640 Kbps to 2.2 Mbps downstream and from 272 Kbps to 1.088 Mbps upstream over an existing line.
VDSL (Very high data rate DSL) - is a developing technology that promises much higher data rates over relatively short distances (between 51 and 55 Mbps over lines up to 1,000 feet or 300 meters in length). It's envisioned that VDSL may emerge somewhat after ADSL is widely deployed and co-exist with it. The transmission technology (CAP, DMT, or other) and its effectiveness in some environments are not yet determined. A number of standards organizations are working on it.

Symmetric DSL (SDSL)

This type of DSL connection have equal bandwidth for upstream and downstream activities. Symmetric DSL connections are more popularly used in business because they have higher requirements or needs when it comes to data transfers as compared to home Internet users. Normally, Symmetric DSL can offer up to 1.5 Mbps both for download and upload. Examples of Symmetric DSL are: HDSL, SDSL/G-Lite and SHDSL .

HDSL (High bit-rate DSL) - is the earliest variation of DSL to be 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. For this reason, the maximum data rate is lower than for ADSL. HDSL can carry as much on a single wire of twisted-pair as can be carried on a T1 line in North America or an E1 line in Europe (2,320 Kbps).
UDSL (Unidirectional DSL) - is a proposal from a European company. It's a unidirectional version of HDSL.
SDSL (Single-line DSL) - is apparently the same thing as HDSL with a single line, carrying 1.544 Mbps (U.S. and Canada) or 2.048 Mbps (Europe) each direction on a duplex line. It is considered to be the "business grade" DSL because of its symmetric speeds. SDSL is slower than ADSL but usually marketed with Service Level Agreement (SLA) such as the network will be guaranteed up for 99.5%, and there will be a 24-hour response time for every problem.

ISDN DSL or IDSL

Another type of DSL Internet Service is the so called IDSL which is actually a hybrid DSL (ISDN technology) that was designed at the same time when the other forms of DSL Internet technologies were made. IDSL however is not used that much because of the low speeds that it offers, actually only a maximum of 128 Kbps.

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