DeVry Institute
Digital Subscriber Lines

The term � xDSL � is a catch-all acronym covering a number of similar yet competing forms of Digital Subscriber Line ( DSL ) technologies. The �x� in xDSL is actually filled with other letters depending on the technology implemented such as HDSL, ADSL, RADSL and VDSL. These will be described fully in the following text.
Whether a small or home business, a telecommuter or a consumer you can use DSL services to deliver multiple forms of data, voice and video at high speeds over your existing copper telephone lines on a continually connected system.
Data networking equipment is required at the customer premise as well as the telephone company�s central office (CO) to enable the end-user to take advantage of DSL technology. Installation is usually a simple and affordable process.
Types of DSL

The major xDSL categories are as follows :
*( HDSL ) HIGH-BIT RATE DIGITAL SUBSCRIBER LINE

*( ADSL ) ASYMMETRIC DIGITAL SUBSCRIBER LINE

*( RADSL ) RATE ADAPTIVE DIGITAL SUBSCRIBER LINE

*( VDSL ) VERY HIGH-RATE DIGITAL SUBSCRIBER LINE
DSL Systems

HDSL :(HIGH-BIT RATE DIGITAL SUBSCRIBER LINE).The grandfather of the DSL systems, it was developed in the early 90s as an phone line and adjusted the delivery rate accordingly. This means the speed can be alternative way to achieve T1 and E1 speeds (1.5 to 2Mbps). The idea was to apply the same modulation technique as was used in ISDN (2B1Q), split the service on two lines (four wires), increase the bits per baud rate and reduce the frequency spectrum needed. All this meant is higher speed and longer loop reach.
SDSL:( SYMMETRIC DIGITAL SUBSCRIBER LINE) was the further devlopment of HDSL has resulted in a Daughter, Single pair HDSL, which is known under the generic name of SDSL.The trade-off between HDSL and the two wires of SDSL is the loop reach, but the difference is so small that it is almost a non-issue.
ADSL, the asymmetric applications, was orginally developed for Video purposes. Experimenting with maximizing loop reach and various line codes it was evident that it was possible to send information at a higher speed from the CO to a remote office or home, than in the opposite direction. (The Internet is a perfect example).
RADSL : (RATE-ADAPTIVE DIGITAL SUBSCRIBER LINE) is an ADSL technology from in which software is able to determine the rate at which signals can be transmitted on a given customer reduced to make it work over longer lines, or increased with better loop conditions. Rate adaptive ADSL supports both symmetric and asymmetric applications, and in a way it unites the two branches.
VDSL :(VERY HIGH-RATE DIGITAL SUBSCRIBER LINE) and is an example of a new range of hybrids that combine the benefits of fiber and copper wire. Promising a much higher data rates over relatively short distance (between 51 and 55 Mbps over lines up to 1000 feet or 300 meters in length). It�s envisioned that VSDL 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 enviroments is not yet determined . Standards and Organizations are still working on it.
While some of these systems may seem to � overlap � in the capabilities of their technologies, it�s likely that they will coexist in a complementary, rather than competitive fashion.
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xDSL
Good site for overall xDSL technology


Supercom '98-DSL Top 10
Great detailed site.

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