ADSL, VDSL, ChocolateDSL...

By Tara Seals

Like a good ice cream shop, the folks responsible for bringing you affordable broadband over copper are constantly inventing new taste sensations. But the acronym-filled world of DSL can be overwhelming. A reasoned look at the not-quite-31 flavors of DSL cuts through the confusion.

The differences between types of DSL hinge mostly on the fact that it is a distance-sensitive technology. The connection speeds depend on how far users are from a special hardware asset known as a DSLAM. As the connection’s length from DSLAM to modem increases, the signal quality decreases and the connection speed goes down. If a user is more than two miles from a DSLAM, he or she cant get high-speed DSL at all.

For now, DSLAMs are typically located in the central offices of the telephone company providing the service. However, a move to putting DSLAMs out in neighborhood phone cabinets a.k.a. the outside plant is under way, meaning that more homes and businesses soon will be closer to the DSLAMs and can take advantage of higher speeds. Also, previously out-of-range locations will now have access.

Standards bodies such as the International Telecommunication Union have worked to provide specifications that improve DSLs performance. As each new standard becomes ratified, it gets a nickname (ADSL, SDSL, etc.) and the phone companies test, trial and in some cases adopt it. Because there is no guarantee they will embrace any one standard, different forms of DSL are available in different geographic areas. For example, very high bit-rate DSL (VDSL) is a standard used in Asia and parts of Canada, but it has yet to make an appearance stateside.

Telcos worldwide are concentrating on implementing VDSL and high-speed forms of asymmetric DSL (ADSL) in order to provide speeds that will support high-margin applications and triple play services multimedia, HDTV, VoIP and high-speed Internet over existing copper infrastructure. But other versions of DSL exist. To pitch your customer the correct solution requires a working knowledge of the options.

ADSL Variations

Most homes and small business users in this country have access to an ADSL line. The limit for ADSL service is 18,000 feet, though for speed and quality-of-service reasons many ADSL providers place a lower limit on the distances for the service. Working on the assumption that most Internet users download much more information than they send, ADSL provides a connection speed from the Internet to the user that can be is three to four times faster than the connection back. ADSL provides 8mbps downstream and 1.5mbps upstream at the top end, although in practice speeds are typically lower. ADSL requires a line splitter that allows different data streams to traverse the same copper pair.

G.Lite (also known as Universal ADSL or DSL Lite) is based on the same underlying technology as standard ADSL, but connects a specific G.Lite modem without a line splitter. Unlike standard ADSL, it operates with Digital Loop Carriers the local loop infrastructure that connects customers located more than 18,000 feet from the central office. That means people in outlying areas can get this form of DSL, but theyre limited to 1.5mbps downstream and 512kbps upstream.

New standards from the ITU, known as ADSL2 and ADSL2+, are under consideration by many telcos here in the United States. Approved last year, these improve the ADSL data rates and reach performance. ADSL2+ more than doubles the downstream data rate to support speeds of 25mbps downloads for 5,000-foot spans. Also, new bonding technologies (encapsulated in the new ADSL2+ standard) mean that two ADSL copper pairs can be combined to boost bandwidth if 5,000 feet from the home, for 32mbps downstream.

VDSL

VDSL is considered the future for broadband over copper. It provides a faster connection, but works only over short distances. Unavailable in the United States for now, VDSL is being tested in some carrier labs and may become more popular as DSLAMs move to the outside plant and closer to customer homes and businesses. It supports 52mbps at up to 1,000 feet.

Going even further, the proposed VDSL2 standard by the ITU brings specifications for ADSL, ADSL2+ and VDSL together to deliver 50mbps to 55mbps of bandwidth in a short-distance scenario, but that standard wont solidify until mid-2005.

Boutique Flavors

While ADSL in all of its various forms is for now the vanilla of the DSL world, and VDSL the chocolate, there are yet other recipes for DSL:

* Symmetric DSL (SDSL) is used mainly by small businesses. Users cannot use the phone at the same time, but upstream and downstream speeds average 1.544mbps. Sometime in the future, a symmetrical DSL that provides 100mbps could appear. Many equipment vendors are working on the technology now and can demonstrate it in a lab situation.

* Consumer DSL (CDSL) provides 1mbps downstream but requires no line splitter.

* High bit-rate DSL (HDSL) also supports symmetric service at 1.54mbps, but it can carry large amounts of data, so it is the DSL service used for T1 lines. HDSL uses two copper pairs instead of one. HSDL2 provides the same speed capabilities, but uses only a single wire pair. Unidirectional DSL (UDSL) is in the proposal stage and is a unidirectional version of HDSL.

* ISDN DSL (IDSL) is simply an always-on alternative to ISDN lines and a data rate capacity of up to 144kbps rather than ISDNs 128kbps.

* Rate-Adaptive DSL (RADSL) is similar to a burstable T1 it monitors the customer phone line and delivers higher rates when possible or necessary. It delivers downstream speeds from 640kbps to 2.2mbps.