The onslaught of new vendors is sure to bring continued variations in the DSL technology. Even as mass deployments of ADSL and SDSL reach the market, new variants are being developed and marketed to fulfill the needs of certain segments of the DSL market.

IDSL provides DSL over ISDN

In some cases, DSL concepts have been applied to existing technologies. For example, ISDN-DSL or IDSL, first emerged as a new spin of the 1980’s technology. IDSL is simply ISDN customer premises equipment, talking to ISDN-compatible line cards that reside on the other end of the copper wire loop, and terminate the ISDN signal independent of the telephone switch. In this scenario, as with all DSL variations, the data service is directed to a WAN data service rather than a switched network.

While IDSL builds on a proven technology, it is functionally a subset of ISDN, in that it forgoes any ability to support telephone service and switched connectivity in general. One key benefit of IDSL is to the service providers seeking to move long duration ISDN data connections to the Internet or remote LAN access servers of the switched network. Another benefit is that it is able to transmit data over copper pairs that are served by digital loop carriers. These devices are often connected to the CO by fiber optic private lines and, as such, cannot carry other DSL signals, such as ADSL and SDSL.

Multirate symmetric DSL

For symmetric applications, Multirate SDSL (M/SDSL) has emerged as a valuable technology in meeting carrier requirements to deliver TDM services on a near ubiquitous basis. Building on the single-pair SDSL technology, M/SDSL supports changing operating line rates of the transceiver and thus, the operating distance of the transceiver.

G.lite for the consumer market

In January 1998, the Universal ADSL Working Group (UAWG) was announced. The result of this group’s work is a new, standards-based subset of ADSL known as G.lite. G.lite was approved as a standard by the ITU (G.992.2) in June 1999 and can offer speeds of up to 1.5 Mbps downstream and up to 512 Kbps upstream. Significantly, G.lite was designed to provide this service over the existing phone lines, without the POTS splitter, usually required by full-rate ADSL solutions. A part of the G.lite standard is a technique known as ‘fast retrain’, which limits the upstream power of the G.lite signal when a telephone handset is in use. In order to minimize interference, it then restores the power when the phone is back on hook.

ReachDSL—another option for business and residential customers

ReachDSL is a symmetric DSL technology that addresses subscriber demand for high-speed DSL services at extended distances. Complementing standard ADSL technology (DMT/G.lite), ReachDSL products support speeds ranging from 128 Kbps to 1 Mbps and have been designed to work over a wider range of line conditions and in wiring premises. ReachDSL solutions are capable of sharing lines, impervious to bridged taps, and are ideal for business, residential and even private network environments.

VDSL delivers video and higher bandwidth

The newest emerging variant of DSL is Very High Speed DSL (VDSL). VDSL systems are still being developed, so the final capabilities are not yet firmly established, but propose standard call for downstream bandwidths of up to 52 Mbps and symmetric bandwidths of up to 26 Mbps. The trade-off of these bandwidths is a much shorter loop reach.

But, the high speed offered by VDSL will bring opportunities for service providers to offer the next generation of DSL services, with video being seen as a prime application. At 52 Mbps, a VDSL line can offer a customer multiple channels of full-quality MPEG-2 video streams and even offer one or more channels of full-quality High Definition Television (HDTV). Some service providers have begun trial deployments of VDSL systems providing these services, with the VDSL endpoint appearing in the residence as a cable TV-like set-top box with an Ethernet or other data interface for connecting to PCs for simultaneous data services.

The DSL series is brought to you in association with Paradyne Corp.