Finding where symmetrical DSL fits

Electronic News, Feb 16, 1998 by Shannon Pleasant

The announcement of the Universal ADSL Working Group (UAWG) consortium promoting standardized ADSL lite (EN, Jan. 26), has made it difficult to discern how symmetrical Digital Subscriber Line (DSL) will fit into the DSL picture.

Digital Subscriber Line (DSL) technology is not a brand new idea. In fact it was the the original approach. The first applications of DSL used a symmetrical version of DSL called HDSL (High Data Rate Digital Subscriber Line) to duplicate T1/E1 services over two copper wire pairs at distances up to approximately 2.3 miles. More recently, the explosive growth of the Internet and corporate intranets, and the resulting bandwidth demand, has introduced an application for DSL with much larger implications and tremendous market opportunity.

Because HDSL requires two copper pairs, HDSL is not ideal for end-user data applications. SDSL (Single Line Digital Subscriber lane) or HDSL-2 was developed to provide symmetrical speeds using only a single pair of copper wire. Because symmetrical service is able to offer much higher speeds than analog modems in both directions, it can, for example, be effectively used by remote LAN users for applications that create significant upstream and downstream traffic.

The first DSL implementations targeting internet and intranet access have been symmetrical. Following SDSL service announcements by several service providers including HarvardNet, WebSprint and TerabitNet, US West introduced "MegaBit Services" in November 1997 in Phoenix, offering up to 704Kbps both upstream and downstream at $125 a month, plus installation. For the service provider, the lower speeds offered by SDSL in comparison to ADSL (which operates at speeds up to 8Mbps) means less strain on the existing backbone infrastructure. Providers therefore do not have to make as significant an investment in infrastructure upgrades to provide faster access; and can therefore get to market quickly.

However, interoperabillty issues have held back the SDSL market just as it has the ADSL market. This year, several vendors have joined together to speed SDSL standardization to jump-start the internet and intranet access market. The HDSL-2 standard being developed allows data to be sent at 1.5Mbps in both directions over a single pair. Consensus has been reached on line coding, system performance issues and forward error correction. The final standard, which is expected to be reached in early 1999 will be another step forward towards mass market deployment.

Several manufacturers have been in the HDSL and SDSL business since its inception. Pairgain, Paradyne, Globepan and Level One have dominated the equipment and chipset markets in the U.S., while Orckit and Metalink have been significant players in the European market. These manufacturers will see an immediate benefit from the development of a standard, allowing further market penetration of SDSL-based services.

Although asymmetrical DSL is expected to eclipse the market for Symmetrical DSL because of its fit with residential consumer demand for high downstream speeds and low upstream speeds required for Internet access, symmetrical services will continue to grow over the next five years. Technology advances in symmetrical DSL will make SDSL increasingly simple to implement and install.