HDSL2: Why 1 Wire is Better Than 2

While the price of raw copper may be low, the cost of installing new copper facilities is higher than ever. That means carriers must do whatever they can to leverage their existing copper infrastructures.

The answer for most telephone companies is high-bit-rate digital subscriber line (HDSL) or the newer, two-wire HDSL2. A widely deployed symmetrical DSL service, HDSL provides full T-1 service, or a bidirectional rate of 1.5M bit/sec, over two copper pairs. In contrast, HDSL2 uses only one copper pair to provide the same level of service.

Small to midsize business can expect to see telcos using HDSL2 as a T-1 replacement that will provide more economical deployments.

By changing the line code from 2B1Q to TC PAM with spectral shaping, HDSL2 not only provides full T-1 service using a single copper pair on loops that meet the Carrier Service Area deployment guidelines, but it also resists noise resulting from coexistence with other DSL services. And because it uses only two wires, HDSL2 consumes half the copper resources of its four-wire predecessor.

Like other flavors of DSL, HDSL2 lets carriers deploy high-speed data services using existing copper facilities. This means service providers don't need to invest time and money to replace copper wires with fiber optic cables, making HDSL2 an excellent solution wherever a large investment in copper has already been made and service needs continue to grow.

The economic benefits of HDSL2 are enormous because providers potentially will be able to generate twice as much revenue from two copper pairs as they can with either HDSL or traditional T-1. Use of HDSL2 also allows the local carrier, which owns the copper wires, to alleviate "copper exhaust," or the shortage of copper facilities. Finally, local carriers can increase revenue by using the existing copper pairs to deploy more services, or by leasing more copper pairs to alternative service providers.

Additional benefits are found in HDSL2's fast deployment and customer turnup, which follow the example set by HDSL. For most carriers, reducing the time technicians spend in the field is key to the success of a new technology.

Likewise, quick turnup is essential for maintaining good customer relations and avoiding lost revenue. In fact, telcos can provision high-speed digital services over existing copper with HDSL2 within 24 hours. That makes it the fastest and most cost-effective option for deploying T-1/E-1 lines.

Vendor interoperability will be another critical factor in the success of HDSL2. Because lack of vendor interoperability creates difficult deployments and limits providers to specific vendors, many providers are wary of venturing into new territory. Although HDSL is the most widely deployed DSL, it is not governed by a standard. The pending HDSL2 standard, as defined by the ANSI T1E1.4 committee, contains sufficient technical detail to let vendors deploy interoperable products.

Toward this end, the industry-led HDSL2 Consortium, composed of leading telecommunications product and chip vendors, will work together with the University of New Hampshire Interoperability Laboratory to test vendor products for interoperability. The combined efforts of the Consortium, the university lab and the committee are expected to avoid interoperability problems such as those associated with early ISDN deployments.

Among the most sophisticated DSL technologies, HDSL2 benefits service providers and subscribers alike. To summarize, this copper-saving technology:

-- Minimizes self-induced noise using offset bandwidths in the upstream and downstream directions.

-- Optimizes performance by reducing noise in the presence of other DSL technologies using "OPTIS" spectral shaping.

-- Allows transport of full T-1 data rates using CSA deployment guidelines on a single copper pair.

Ensuring healthy competition through interoperability, HDSL2 benefits service providers as well as other market segments. Because HDSL2 chipsets and products will be available from multiple sources in the near future, competition will increase as service providers demand the best products for the best prices.

This will result in economical deployments, and in lower costs to consumers for T-1 services delivered by HDSL2.

(Atwell is a product line manager at Adtran, a maker of transmission products for high-speed digital communications. He can be reached at keith.atwell@adtran.com.)

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