Tài liệu HDSL2 TECHNICAL BRIEF doc

A new standard has emerged in delivering T1s over unbundled copper called HDSL2... To meet these requirements of change, the industry has begun a migration from two copper pair solutions

T H E S I N G L E L O O P A D V A N T A G E O V E R S D S L

Overview

Demand for high-speed data services and Internet access is on a dramatic

upswing in the business world As a result, telecommunications service

providers have widely adopted T1 lines as the preferred vehicle to provide

the speed and bandwidth to deliver these vital services In 1997, there were

approximately 1.7 million T1s installed in the United States Conservative

estimates show T1 deployment growing at a healthy 30 to 40 percent a

year

High bit-rate digital subscriber line (HDSL) technology is the dominant

choice among carriers for delivering T1s In fact, more than 50 percent of

the T1s being installed today on copper employ HDSL A new standard has

emerged in delivering T1s over unbundled copper called HDSL2

HDSL technology has proven to be highly reliable and has played a major role in easing many time and cost issues associated with T1 line provisioning and installation For example, an HDSL T1 solution can reach 12,000 feet For the same distance, the old repeatered T1 method would require labor-intensive installation of a repeater module every 3,000 feet

In the telecom world, carriers are constantly under pressure to deliver services more cost effectively, while reducing installation intervals Another pressure point is the carriers’ need to conserve their preciously limited but highly coveted copper resources

To meet these requirements of change, the industry has begun a migration from two copper pair solutions such as HDSL, to single-pair solutions The theory is that these single-pair offerings will provide the same data rates as two-pair technologies while using half the copper resources

Equipment vendors are currently working together in the American National Standards Institute (ANSI) T1E1.4 subcommittee to develop HDSL2, a next-generation, single-pair version of HDSL HDSL2 provides full duplex 1.544 Mbps transmission over a single pair

to an unrepeatered carrier serving area (CSA) distance of 12,000 feet HDSL2 will be spectrally compatible with other services in the same cable, including HDSL, ADSL, T1 and ISDN It will also be able to meet a 5 dB noise margin in worst-case crosstalk and have a latency of less than 500 microseconds Another goal in the ANSI committee is for HDSL2 to be vendor-interoperable

The plan is to bring HDSL2 standards-compliant systems to market for testing and commercial rollout by the end of 1998

Single-pair digital subscriber line (SDSL) is another technology that delivers data over fewer copper pairs Several different vendors have developed solutions claiming to fit this technological slot There is confusion in the industry, however, about what the true definition of SDSL is None of these solutions offer the performance quality or standards committee cooperation inherent with HDSL2

A “Single” Solution

Single-Pair Confusion

Technology Data Rate Mode

Distance Limit from Central Office

Number of Copper Pairs

HDSL2 SDSL

1.544 Mbps 1.544 Mbps

Duplex Duplex

12,000 feet 10,000 feet

1 1

The first definition of SDSL could be described as “HDSL divided by two.” It essentially takes one loop from the existing two-loop HDSL approach As a result, it provides 784 kbps data rate, or half of the full 1.544 Mbps of HDSL With this 50 percent data rate, this definition clearly does not stack up to HDSL2

The second SDSL definition includes a single HDSL loop on which the clock has been sped up at least two times This allows transport of 1.544 Mbps over the span Another approach is the proprietary CAP over a single loop, with which a carrier can also achieve full DS-1 transport at 1.544 Mbps

The HDSL Standby

These latter two approaches also do not meet HDSL2 specifications First, neither is able

to achieve full 12,000 foot CSA loop reach without worst-case crosstalk Additionally, they are not spectrally compatible with other services As a result of not meeting these criteria, these SDSL approaches are not being considered by ANSI standards committees

Equipment vendors understand the importance of maximizing existing copper reserves

to carriers while optimizing equipment and installation costs This is why most of the leading vendors – usually fierce competitors – are working hand-in-hand in the T1E1.4 committee to develop a well-defined, interoperable standard for single-pair copper service delivery That standard is HDSL2 Because of its rigid performance guidelines, it will ensure carriers the most optimal network performance

ADC Telecommunications is taking a lead role in the development of HDSL2 In fact the lead editor of the HDSL2 standard is an ADC engineer When the technology is ready for the commercial market, vendors will integrate HDSL2 into existing platforms for

will accommodate HDSL2, allowing carriers to maximize the existing infrastructure they’ve grown comfortable with and relied upon

The applications of both HDSL2 and its predecessor are the same HDSL will still have a role as well in T1 deployment It will be required for circuits of longer distances, where there’s a requirement to power a significant amount of components in the outside plant HDSL2 is a natural for short distances or unbundled copper T1 deliveries Both will accommodate frame relay for T1, Internet interconnection, LAN extension, remote LAN access and ISDN

Despite concerns that there is a lot of confusion and misconception in the market about the various “single-loop” service delivery technologies, the emerging HDSL2 technology offers clear-cut advantages for carriers when compared with the several divergent SDSL offerings:

• HDSL2 is an emerging ANSI standard This offers interoperability between vendors and the thorough technical review of the ANSI T1E1.4 subcommittee Conversely, SDSL solutions are proprietary When all is said and done, standard solutions are always more cost-effective for carriers

• HDSL2 has full CSA loop reach, even in the presence of worst-case crosstalk SDSL has unacceptably lower performance under the same conditions

• Some SDSL solutions are not spectrally compatible with other services, whereas HDSL2 is engineered to be compatible

HDSL2 is the natural evolution of and improvement upon the well-established HDSL technology, which carriers use for a majority of today’s T1 deployment The multi-vendor cooperation and painstaking testing during the standardization process will ensure that HDSL2 brings the same quality and reliability as its predecessor plus many more copper-saving enhancements For carriers considering single-loop T1 technolo-gies, HDSL2 is the clear choice

One Pair, One Real

Choice

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