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Technology update from DSL Forum

David Greggains highlights interoperability and takes a look at the standards that take ADSL to a New Level with ADSL2

Equipment interoperability is a foundation of mass-market success for every technical advance. Video recorders and fax machines are just two of today’s ubiquitous home and business technologies that demonstrated in their early days the need for international standardisation and interoperability. For the continued dynamic development of broadband DSL around the world, interoperability is no less important. So, the international DSL Forum is working hard to bring about ubiquitous interoperability in the DSL field, focusing specifically on facilitating testing excellence, and setting the bar for equipment specifications. It provides recommendations for cross-vendor interoperability, ADSL Implementation Conformance Statements (ICS), test procedures and configurations with comprehensive documentation to ensure that different test institutions should achieve similar results. Outcomes of this work also include a series of Technical Reports published by the DSL Forum at http://www.dslforum.org To achieve Technical Report status, industry consensus is reached through a final vote from DSL Forum members on contributions submitted on a Working Text. These Technical Reports are kept under constant review by the forum’s technical working groups to accommodate new standards and developments in the technology as part of its continuing fast-track approach to delivering best practices and accelerating global mass-market deployment of DSL.

A key Technical Report, TR48, addresses the need for any-to-any interoperability for ADSL, the most popular flavour of DSL, by specifying a core test plan for customer premise equipment to ensure it interoperates with a range of central office equipment. Current efforts are centered around updating this Technical report and also on developing additional specifications for SHDSL and VDSL equipment interoperability.

Another interoperability initiative from the DSL Forum is the Independent Testing Labs (ITL) programme. Using a single industry-wide Interoperability Test Plan [TR48], the ITL Programme has been designed from the outset to enable DSL Forum approved testing labs to administer the programme virtually independently of the DSL Forum. Under a legal agreement, labs follow the DSL Forum’s defined test plan and provide non-attributable statistics that will give the Forum and its members a tool to monitor industry progress towards any-to-any interoperability.

This same test plan, TR-48, was specified by the International Telecommunications Union [ITU] in the latest DSL technology standard. The ITU has recently consented standards for both ADSL2 and ADSL2plus that will help deliver higher rates, longer reach and improved diagnostics.

“These new standards are an important next step in the development and deployment of DSL technology,” said Tom Starr, President of the DSL Forum. “They build on existing ADSL work, allowing efficient transition to the enhanced performance of ADSL2 standards. The direct reference to our Technical Report TR-48 is a first and we are rightly, proud of that. By using our interoperability test plan as reference in the new standards, equipment providers can work on ADSL2 products in the context of familiar protocols.”

This new family of ADSL standards called "ADSL2" add features and functionality to boost ADSL's performance, improving interoperability and supporting new value added applications, services and deployment scenarios. ADSL2 is approved by ITU and is designated as G.999.3 and G.999.4. Besides improvements in performance and data rates, the technology has features like rate adaptation, diagnostic and power management that allow greater service package flexibility. ADSL2 tackles inconsistent performance over long distances by making improvements in modulation.

Meanwhile the ITU standardised symmetric DSL, SHDSL, is going through trials around the world, the expectation is that this will be focussed in deployment on small and medium sized business users. VDSL has already begun to take off, by supporting higher bandwidth applications it extends the potential for DSL’s mass-market growth around the world. South Korea is leading the way with half a million subscribers to this service. Those subscribers are now connected to streaming videos and online gaming capacities that film fans and gamers can currently only dream of in most countries.

Making it work – Testing and Interoperability
The first step of the DSL Forum’s plan to ensure ADSL2 Physical Layer Interoperability was successfully completed at a multi-vendor ADSL2 “plugfest” interoperability event conducted earlier this year by the University of New Hampshire’s InterOperability Laboratory (UNH-IOL). Seven ADSL silicon vendors participated in the DSL Forum sponsored interoperability plugfest, which was based on the DSL Forum’s internal “Proposed Draft 013 - Interoperability Test Plan for ADSL2 Plugfests” (PD-013). PD-013 provides a suite of basic physical layer tests to ensure interoperability between multiple chipsets and systems supporting ADSL2.

A series of equipment interoperability testing events, arranged by the DSL Forum, have been held to set the performance benchmarks for any-to-any interoperability for both customer premises [CPE] and central office equipment [Digital Subscriber Line Access Module - DSLAM]. Those plugfests have played a major role in facilitating multiple company equipment testing under clear criteria and confidentiality.

Forthcoming ADSL2 and ADSL2plus equipment will interoperate with existing ADSL equipment, allowing carriers to roll out new high-speed services while gradually upgrading their legacy infrastructure. The added value that asymmetric DSL can deliver using these new standards is significant.

Key enhancements provided by ADSL2 and ADSL2PLUS:

  • Better rate and reach: Improved modulation efficiency reduces framing overhead, achieves higher coding gain, improves the initialisation state machine, and provides enhanced signal processing algorithms. ADSL2 increases downstream data rates to more than 12M bit/sec, compared to between 8M and 10M bit/sec for original ADSL and ADSL2 extends reach by approximately 600 feet.
  • Diagnostics: Real-time performance-monitoring capabilities provide information regarding line quality and noise conditions at both ends of the line. Service providers can use this data to monitor the quality of the ADSL connection and prevent service failures. Carriers also can use the data to determine if a customer qualifies for higher data-rate services.
  • Channelisation: ADSL2's channelisation capability provides support for Channelised Voice over DSL (CVoDSL), a method to transport derived lines of TDM (time-division multiplexing) voice traffic transparently over DSL. CVoDSL transports voice within the physical layer, letting derived voice channels ride over DSL bandwidth while maintaining both plain old telephone service (POTS) and high-speed internet access.
  • Power enhancements: Two-power management modes help reduce power consumption while maintaining ADSL's always-on functionality for users.
  • Bonding for higher data rates: The new standards support the ATM Forum's inverse multiplexing over ATM (IMA) standard developed for traditional ATM architectures. Through IMA, ADSL2 chipsets can bind two or more copper pairs in an ADSL link. The result is fibre-like data rates over existing copper lines.
  • Improved interoperability: Modem initialisation procedures are clarified, which improves interoperability and provides better performance when connecting ADSL transceivers from different chip suppliers.
  • Fast startup: A fast start-up mode reduces initialisation time from about 10 seconds to less than three seconds.
  • All-digital mode: An optional mode allows for transmission of data in the POTS portion of the phone line. This adds 256K bit/sec to the upstream data rate, which can be an attractive option for businesses that have voice services on different phone lines and value the additional upstream bandwidth.
  • Packet-based services: Packet-based services such as Ethernet can be transported over ADSL2.

About the DSL Forum
DSL Forum is an international industry consortium of nearly 200 leading service providers, equipment manufacturers and other interested parties, focused on developing the full potential of broadband DSL to meet the needs of the mass market. With the established goal of 200 million customers by 2005, DSL Forum works to streamline processes, develop specifications and share best practices that set the stage for effective deployments, and explosive global DSL growth. By developing new standards and embracing new applications, DSL Forum is tailoring DSL to meet the needs of the next generation of multi-media services and the online community. Established in 1994, more information about the DSL Forum is available from its industry site www.dslforum.org. Information tailored to end-users is available on www.dsllife.com.

DSL – more than a phone line – it’s a global solution.

Outcomes of that work are published as Technical Reports for use throughout the global industry and are available from our website www.dslforum.org. Further information on the DSL Forum, its work, members and meeting schedule can be found at www.dslforum.org. DSL Forum’s website dedicated to providing information to end-users can be found at www.dsllife.com

David Greggains
David Greggains is vice president, operations of the DSL Forum, (http://www.dslforum.org), a consortium of around 200 businesses focused on developing the full potential of broadband DSL. He is also a director of Gorham & Partners Ltd, management consultants.

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