ADSL2 boosts data rate and reach

By Peter Leblanc, Network World
January 20, 2003 12:10 AM ET

This vendor-written tech primer has been edited by Network World to eliminate product promotion, but readers should note it will likely favor the submitter's approach.

A new family of ADSL (asymmetric digital subscriber line) standards called "ADSL2" adds features and functionality that boost ADSL's performance, improve interoperability, and support new applications, services and deployment scenarios.

The ADSL2 G.992.3 and G.992.4 standards, recently approved by the International Telecommunication Union, improve data rate and reach performance, dynamic rate adaptation and diagnostics, as well as include a power-saving standby mode. Another forthcoming standard, ADSL2+ (G.992.5), more than doubles the downstream data rate of ADSL to 25M bit/sec. The ITU expects to finalize the ADSL2+ standard this month.

ADSL2 addresses the growing demand for bandwidth to support services such as video. Forthcoming ADSL2 and ADSL2+ gear will interoperate with existing ADSL equipment, allowing carriers to roll out new high-speed services while gradually upgrading their legacy infrastructure.

Some of the enhancements ADSL2 and ADSL2+ offer include:

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Better rate and reach: Improved modulation efficiency reduces framing overhead, achieves higher coding gain, improves the initialization state machine, and provides enhanced signal processing algorithms. ADSL2 increases downstream data rates to more than 12M bit/sec, as compared to between 8M and 10M bit/sec for original ADSL. 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 the 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.
Channelization: ADSL2's channelization capability provides support for Channelized Voice over DSL (CVoDSL), a method to transport derived lines of TDM 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 fiber-like data rates over existing copper lines.
Improved interoperability: Modem initialization 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 initialization time from about 10 seconds to less than 3 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.
ADSL2+: The ADSL2+ standard doubles the maximum frequency used for downstream data transmission from 1.1 MHz to 2.2 MHz. This effectively provides downstream data rates of 25M bit/sec on phone lines as long as 5,000 feet.