higher data rates
ADSL2 AND ADSL2+ THE NEW ADSL STANDARDS
As the number of users of ADSL chipsets based on first-generation ADSL standards surpasses 25 million, the ITU has completed and approved thenewest revision of international ADSL standards. The new standards are referred to as "G.dmt.bis" and "G.lite.bis" at the ITU, but will become better known as "ADSL2." By the first quarter of 2003, leading silicon vendors are expected to have solutions available that support and interoperate with ADSL2 equipment as well as legacy ADSL equipment.
ADSL systems, on long lines where the data rate is low(e. g. 128 kbps), a fixed 32 kbps (or 25% of the total data rate) is allocated to overhead information. In ADSL2 systems, the overhead data rate can be reduced to 4 kbps, which provides an additional 28 kbps for payload data.
ADSL2 (ITU G.992.3 and G.992.4) adds new features and functionality targeted at improving performance and interoperability, and adds support for new applications,services, and deployment scenarios. Among the changes are improvements in data rate and reach performance, rate adaptation, diagnostics, and stand-by mode, to name a few.
Rate and Reach Improvements
ADSL2 has been specifically designed to improve the rate and reach of ADSL largely by achieving better performance on long lines in the presence of narrowband interference. ADSL2 accomplishes this byimproving modulation efficiency, reducing framing overhead, achieving higher coding gain, improving the initialization state machine, and providing enhanced signal processing algorithms. As a result, ADSL2 mandates higher performance for all standard-compliant devices.
ADSL2 provides better modulation efficiency by mandating fourdimensional, 16-state trellis-coded and 1-bit quadrature amplitudemodulation (QAM) constellations, which provide higher data rates on long lines where the signal-tonoise ratio (SNR) is low. In addition, receiver-determined tone reordering enables the receiver to spread out the non-stationary noise due to AM radio interference to get better coding gain from the Viterbi decoder.
On long lines where data rates are lower, ADSL2 achieves higher coding gain from theReedSolomon (RS) code. This is due to improvements in the ADSL2 framers that improve flexibility and programmability in the construction of the RS codewords.
Additionally, the initialization state machine has numerous improvements that provide increased data rates in ADSL2 systems. Examples include:
Power cutback capabilities at both ends of the line to reduce near-end echo and theoverall crosstalk levels in the binder.
ADSL2 systems reduce framing overhead by providing a frame with a programmable number of overhead bits. Therefore, unlike the first-generation ADSL standards where the overhead bits per frame are fixed and consume 32 kbps of actual payload data, in the ADSL2 standard the overhead bits per frame can be programmed from 4 to 32 kbps. In first-generation
IDetermination of the pilot tone location by the receiver in order to avoid channel nulls from bridged taps or narrow band interference from AM radio.
ADSL2 and ADSL2+: The New ADSL Standards White Paper
Control of certain key initialization state lengths by the receiver and transmitter in order to allow optimum training of receiver and transmitter signal processing functions.Diagnostics
Determining the cause of problems in consumer ADSL service has at times been a challenging obstacle in ADSL deployments. To tackle the problem, ADSL2 transceivers have been enhanced with extensive diagnostic capabilities. These diagnostic capabilities provide tools for trouble resolution during and after installation, performance monitoring while in service, and upgrade qualification....