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ADSL2 and ADSL2+

The ADSL2 and ADSL2+ standards were adopted by the ITU in July 2002. ADSL2 improves the data rate and reach performance, diagnostics, standby mode, and interoperability of ADSL modems.

ADSL2+ doubles the possible downstream data bandwidth, enabling rates of 20 Mbps on telephone lines shorter than 5,000 feet (1.5 km).

First-generation ADSL standards require fixed 32-bit overhead framing on all ADSL packets. On long lines with low rates of 128 Kbps, the overhead represents 25 percent of the available bandwidth. ADSL2 standards allow the overhead per frame to be a programmable value between 4 Kbps and 32 Kbps, to provide up to 28 Kbps more bandwidth for payload data.

ADSL2 uses seamless rate adaptation (SRA) to change the data rate of a connection during operation with no interruptions or bit errors. The ADSL2 transceiver detects changes in channel conditions—for example, the failure of another transceiver in a multicarrier link—and sends a message to the transmitter to initiate a data rate change. The message includes data transmission parameters such as the number of bits modulated and the power on each channel. When the transmitter receives the information, it transitions to the new transmission rate.

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