Specified by the ITU-T in Recommendations G.992.3 and G.992.4 (July 2002), ADSL2 supports increased data rates of as much as 12 Mbps downstream and 1 Mbps upstream, depending on local loop length and quality.ADSL2 achieves the higher downstream data rates by increasing the frequency band from 552 kHz (ADSL) to 1.1 MHz, and improving modulation efficiency through the introduction of trellis-coded modulation (TCM) quadrature amplitude modulation (QAM) constellations. In combination, these modulation techniques yield higher throughput on long loops with low signal-to-noise ratio (SNR). ADSL2 also uses receiver-determined tone reordering of the discrete multitone (DMT) channels to spread out the noise from AM radio interference and, thereby, to realize greater coding efficiency, which yields higher throughput. ADSL2 systems feature reduced framing overhead, enhanced power management, faster startup, seamless rate adaption, and improved diagnostics. ADSL2 also features an all-digital mode, in which the analog voice channel can be used for digital data transmission, thereby increasing aggregate upstream data transmission rates by as much as 256 kbps.ADSL2 adds a packet mode capability that enables packet-based services such as Ethernet. On long loops, ADSL2 can increase the data rate by as much as 50 kbps and extend the reach by about 600 feet (200 meters). ADSL2 supports bonding in asynchronous transfer mode (ATM) mode, based on the MFA Forum specification for Inverse Multiplexing over ATM (IMA). This allows two ADSL pairs to be bonded together to yield roughly double the single-pair rate. See also ADSL, AM, analog, ATM, channel, digital, DMT, downstream, Ethernet, frequency band, IMA, interference, local loop, MFA Forum, modulation, overhead, QAM, rate adaption, SNR, TCM, throughput, and upstream.
Specified by the ITU-T in Recommendation G.992.5 and G.992.4 (January 2003), ADSL2+ doubles the downstream data rate, in comparison to ADSL2, to as much as 24.5 Mbps over local loops up to approximately 5,000 feet (1,500 meters) in length.The upstream rate remains at a maximum of 1 Mbps. In order to achieve this enhanced data rate, ADSL2+ increases the downstream frequency range to 2.2 MHz and increases the number of subcarriers to 512.The analog POTS channel remains at 4 kHz and the upstream data channel remains capped at 140 kHz. See also ADSL, ADSL2, analog, downstream, frequency, local loop, POTS, subcarrier, and upstream.