To achieve these goals, ADSL2+ employs a dual approach; firstly, by adopting the improved modulation mechanism of the ADSL2 standard (ADSL-compatible), it improves the bit carrying efficiency per sub-carrier frequency; secondly, by doubling the downstream spectrum bandwidth1 of the ADSL/ADSL2 specifications, it effectively doubles the downstream bitrate it can offer. Both these choices are intended to endower ADSL2+ with greatly improved access rates, while maintaining compatibility with both the ADSL and ADSL2 standard.

Furthermore, ADSL2+ has included the reach extension model described in the ADSL2 Annex L standard (studied by Ouyang, Duvaut, Moreno, & Pierrugues, 2003), as well as the increased upstream option introduced in the ADSL2 Annex J standard. The latter feature has been included as the Annex J/M of the ADSL2+ standard (ITU-T G.992.5, 2003). The importance of these two features is demonstrated by Bong, Taek, Ginis, and Cioffi (2002) and Androulidakis, Kagklis, Doukoglou, and Skenter (2004b) in their respective work. The former has shown that an ADSL2+ Annex L transceiver can deliver an improvement in reach for distances beyond 5.5km, allowing for broadband connectivity even in remote areas. The latter has demonstrated how an ADSL2+ Annex J/M transceiver can offer the necessary high upstream rates for customers needing low-cost symmetric broadband access similar to a G.SHDSL service (ITU-T, G.991.2, 2001). The ADSL2+ standard has incorporated other improvements, including:

• Better diagnostic methods such as SELT described in the Texas Instruments White Paper (2003), or DELT, (ITU-T G.992.5, 2003)
• Improved power management capabilities (the L0/L2/L3 power management scheme) as well as
• Line bonding functionality based on an ATM IMA layer, similar to the method described by the Aware White Paper (2004) and evaluated by Androulidakis, Kagklis, Doukoglou, and Sykas (2005)

All these features indicate that the new standard holds much promise, especially in view of the move to bandwidth-intense multimedia-based services targeted at the residential market. Given the rising demand for advanced “triple play” services, which the current ADSL technology cannot fully support, and the fact that many content providers seek entrance into new markets (e.g., DVD- or HDTV-quality video broadcasts) through IP-based networks, the move to ADSL2+ technology seems perfectly justified. It is expected that deploying ADSL2+ on a massive scale for residential broadband access will address the inflated bandwidth requirements in the access network at a low cost per customer.