To support the extra traffic generated by the numerous residential clients, backbone network infrastructures have evolved to a level capable of sustaining high volumes of traffic belonging to bandwidth-intense applications. On the other hand, however, the currently mass-deployed residential broadband access technologies are considered inadequate for supporting the increased needs of high-end multimedia/audiovisual services.

Since it first became available, the ADSL technology (ITU-T G.992.1, 1999) has become the dominant technology for residential broadband access, deployed on a massive scale in order to address the Internet-access needs of the majority of residential end users. The data rates supported by ADSL and currently offered by most access service providers typically fall in the range of 512Kbps up to 8Mbps for the downstream direction, and 128Kbps up to 768Kbps for the upstream direction, with the actual figures depending on a variety of technology- and business-specific considerations.

Even though such speeds represent a significant improvement over typical PSTN/ISDN-based connectivity (i.e., 56/128Kbps), a typical ADSL line is unable to provide users with enough bandwidth for upstream-intense services (e.g., peer-to-peer networking) and ultrahigh-quality audio-visual content access (e.g., HDTV). Indeed, it is a fact established by monitoring current network usage practices that the majority of residential users (i.e., “mainstream” users) require quasi-symmetric access, with high upstream and downstream bitrates, in order to enjoy such services as bi-directional video conferencing, peer-to-peer networking, or online entertainment. Furthermore, Wolfe (2003) claims that the emergence of HDTV-based applications is expected to give rise to a category of “power” residential users, who will require very high downstream access rates in order to enjoy premium services. In this context, the ADSL technology as it stands today is not a suitable choice for addressing the needs of both “mainstream” and “power” users and their respective “profile” services.

With this in mind, enhancements were made to the original ADSL technology that produced ADSL2 as an immediate successor (ITU-T G.992.3, 2002). ADSL2 offered higher bitrates in both the downstream and upstream directions by using a more efficient G.DMTbased modulation mechanism, as described in the Aware White Paper (2003). The new modulation scheme was applied to the same frequency band as in the original ADSL standard (i.e., up to 1,1MHz), thus ensuring backward-compatibility with existing equipment. However, ADSL2 never achieved prominence and has come to be considered a transitional technology. Consequently, further improvements to the original ADSL specification have produced the next successor standard referred to as ADSL2+ (ITU-T G.992.5, 2003).