Digital Subscriber Line (DSL) has become popular in recent years because it can provide high-bandwidth connections between a telephone switching station and a home or office over existing telephone lines. DSL technologies use sophisticated modulation schemes to pack data onto copper wires of plain old telephone system (POTS). Among the exiting DSL technologies, Asymmetric Digital Subscriber Line (ADSL) is the most practical since it fits average users' need for greater downstream bandwidth than upstream bandwidth.
Discrete Multi-tone (DMT) is a method of modulating a DSL signal for transmission on a usable frequency spectrum divided into a plurality of sub-carriers or frequency channels. For example, a spectrum may be divided into 256 channels (“bins”) of 4.3125 kHz each. The center frequency of a bin is 4.3125 kHz multiplied by the bin number. Thus, a specific bin may be referred to by its bin number instead of its actual frequency range. Dividing the frequency spectrum into multiple channels reduces cross-talk in signal transmissions. In a DMT system, a usable frequency spectrum is typically allocated to the upstream and downstream transmissions based on a standard scheme. For example, according to International Telecommunication Union (ITU) standard G.992.1 (Annex A), the downstream transmission may occupy bins 6 through 255 and the upstream transmission may occupy bins 6 through 31. In a common operation mode, to reduce near-end cross-talk (NEXT), the downstream transmission may start from bin 33. Other standards or schemes also exist that allow an overlap in bin allocations to the upstream and downstream traffic.
However, none of the fixed bin allocation schemes can consistently achieve optimal data rates. Due to the ever-changing operation environment (e.g., noise level, loop attenuation and echo rejection rate), quality of a connection based on a particular scheme usually drifts over time. Especially when the cross-talk noises become highly asymmetric, it may be difficult to rely on a fixed bin allocation scheme to maintain link performance. As a result, the available bandwidth is not fully utilized and customers may become dissatisfied with inconsistent link performance.
In view of the foregoing, it would be desirable to provide an efficient and cost effective solution for bin allocation that overcomes the above-described inadequacies and shortcomings.