d overlaps only with immediately adjacent subchannels. When a signal is transmitted over a long copper loop (e.g. several miles), the higher frequency components of the signal attenuate significantly more (tens of dB) than the lower frequency components. Narrowband interferers from AM or amateur radio signals also affect the transmission by destroying the signal in parts of the band. Multicarrier technology, called Discrete Wavelet Multitone (DWMT), provides subchannel isolation that is superior to DMT. DWMT uses an advanced digital wavelet transform instead of the Fourier transform used in DMT. Echo CancellationThe T1.413 standard for ADSL defines two categories of modems: frequency division multiplex (FDM) modems (Category I) and echo cancellation modems (Category II). FDM systems allocate separate frequency bands for upstream and downstream transmissions. Echo canceled systems send upstream and downstream signal over the same frequencies. Since the attenuation of a signal over a copper line increases with frequency, it is desirable to transmit data using a frequency band that is as low as possible. In an ADSL system, the lowest attenuated frequencies begin right after the POTS band. In FDM system, the lower frequency band is used for upstream transmission while the downstream transmissions are allocated to the higher attenuated frequencies. Some xDSL transceivers use echo cancellation (similar to the echo cancellation utilized in the standard V.34 28.8kbps duplex modem) to exploit the lower attenuated frequencies and increase its downstream performance. By utilizing the lower frequencies for both upstream and downstream performance, the transceiver can deliver higher downstream performance, particularly on the longer loops where the higher frequencies become severely attenuated. In an effort to promote interoperability among FDM and EC systems, the echo-canceled transceivers can be configured to operate in an FDM mode in order to com...
