ticarrier technology, called Discrete Wavelet Multitone (DWMT), provides sub channel isolation that is superior to DMT. DWMT uses an advanced digital wavelet transform instead of the Fourier transform used in DMT. Echo Cancellation The 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 between FDM and EC systems, the echo-canceled transceivers can be configured to operate in an FDM mode in order to communicate with a category I (FDM) modem. Conclusion the twisted pair wires between the telephone central office and end users of telecommunication services has a great deal more information capacity than used for the regular voice services. Several base band and pass band transmission systems collectively referred to as xDSL, have been developed over the last ten years that enable up ...
