ADSL uses various techniques to determine the optimum bit transfer for the given transmission conditions. Thus ADSL is a variable bit rate asymmetric transmission DSL technology. The upstream rates in ADSL are lower as compared to downstream rates, thus downstream communication utilizes more bandwidth than upstream communication. The frequency range for upstream transmission lies between 25-138 kHz. In the case of downstream transmission, utilized frequency ranges depend on the mode of operation of transceivers.
Full overlap between upstream and downstream bands is used in circumstances where larger amounts of bandwidth are required. However this might lead to line noise and sophisticated echo canceling techniques need to be implemented for noise reduction. Full overlap between upstream and downstream bands utilized frequencies in the range between 25 kHz to 1.14 MHz. In the case of Frequency Division Duplex (FDD) non overlapping downstream and upstream bands are used and they utilized frequencies starting from 138 kHz for downstream communication (Jones, 2006; Stallings, 1998).
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