After a long standards battle between the VDSL Alliance, a partnership between Alcatel, Texas Instruments and others, which supports VDSL using a carrier system called Discrete MultiTone (DMT), and the VDSL Coalition, led by Lucent and Broadcom and proposing a carrier system that uses a pair of technologies called Quadrature Amplitude Modulation (QAM) and Carrierless Amplitude Phase (CAP), DMT won out. According to equipment manuafacturers, most ADSL equipment today uses DMT technology.
DMT divides signals into 247 separate channels, each 4 kilohertz (KHz, or 1,000 cycles per second) wide. One way to think about it is to imagine that the phone company divides your copper line into 247 different 4-KHz lines and attaches a modem to each one. You get the equivalent of 247 modems connected to your computer at once!
Each channel is monitored and, if the quality is too impaired, the signal is shifted to another channel. This system constantly shifts signals, searching for the best channels for transmission and reception. In addition, some of the lower channels (those starting at about 8 KHz) are used as bidirectional channels, for both upstream and downstream information. Monitoring and sorting out the information on the bidirectional channels, and keeping up with the quality of all 247 channels, makes DMT more complex to implement than other carrier technologies, but also gives it more flexibility on lines of differing quality.
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