The DMT System of Splitting the Signal

How does DSL support both voice and data on the same line? A device called a "splitter" assigns voice and data transmissions to different frequencies. Digital bits travel on the inaudible frequencies of your phone line. That keeps them separate, and that's why all the signals—voice and data—can work in parallel without colliding with each another. A DSL modem spreads signals over many frequencies on the line, carrying many times more information than an analog dial-up modem.

DMT (Discreet Multi-Tone) Modulation uses multiple narrowband carriers, all transmitting simultaneously in a parallel transmission mode. Each of these carriers carries a portion of the information being transmitted. These multiple discrete bands—or in the world of frequency-division multiplexing, subchannels—are modulated independently of each other, using a carrier frequency located in the center of the frequency being used. These carriers are then processed in parallel form.

In order to process the multicarrier frequencies at the same time, a lot of digital processing is required. In the past, this was not economically feasible, but integrated circuitry has made this more practical.

The American National Standards Institute (ANSI) selected DMT with the use of 256 sub carriers, each with the standard 4.3125-kHz bandwidth. One way to think about it is to imagine that the phone company divides your copper line into 256 different 4-kHz lines and then attaches a modem to each one. You get the equivalent of 256 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 between different channels, 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 upstream and downstream information. Monitoring and sorting out the information on the bidirectional channels, and keeping up with the quality of all 256 channels, makes DMT complex to implement, but gives it more flexibility on lines of differing quality.