The basic idea of DMT is to split the available bandwidth into a large number of subchannels. DMT is able to allocate data so that the throughput of every single subchannel is maximized. If some subchannel can not carry any data, it can be turned off and the use of available bandwidth is optimized. The examples in figure give an idea about of the functionality of DMT. 
Figure: Examples of DMT
First an equal number per tone is transmitted to measure the characteristics of the line. The processing of the signal takes place in ATU-R, and the optimized bit distribution information will be delivered for ATU-C by using the same phone-line at a secure low speed.
The first example describes a segment of 24-gauge twisted pair phone-line. Low frequencies are eliminated by the transformer coupling. The attenuation at the higher frequencies depends on the length of the phone-line.
The second example includes the notch in spectrum that is illustrative of bridge taps and also the interference of an AM radio station.
A third example shows that DMT is also an interesting possibility for other transmission channels, such as coaxial cable-TV networks, as well.