ADSL divides the high frequency of telephone lines into many tones.

ADSL divides the high-frequency part of a telephone line into many tones. Each tone carries a certain bit bandwidth, and the total bandwidth that all tones can carry is the maximum bandwidth of this ADSL line. Each tone has a certain background noise. After the bandwidth occupied by this background noise is removed, the remaining part can transmit data. Therefore, the maximum bandwidth that the tone can carry after the noise is removed is calculated. While telephone lines work in harsh environments, and interference is constantly changing. Therefore, the noise of each tone cannot be fixed on a certain dB. Therefore, we must keep the margin when allocating bits, that is to say, the bandwidth of each tone must be the total bandwidth-noise-margin. The purpose of this margin is that if the tone noise suddenly increases, the data can be temporarily used to ensure normal transmission without going offline.
Therefore, we can know that if the signal-to-noise ratio of the line is large, the actual available bandwidth is small, and vice versa, the actual available bandwidth is large, which is the relationship between the two.
Generally, it is called noise tolerance (SNR limit ).
Each tone must have a certain margin for noise tolerance without data transmission. The signal-to-noise ratio of each tone is different. Generally, modem displays the worst data.