Calculate the maximum information transfer rate C formula: C = H * log ?(1 + S / N) (bit / s). In the formula: H is the channel bandwidth, S is the signal power (watt), N is the noise power (watt) in the formula of the S / N is for the signal and noise power ratio, for the dimensionless unit. For example, S/N=1000 (i.e., the signal power is 1000 times the noise power). Here C=64kb/s; H is the channel bandwidth, here H= 3kHz, substituting into the above formula, we can calculate SNR (S/N) = 2641634, which converted to dB form, is 64.2 dB.

This result shows that in the 3kHz telephone channel want to transmit 64kb/s data need very high signal-to-noise ratio. In addition, channel capacity is not only proportional to channel bandwidth, but also depends on the system SNR and the type of coding technique.

The usual audio telephone connection supports a bandwidth of B=3kHz, and the typical signal-to-noise ratio for a typical link is 30dB, i.e., S/N=1000, so there is C=3000×log2(1+1000), which is approximately equal to 30kbps, so if the telephone network's signal-to-noise ratio is not improved or if no compression method is used, the "cats " will not reach higher rates.

After the emergence of the Integrated Services Digital Network (ISDN), the digitization of subscriber lines has developed tremendously: the 3 kHz limit on audio bandwidth has been eliminated, making full use of twisted-pair bandwidth, and transmitting data at a rate of 144 kbps (2B+D).

But the rate of ISDN for broadband services is still far from enough, higher-speed digital subscriber loop technology came into being, which is now used more is ADSL (asymmetric digital subscriber line loop). ADSL uses frequency division multiplexing technology, in the traditional telephone bandwidth (0 ~ 4 kHz) at the same time, in addition to open up the 10 ~ 130 kHz and 130 ~ 1100 kHz two bands respectively. ADSL uses frequency division multiplexing technology to retain the traditional telephone bandwidth (0~4kHz) while opening up two other frequency bands, 10~130 kHz and 130~1100 kHz, for upstream and downstream data transmission respectively. In addition, ADSL also adopts a new digital modulation and demodulation technology, which expands the transmission bandwidth and revolutionizes the modulation technology, making it possible to reach a rate of 1Mb/s for the upstream, and a rate as high as 8Mb/s for the downstream.

References:

Shannon's Formula_Baidu Encyclopedia