Network-the last mile in the telephone industry and ADSL notes

Statement:
The following section describes some of my clues on the Web book. The principle is as follows. Because the Web book is too early, some technologies may have broken through some limitations.
One mile in the telephone industry, people who have learned the internet know that our telephone line to the telecommunications company is a simulated line, that is, we can only transmit analog signals, the bandwidth we use to access the Internet, this line is also required for transmission. A modulation and demodulation technique is involved here.
How can we transmit digital signals over traditional telephone lines. As we all know, before we had a computer, the telephone line base was designed to transmit voice, and the voice frequency was generally 3100Hz, that is to say, the telephone company needs to filter out all the waves below 300Hz and above Hz on traditional lines. If the bandwidth of this line is artificially changed to 3100Hz, the bandwidth is generally considered to be 4000Hz. According to the nequest theorem, only two times of bandwidth sampling is allowed, in order to reconstruct all the signals, the sampling rate is generally 8000 Porter. In each sample code, according to the U.S. standard, there are 7 valid data and 1-bit verification data, so that the data transmission rate is 8000 × 7 = 5.6 kbps. This is the data transfer rate that traditional telephone lines bring to us.
The Chinese version of ADSL is called asymmetric digital user lines. The so-called asymmetric Mode means that the uplink speed is different from the downlink speed. Why is it different? Because people have different needs, generally speaking, we have a higher chance of downloading from the Internet than uploading. For example, if we want to access a webpage, the request may only contain a few KB of data, however, the data that the server returns to us is much larger than this one. How to design it.
Generally, the line itself has a bandwidth of 1 MHz, and the Telecommunications Department adopts a method called DMT (discrete multi-channel modulation. Divide this 1.1mhz into 256 independent channels, and the bandwidth of each channel is 1.1 × 1000 × 1000Hz/256 = 4296.8Hz (4312.5Hz in the book ), channel 0 is used in traditional telephone services. This bandwidth is also suitable for human voices. Channel 1-5 is used as a segmentation function, generally to avoid interference between traditional services and data services. Among the remaining 250 items, one is for uplink data control, the other is for downlink data control, and the other is for uplink data volume. The sampling rate of each channel is 4000 Potter, and each Porter transmits 15-bit data, the downlink data transmission rate can reach: 4000 × 15 × (250-32-1-1) = 12.96 Mbps (224 downstream channels in the book, with a value of 13.44 Mbps). According to the actual signal, the transmission rate can reach 8 Mbps.
Each port in the above channel only transmits 15-bit data. If there are more data, I have not actually studied this. It is technically and practically impossible. I do not know, however, with 15-bit data, traditional telephone lines can have 8 Mbps of bandwidth. The standard is also 8 m.