为什么DSL能达到1M或2M？

hppyhjh

the subscriber loop, which is the twisted pair of 22- to 26-gauge copper wires
going from your telephone to the local telephone exchange. These were originally
designed to handle a rather narrow band of frequencies. That is, they were
designed to handle analog voltages in the 20 to 50-volt range with frequencies of
less that 4,000 hertz. This sort of wiring cannot be easily convert to carrying even
voice-grade digital communications at 64,000 bits per second.

这是在网上看到的一段话，认为用户线原来的设计，是用来传输 20~50的电压、低于4kHz频率
的模拟信号
但是如今的DSL技术，可以在用户线上传输1M或2Mhz频率的信号
为什么能取得这样的结果，谁能给我通俗的说明一下？并且将来会达到3M，4M吗？