In addition to being used to transmit telephone signals, telephone lines can be used in the following areas: 1. Communication field: In the field of digital communication, telephone lines are widely used for the connection between digital audio equipment and modems. Especially in the field of DSL (Digital Subscriber Line), telephone lines are more widely used. 2. security field: that telephone line can be use not only for transmitting video signal,It can also be used to transmit audio signals. In the field of security, telephone lines are widely used in real-time audio monitoring in video surveillance systems to achieve remote monitoring of the monitoring area. 3. Medical field: Telephone lines are widely used in medical places. Some medical devices are equipped with telephone line interfaces to facilitate timely patient diagnosis. 4. Home field: The telephone line is not only a tool for transmitting telephone signals, but also very important in the construction of home network. Home users can realize the functions of Internet access and conventional communication through the telephone line interface.