为解决对公共交换电话网络（PSTN）用户线路长短不一难以驱动的问题，提出了一种自动测量用户线路长度 和电路切换的方法。通过建立用户线路模型，并根据线路传输信号类型进行简化分析。利用直流算法测量出用户线路长度， 并分别针对“振铃”、“馈电”两种不同用户线路状态进行电路和参数的切换。实验结果表明，本方法可以降低用户线路接口电 路（SLIC）的功耗，延长用户线路距离，提升用户体验。

To solve the problem of driving different length of public switched telephone network (PSTN) subscriber line, a method of automatic measurement of subscriber line length and switching circuit is presented. By the construction of subscriber line model, the line model is simplified according to the type of signal. A DC algorithm is used to measure the length of the subscriber line. Circuit and parameter profiles are switched automatically according to different line state of“ring" and "feed". Experiments show that the proposed method can reduce the power consumption of the subscriber line interface circuit (SLIC), extend the subscriber line distance, and enhance the subscriber’s experience.