打过电话的人都知道，通话过程中必须传送和交换必要的信号。例如：当用户摘机听到“拨号音”信号，主叫用户必须拨“被叫用户号”才能接通被叫用户。“拨号音”和“被叫用户号”就是电话网中的一种信号。信号是通信网的重要组成部分，电话网的信号与交换机采用的控制技术有关 Anyone who has made a phone call knows that the necessary signals must be transmitted and exchanged during the call. For example, when the user hears “dial tone” signal, the calling user must dial “called user number” to connect the called user. “Dial tone” and “called user number” is a signal in the telephone network. Signal is an important part of the communication network, telephone network signals and switch control technology used