北美洲电话用户数目已远超过一亿。每个用户在需要时都希望能接通任何一个其他用户。电话交换的最根本要求就是使每个电话用户能和北美洲内、甚或和世界上任何用户接通电话。在电话交换问题上有三个基本方面。第一,用以连接电话的交换设备的制式。第二,局内连接线路或中继线的连接网络。第三,交换设备为某一特定的通话按正常途径进行接续的控制方法。 North American phone users have far exceeded 100 million. Each user wants to be able to reach any one other user when needed. The most fundamental requirement for telephone exchange is to enable each telephone user to make calls with any user in North America, or even in the world. There are three basic aspects to the telephone exchange problem. First, the format of the switching equipment used to connect the phones. Second, the intra-office connection line or trunk connection network. Third, switching equipment for a particular call by the normal way to continue the control method.