历史概况在过去的几十年里,公共交换电话网(PSTN)的发展遍及世界几乎所有国家和地区。公共交换电话网的普及满足了人们之间话音通信的需求。然而随着电子计算机的发展及普及,公共交换电话网的应用已不仅仅局限于普通的电话通信,越来越多 Historical Facts and Figures Over the past few decades, the Public Switched Telephone Network (PSTN) has been developed in almost all countries and regions in the world. The popularity of public switched telephone networks addresses the need for voice communications between people. However, with the development and popularization of electronic computers, the application of the public switched telephone network has become more and more limited to ordinary telephone communications