目前世界各国电信网都是以电话网为基础。各国电话网结构的分级名称和局级数虽有不同,但大体上分为两类:一类如日本、英国等领土较小国家,实行包括市话端局在内的“四级中心交换局”结构;另一类如美国,在五十年代也曾实行“四级制”,以后由于领土较大,随着业务量和通信设备的大量增加,改 At present, all the telecommunication networks in the world are based on the telephone network. Although the hierarchical names and bureau levels of telephone network structures in various countries are different, they are broadly divided into two types: one is the smaller territories such as Japan and the United Kingdom, and the “four-level central exchange offices” Structure; the other category, such as the United States, also implemented the “four-tier system” in the 1950s. Since then, due to its large territory, as the volume of business and communications equipment has increased substantially,