前言面对日益动态变化的电信环境，传统的准同步数字体系（ＰＤＨ）正暴露出一些难以克服的固有弱点，诸如缺乏标准接口，僵硬的复用结构和极其有限的网管能力等。显然，要想在原有技术体制和框架内解决这些问题是事倍功半、得不偿失的，唯一的出路是从技术体制上进行根本... Preface With the ever-changing telecom environment, the traditional PDH is experiencing some insurmountable inherent weaknesses such as the lack of standard interfaces, rigid multiplexing architecture and extremely limited network management capabilities. Obviously, in order to solve these problems in the prior art system and framework, it is more effective and less profitable, and the only way out is fundamentally from the technical system.